ACTA BIOLOGICA SLOVENICA 
 LJUBLJANA 2022              Vol. 65, Št. 1: 48-69
One year spread and insight into ecology of invasive Impatiens glandulifera in 
Ljubljansko barje area (Central Slovenia)
Enoletna dinamika širjenja in vpogled v ekologijo invazivne tujerodne vrste 
Impatiens glandulifera na območju Ljubljanskega barja (osrednja Slovenija)
Azra Šabića*, Nejc Joganb
a Agricultural Institute of Slovenia, Hacquetova ulica 17, 1000 Ljubljana, Slovenia
b Biotechnical Faculty, University of Ljubljana, Jamnikarjeva 101, 1000 Ljubljana, Slovenia
*Correspondence: Azra.Sabic@kis.si
Abstract: Impatiens glandulifera Royle (Himalayan balsam) is an annual plant, 
native to humid parts of the Himalayas. Brought to Europe in the XIX century, it has 
since successfully naturalized and spread throughout the continent, becoming one of 
the best-known invasive plants. Even though it has been thoroughly studied by many 
authors, some aspects of its biology and ecology remain unclear and debatable, such 
as its spreading dynamics, negative impacts in invaded ecosystems and ecological 
adaptability regarding moisture, nutrients, and light. This 2019 field study from the 
Ljubljana Marsh (Slovenia) has proved that Himalayan balsam successfully develops 
under mesophilic conditions, where it easily compensates moderate deviations from 
its ecological optimum. The species thrives in riparian zones, out-competing native 
vegetation, trait shown as potentially the biggest negative impact of chosen species in 
this context. This study additionally gave us a different insight into - usually highly 
emphasized - dynamics of species’ spreading. Spreading was not either dependent on 
hydrochory or as drastic as mentioned in literary sources, giving it a secondary role 
in species’ invasiveness on the chosen area. Lastly, we proposed rough estimates of 
eradication expenses for given area, based on our field results of species’ abundance 
and distribution on chosen area.
Keywords: ecology, Himalayan balsam, Impatiens glandulifera, invasive species, 
plant invasions
Izvleček: Impatiens glandulifera Royle (žlezava nedotika) je enoletna rastlina, 
domorodna v vlažnih predelih območja Himalaj. Vrsta je bila vnešena v Evropo v XIX. 
stoletju, kjer je od takrat, kot ena izmed najbolj znanih tujerodnih vrst, postala uspešno 
naturalizirana in invazivna, ter razširjena po vsej celini. Nekatere njene biološke last-
nosti še vedno ostajajo precej nejasne in vprašljive. To se nanaša predvsem na njeno 
invazivnost in morebitni vpliv v ekosistemih, dinamiko širjenja in možnost prilagajanja 
na različne okoljske dejavnike, kot so vlaga, hranila in svetloba. Terenska raziskava iz 
2019, opravljena na območju Ljubljanskega barja v osrednji Sloveniji, je pokazala da 
žlezava nedotika uspeva v zmernih pogojih, kjer uspešno prenaša zmerna odstopanja 
od svojega ekološkega optimuma. Vrsta je najbolj uspešna v obrežnem pasu, kjer je 
49Šabić and Jogan: Impatiens glandulifera in Ljubljansko barje 
precej bolj kompetitivna kot domorodna vlagoljubna vegetacija, kar bi v tem kontekstu 
lahko definirali kot največji negativni vpliv izbrane tujerodne vrste. V tej študiji smo 
tudi ugotovili, da ima dinamika širjenja sekundarni pomen v njeni invazivnosti na 
Ljubljanskem barju, ter da širjenje ni odvisno od hidrohorije, niti ni toliko uspešno kot 
navajajo nekateri drugi literaturni viri. Na podlagi rezultatov o razširjenosti in velikosti 
lokalnih populacij žlezave nedotike na Ljubljanskem barju smo pripravili ocene stroškov 
odstranjevanja vrste in predlog monitoringa stanja v prihodnjih letih.
Ključne besede: ekologija, Impatiens glandulifera, invazivne tujerodne rastlinske 
vrste, žlezava nedotika
Introduction
Short introduction to biological and plant 
invasions
Biological invasions are one of the most covered 
topics in modern science and an important negative 
consequence of globalization (Lambdon et al. 2008, 
Perglová et al. 2009, Vilà et al. 2010, Vilà et al. 2011, 
Bieberich et al. 2020). Invasive species are diverse 
and known for their specific biological traits, which 
make them stronger competitors in comparison to 
native species and facilitate their success in newly 
invaded habitats (Čuda et al. 2014, Enders et al. 
2020). Many studies have proven negative impacts of 
invasive species on environment and society, which 
are derived from their disruption of ecosystem ser-
vices. The disruption comes from changes in native 
species’ diversity, trophic networks, biogeochemical 
cycles, and habitat structure in invaded ecosystems, 
all of which are confirmed consequences of biologi-
cal invasions (Hejda et al. 2009, Vilà et al. 2011, 
Pyšek et al. 2012, Blackburn et al. 2014). Impacts 
of invasive species are however context-dependent 
and determined by various aspects of individual 
case of species’ invasion, such as its residence time 
and naturalization status, biological traits of given 
species and biotic and abiotic conditions found in 
ecosystem they invade (Richardson and Pyšek 2006, 
Pyšek et al. 2012, Blackburn et al. 2014). Different 
ecosystems show different levels of vulnerability 
regarding biological invasions. Generally, it is 
proposed that ecosystems with higher number of 
endemic species are more susceptible to the negative 
impacts of biological invasions. On the other hand, 
ruderal vegetation is often predominantly composed 
of neophytes, to which frequent disturbances found 
in urban areas are suitable (Zelnik 2012). Due to 
both natural disturbances and human pressures put 
on them - such as building dams, flow regulations, 
degradation, eutrophication or draining, riparian 
habitats are exceptionally sensitive to biological 
invasions (Schnitzler et al. 2006, Richardson et al. 
2007, Nobis et al. 2017). Additionally, permanent 
presence of water is reducing drought stress, which 
is an important limiting factor for some alien plants 
from more humid climates.
General traits of Himalayan balsam
One of the invasive species commonly found 
in riparian habitats, where it triggers conservation 
issues, is Himalayan balsam, Impatiens glandulifera 
Royle (Hejda and Pyšek 2006). Himalayan balsam 
is a summer therophyte, up to 2.5 (4) meters tall 
and known as the tallest annual plant in Europe. It 
is easily distinguished by its zygomorphic, pinkish-
purple flowers and opposite, lanceolate-shaped, 
serrated leaves. Complex, multicellular, dark-purple 
tipped glands develop at the leaf base and serve as 
extrafloral nectaria, to which the species owns its 
scientific name. In Europe, seeds of Himalayan 
balsam start germinating in early spring, usually 
in March (Balogh, 2008), and according to the 
experimental study by Perglová et al. (2009), they 
require stratification for germination. Seedlings 
are very vulnerable to late frosts, snails, and fungal 
infections, as seen in a study by Prowse (1998), 
in which only about 5-10% of observed young 
plants survived. Flowering begins 13 weeks after 
the germination and it is the most intense in July 
and August, even though it can last until October 
or November. Individual flowers last for 2-3 days. 
Fruit and seed development end approximately 13 
weeks after the flowering and are followed by plant 
50Acta Biologica Slovenica, 2022, 65 (1), 48-69
maturation and seed dispersal, which take place 
in autumn and last until November. Fruit type is a 
capsule which can contain up to 16 seeds, so whole 
plant can produce up to 2500 seeds. Upon matura-
tion, capsules burst explosively, shooting seeds up 
to 7 meters away from the parent-plant (Balogh, 
2008). This mechanism is known as ballistochory 
(Elst et al. 2016). By this natural mechanism, local 
populations spread their range by 3-5 meters per 
year on average, even though spreading is often 
enhanced by human activities, animals, or waters 
(Balogh 2008). Often cited study by Perrins et al. 
(1993) reports on Himalayan balsam’s spread by 
rivers in England. Authors have calculated the 
maximum downstream spreading rate of 38 km/year, 
although they note that the realistic enhancement 
by hydrochory is around 2.6 km/year. Čuda et al. 
(2017a) additionally state that flooding might have 
an important role in species’ spreading. In the soil 
seed bank, some seeds can survive for 4 (Skalová 
et al. 2019) or even 6 years (Schuldes 1995). 
Himalayan balsam is often found in humid 
and half-shaded habitats, in both its native and 
non-native range. In Europe, it usually inhabits 
temperate areas with rainfall values of at least 250 
mm/year, optimally 1000 mm/year, with an average 
annual temperature between 6 and 12 ˚C. Species 
avoids direct sunlight, unless provided with adequate 
sources of water, since it is defined as hydrolabile. 
It is commonly found in riparian zones and damp 
ruderal communities (Balogh 2008), but it can 
also be found in other types of ecosystems, like 
forest fringes, arable fields, and meadows (Čuda 
et al. 2017a). Species is found growing in alluvial 
soils, loam, and humic clay, avoiding both acidic 
and alkaline soils (Balogh 2008). Species is often 
regarded as nitrophilous (Hejda and Pyšek 2006). 
According to Beerling and Perrins (1993), optimum 
soil nitrate values for Himalayan balsam range 
from 0.5±0.3 µg/g to 5.5±1.3 µg/g. Andrews et al. 
(2005) have confirmed that species has an ability 
of nitrates accumulation, what possibly stimulates 
species’ extensive growth in shaded habitats, whereas 
Andrews et al. (2009) report on positive effect of 
higher nitrates value on seed germination, later 
confirmed again by Skalová et al. (2019).
Often-used methodology used for describing 
species’ ecological preferences are indicator values 
(see Bartelheimer and Poschlod 2016, Chytrý et al. 
2018). In this study, we present and use modified, 
Ellenberg-type indicator values (shortened as EV 
in continuation) by Chytrý et al. (2018), which 
describe Himalayan balsam as transitional between 
the plants of half-shade and half-light, rarely found 
on less than 20% of full light intensity (EV for 
light 6), transitional between mesothermic and 
thermophilous plant species (EV for temperature 
6), transitional between indicators of wet and moist 
soils (EV for moisture 8), indicator of neutral soils, 
avoidant of extremely acid soils (EV for pH 7) and 
the favourer of fertile soils (EV for nutrients 7).
Introduction of Himalayan balsam in Europe and 
its current invasiveness status
Native to the Western Himalayas, species was 
introduced to Europe in 1839 as an ornamental 
plant, whose seeds were sent to Kew Garden from 
Kashmir by dr. J. F. Royle. Soon after the intro-
duction in 1848, Himalayan balsam escaped the 
cultivated area and was proclaimed as successfully 
naturalized in United Kingdom by 1855. In the 
continental Europe, first records on naturalization 
date from 1897, although the species probably 
didn’t become invasive in Europe until the last three 
decades of previous century (Balogh 2008). Today, 
Himalayan balsam is widely distributed across the 
Northern Hemisphere, in zone between 40˚ and 65˚ 
of geographic latitude, which includes most parts 
of temperate Europe and North America, along 
with some parts of Asia where it’s not native, such 
as Russia and Japan (Drescher and Prots 2003). 
Results of DAISIE project report on its presence 
in more than 30 European countries, out of which 
25 consider it as at least naturalized allochthonous 
species, what puts Himalayan balsam on the list 
of 150 mostly widespread alien plants in Europe 
(Lambdon et al. 2008). Due to its progressive spread-
ing and impacts on biodiversity and subsequently 
economy, it was included in the first edition of the 
List of invasive species of Union concern (European 
Commission 2017). Therefore, all member states 
of the European Union must act towards prevent-
ing its further introductions and spreading, while 
simultaneously working on adequate management 
and monitoring methodology (ibid.; Commission, 
2017). In Slovenia, it was first reported in 1935 
(Petkovšek 1966), whereas oldest findings on 
51Šabić and Jogan: Impatiens glandulifera in Ljubljansko barje 
northern margins of capital city Ljubljana date 
from Zalokar, from 1939 (data from University 
of Ljubljana’s Herbarium). Species has been suc-
cessfully spreading since, mostly along main water 
bodies, as seen in older publication by Prekoršek 
(1964), who claimed its presence by rivers Sava, 
Savinja, Drava and Mura, and is today widespread 
across the country (Jogan et al. 2001). Zelnik (2012) 
lists it as one of the most invasive alien plants in 
Slovenia, found in an array of habitat types, such 
as riparian zones, floodplains and swamps, forests, 
urban areas, and managed ecosystems.
Studies of Himalayan balsam’s invasiveness
Although much is known about the species, 
some aspects of its ecology and invasiveness are still 
insufficiently understood and sometimes debatable. 
For instance, while some authors report on its nega-
tive impact in invaded habitats (Hulme and Bremner 
2006, Kiełtyk and Delimat 2019), other studies 
deny it (Hejda and Pyšek 2006, Hejda et al. 2009, 
Diekmann et al. 2016, Čuda et al. 2017b, Bieberich 
et al. 2020). Notably, studies were carried out in 
different habitats and biogeographic regions, so the 
impact should be interpreted as context-dependent. 
For instance, Hulme and Bremner (2006) found that 
Himalayan balsam’s worst negative impact was on 
the native heliophytes in its undergrowth in ripar-
ian zones, whose development was disturbed due 
to Himalayan balsam’s rapid growth and height. 
Kiełtyk and Delimat (2019) confirmed species’ 
impact on reduced native biodiversity in temper-
ate meadows, while its impact was negligible in 
mesophilic forests (Čuda et al. 2017b). Apart from 
invasiveness, other questionable traits of the spe-
cies are its nitrophily - considering the fact that it 
survives in wider range of nutrients availability in 
soil (Beerling and Perrins 1993, Hejda and Pyšek 
2006, Clements et al. 2008), and its dependence 
on moisture and riparian zones. Some authors 
hypothesise that latter might be an evolutionary 
adaptation which would enhance spreading along 
the watercourses (Čuda et al. 2014). Results of study 
by Elst et al. (2016) have shown that Himalayan 
balsam could be pre-adapted to invasions, since 
both native and non-native population express 
phenotypic plasticity, regardless of environmental 
conditions they grew in. 
Aims of the study
Aim of this study was to get a clearer insight into 
dynamics of species’ seasonal spreading in chosen 
area, since it corresponds with its described ecologi-
cal optimum and it might show potential impact of 
water bodies in seed spreading. Additionally, due 
to differing literary sources on ecological traits of 
chosen species, we studied its ecology throughout 
its accompanying flora, for which we hypothesised 
that it would include widespread, ruderal species and 
neophytes. Due to the state of Himalayan balsam’s 
widespread and continuously growing populations in 
Europe and stance of European conservation politics 
on it, we proposed simplified eradication expenses 
estimation, based on field findings, along with an 
adequate eradication plan for initial and following 
upcoming management actions. 
Materials and methods
Study area
Field sampling was performed during the sum-
mer of 2019, in area of Nature Park Ljubljana Marsh 
(in continuation: Ljubljana Marsh; in Slovenian: 
Krajinski park Ljubljansko barje), which holds the 
basic level of nature protection as a nature park. 
Almost the entire area of Ljubljana Marsh was 
included in Natura 2000 network in 2004 and it 
has had a status of a protected area since 2008. It 
is a floodplain of river Ljubljanica and some of its 
tributaries, noted for its irregular, seasonal flooding 
and mosaic of different habitats including wetland 
meadows, riparian forests, and remains of peat bogs. 
The area has been under heavy human pressures, 
which include river flow regulation, peat excava-
tion, intense agriculture, and prolonged draining in 
recent times (Zorn and Šmid Hribar 2012). Human 
activities have also contributed to spreading of 
invasive species. Based on official reports on the 
distribution of Himalayan balsam in the previous 
season (Lozej 2018), we designed a systematic 
census of Himalayan balsam’s population in 2019, 
to compare states of populations between the two 
consecutive seasons and to see, whether theoretical 
claims of species’ spreading could be applied to 
chosen area. Our methodologies differed slightly: 
our inventory distinguished each locality its by 
52Acta Biologica Slovenica, 2022, 65 (1), 48-69
size and shape, followed by partial or complete 
inventory of accompanying flora, whereas previous 
report focused mostly on geographic mapping of 
entire population (Lozej 2018).
Field work
We sampled throughout July, August, and 
September of 2019, at the peak of the species’ 
development. Field data (geographical coordinates) 
from previous report (Lozej 2018) were used, which 
were all reviewed for presence of Himalayan balsam, 
along with the other potential localities near them, or 
elsewhere considered suitable for the species based 
on its biological traits. Entire road and cart-track 
network of the study area has been systematically 
monitored. Every confirmed locality was marked 
as either a point - when less than 50 meters long, 
or as either linear or polygon – both longer/wider 
than 50 m, typified according to the spatial traits 
(shape) of a given local stand. Two stands (localities, 
populations s.l.) were distinguished as separated, if 
there was approximately 10 meters between them 
without any flowering individuals of Himalayan 
balsam. Size of each stand was roughly estimated 
on a scale which consisted of 4 size classes: l-10, 
10-100, 100-1000 and over 1000 fully developed 
plants. Fully developed, flowering individuals were 
considered for localities’ distinguishment and size 
estimations, since Himalayan balsam is an annual 
therophyte which spreads primarily by its seeds. 
Therefore, flowering individuals represent adequate 
basis for population size estimations and stand as a 
reference for discussions regarding population dy-
namics in forthcoming vegetation seasons. To get a 
better insight in species’ ecology, the general habitat 
type was also determined, based on the predominant 
vegetation type, proximity to agricultural areas, 
travel infrastructure and water bodies. No detailed 
analysis (mapping) of habitat types were carried 
out, due to the complexity of study area, intercon-
nectedness of its many habitat types and changes 
in their structure, whose causation goes beyond 
this specific study. Each locality was described 
as accessible, inaccessible, or partially accessible, 
for purposes of further monitoring propositions 
and calculations of potential eradication expenses. 
Qualitative inventory of Himalayan balsam’s ac-
companying flora was carried out on 67.5% (370 
of 548 in total) of the locations (following nomen-
clature in Martinčič et al. 2007), whereas no cover/
abundance examination was carried out for those 
species. Inventories of accompanying flora were 
carried out based on accessibility and the general 
state of given stand (for example, overgrown by 
Himalayan balsam, or small plant stands where only 
few plant individuals of any species were present). 
In case of presence of any other invasive alien plant 
species within stands of Himalayan balsam, those 
were always documented, regardless of whether 
the inventory of the entire accompanying flora was 
carried out or not. 
Data analysis
Spatial data was processed in QGIS programme, 
version 3.18.1 (QGIS.org 2022), which was used to 
create maps of species’ current distribution and to 
compare Himalayan balsam’s population dynamics 
between two consecutive seasons. Distribution map 
in form of population hotspots was created using 
the built-in function in QGIS, which considers 
both the density of stands on a certain area, as well 
as their maximum size, as described above in the 
field work methodology. For ecological analyses, 
we used modified Ellenberg-type indicator values 
(Chytrý et al. 2018), applied to significantly abundant 
taxa, found at 11 or more locations where qualita-
tive vegetation surveys were carried out (f ≥ 11). 
Species which occurred sporadically in Himalayan 
balsam’s accompanying flora (f ≤ 10) were omit-
ted from analysis, due to their low abundance in 
microsites where Himalayan balsam was growing, 
so their co-existence wasn’t considered a possible 
indicator of Himalayan balsam’s ecological prefer-
ences. Average values for Ellenberg-type indicator 
values in Himalayan balsam’s accompanying flora 
for light (L), temperature (T), moisture (M), pH 
reaction (R) and nutrients (N) were calculated, 
along with percentages of each individual value 
for each ecological condition. Average values were 
then compared to the Himalayan balsam’s theoreti-
cal optimum for given ecological factor, following 
Chytrý et al. (2018). Observed average values in 
Himalayan balsam’s accompanying flora were then 
compared to the average values for each ecological 
condition for dataset of entire flora Ljubljana Marsh 
53Šabić and Jogan: Impatiens glandulifera in Ljubljansko barje 
flora (data from Jogan et al. 2001). Comparisons 
between Himalayan balsam’s stands and Ljubljana 
Marsh flora were carried out to get a robust estima-
tions of species’ possible ecological niche profil-
ing on chosen area, since no similar studies of 
Himalayan balsam’s accompanying flora ecology 
were carried out previously in Slovenia. Additionally, 
general ecology of Ljubljana Marsh fits into basic 
ecology of Himalayan balsam (described above), 
so its distribution and co-existence with specific 
species might provide additional information on 
species’ biology. Due to those reasons, all species 
in Ljubljana Marsh flora dataset were included 
in analysis. Species without modified Ellenberg 
values were left out of calculations. Distribution 
of significantly abundant accompanying flora on 
Ljubljana Marsh was used to create a simple model 
of Himalayan balsam’s prospective distribution in 
Slovenia. Occurrence of those species in floristic 
quadrants at Ljubljana Marsh was extrapolated to 
entire Slovenia, using data from Slovenian Centre for 
fauna and flora cartography, available at Bioportal.
si (2022). Suitability of conditions was simply esti-
mated as portion of present taxa of accompanying 
flora in quadrants of Slovenia.
Eradication expenses were estimated from 
our field results, based on values calculated from 
combinations of local populations’ accessibility and 
sizes. We assumed that partially accessible and inac-
cessible populations would require solely manual 
eradication work, whilst accessible ones could be 
managed by machines. Hypothetical wage of 10 
EUR/hour was used in both cases, fair for the work 
effort needed for theoretical hand-eradication of one 
plant per minute. For accessible stands, we made a 
simple calculation by multiplying their frequency 
with hypothetical wage of 10 EUR/hour. Partially 
accessible and inaccessible stands were analysed in 
more detail, taking into consideration their frequen-
cies combined with Himalayan balsam’s stands size 
for each stand category, multiplying both minimum, 
maximum sizes of stands (10, 100, 1000 and 10 000 
for stands for over 1000), along with combinations 
of minimum and maximum sizes in intermediate 
stands. Minimum stand size wasn’t considered only 
for populations of size range 1-10, due to low prob-
ability of finding single individual of Himalayan 
balsam in a stand, whilst stands with over 1000 
were defined with hypothetical upper stand size of 
10000. Those numbers were then divided with 60, 
resulting in hypothetical hours needed for manual 
eradication, then multiplied with hypothetical 10 
EUR/hour wage. Numbers were added to the value of 
eradication expenses for accessible stands, resulting 
in price ranges. To achieve the completely effective 
prevention of future flowering of a local population, 
consecutive approaches are needed in the following 
years, due to soil seed-bank persistency (Schuldes 
1995, Skálová et al. 2019). We predicted the rough 
estimate needed to cover those expenses in future 
growing seasons, which would in practice depend on 
the success of the previous eradication and should 
be significantly lower than the starting price point. 
Results
Distribution and estimation of spreading
In 2019, Himalayan balsam was found on 548 
localities in Ljubljana Marsh. Most of the localities 
were situated in the northern part of area, by the 
water bodies - usually streams, creeks and drain-
age ditches. Some of the biggest stands (written 
as numbered on Fig. 1) were found in villages 
Vnanje Gorice and Notranje Gorice by the stream 
Drobtinka (1), in Plešivica by the stream Veliki 
Graben (2), in Brezovica by the stream Radna (3), 
in area of Rakova Jelša by the stream Curnovec 
(4) and in village Črna vas, by the rivers Iška and 
Ižica, and streams Lahov graben and Prošca (5). On 
the southern parts of Ljubljana Marsh, Himalayan 
balsam occurrence is sparse, in form of scattered 
localities by rivers Borovniščica and Iška, by the 
the stream Draščica and in the Draga area, by the 
pond Veliki Ribnik. Due to uneven distribution and 
highlighted concentrated in the northern part of the 
area, we have decided to present species’ distribution 
in forms of both common map and hotspots map, 
as shown below on Fig. 1 (locations are numbered 
as written above).  We would additionally empha-
sise local stands 6*, located near the old riverbed 
of Ljubljanica river (in Slovenian: Stara struga 
Ljubljanice), and 7*, situated at the very border 
of Ljubljana Marsh, by the waste collecting and 
management centre Barje (in Slovenian: Zbirni 
center Barje). Those aren’t shown as true hotspots 
though, due to their smaller size compared to other 
stands which expand over larger area.
54Acta Biologica Slovenica, 2022, 65 (1), 48-69
Figure 1: Distribution of Himalayan balsam on Ljubljana Marsh (bordered with black) in season 2019, given in 
form of individual population locations, categorized by size classes (A) and hotspots (B). Hotspots are 
numbered as: Notranje and Vnanje Gorice (1), Podplešivica (2), Brezovica (3), Rakova Jelša (4) and 
Črna vas (5).  Additional mention goes to smaller areas with densely distributed populations, by the 
oldriver bed of Ljubljanica river (6*), and by the waste collecting and management center Barje (7*).
Slika 1: Razširjenost žlezave nedotike na območju Ljubljanskega barja (omejeno s črnim poligonom) v letu 2019, 
prikazana v obliki posamičnih nahajališč populacij, razdeljenih po velikostnih razredih (A) in vročih 
točk (B). Vroče točke so označene kot: Notranje in Vnanje Gorice (1), Podplešivica (2), Brezovica (3), 
Rakova Jelša (4) in Črna vas (5). Poudarjamo še dve manjši lokaliteti z večjo gostoto lokalnih populacij, 
kot sta Stara struga Ljubljanice (6*) in Zbirni center Barje (7*).
55Šabić and Jogan: Impatiens glandulifera in Ljubljansko barje 
Majority of the localities – 442, (80.7%) 
were marked as points - stands smaller than 50 
meters. We recorded 82 linear ones (15%) and 24 
polygons (4.3%). Around 90% of all localities had 
up to 1000 individuals, whereas the most frequent 
size class was the second one (10-100), with 223 
localities (40.7%), followed by the first one (1-10), 
with 152 (27.7%) and the third one (100-1000) 
with 122 localities (22.2%). Fifty-one localities 
counted more than 1000 individuals (9.4%). We 
found Himalayan balsam in 9 different habitats, 
listed according to their frequency: roads, ditches, 
fields, hedges, forests, grasslands, riparian zones, 
banks, and embankments. About a half (49.8%) 
of the stands were located by roads, followed by 
drainage ditches, where around a quarter (26.8%) 
of stands were found.  Other common habitat types 
included fields (8%), where we found species as 
either a weed growing among crops or at the field 
margins, and hedges (7.1%), fragments of bush 
vegetation specific for study area, which usually 
divide individual parcels. Himalayan balsam 
grew in other habitat types on Ljubljana Marsh 
only sporadically. 
Comparison of 2018 and 2019 spatial data 
showed that the general pattern of Himalayan 
balsam’s population on study area in two consecu-
tive seasons was very similar, without observed 
long-distance spreading. Our study has shown 
that Himalayan balsam can successfully spread 
up to 150 meters in one year (Fig. 2), whereas 
the average distance of newly recorded popula-
tions from the nearest localities known in 2018 
was 122 meters. 
Figure 2: Frequency distribution of measured distances between the newly recorded locations and the nearest 
previously known populations, regarding the spreading of Himalayan balsam in 2018-2019 period.
Slika 2: Porazdelitev pogostosti izmerjenih razdalj med na-novo zabeleženimi populacijami v letu 2019 in njim 
najbližjimi populacijami, zabeleženimi v letu 2018, s ciljem ugotavljanja dinamike širjenja žlezave 
nedotike v obdobju 2018-2019.
56Acta Biologica Slovenica, 2022, 65 (1), 48-69
Accompanying flora, statistical and ecological 
analysis
Inventory of Himalayan balsam’s accom-
panying flora resulted in presence of 238 other 
plant taxa. Identification was to the species-level, 
unless specimens were unreachable or underde-
veloped. On average, there were 11 other taxa in 
communities beside Himalayan balsam, whereas 
the maximum number was 53. Five of the most 
common accompanying species were European 
dewberry Rubus caesius L. (on 250 locations), 
common nettle Urtica dioica L. (221), cock’s-
foot Dactylis glomerata L. [s.str.] (200), giant 
goldenrod Solidago gigantea Aiton (139) and 
large-flowered hemp-nettle Galeopsis speciosa 
Mill. (114). The most common woody species 
were common alder Alnus glutinosa (L.) Gaertn. 
(93), white willow Salix alba L. (76), European 
ash Fraxinus excelsior L. (49), brittle willow Salix 
fragilis L. (36), goat willow Salix caprea L. (17) 
and black poplar Populus nigra L. (14). Alongside 
Himalayan balsam, on 285 of its stands (52 % in 
total), at least one of 26 other alien plants species 
was present (27 taxa, if we include inconclusively 
determined goldenrods, due to inacessibility). Apart 
from the already mentioned S. gigantea – which 
was the most common allochthonous species, other 
significantly abundant alien plants were Canadian 
goldenrod S. canadensis L. (80), small balsam 
Impatiens parviflora DC. (62), Japanese knotweed 
Fallopia japonica (Houtt.) Ronse Decr. (56) and 
annual fleabane Erigeron annuus (L.) Pers. (52). 
Ecological analysis of Himalayan balsam’s 
accompanying flora showed that average values 
of four of the observed environmental variables 
(temperature, pH reaction, light, and nutrients) 
were approximate to the given theoretical optimum 
(Ellenberg-type ecological indicator value) of 
Himalayan balsam for each variable (according 
to Chytrý et al., 2018), as seen in Tab. 1 below. 
Values for temperature and pH reaction had the 
lowest variance of all the results (0.45 in both 
cases), whereas the variance was the highest for 
moisture (2.15). In Tab. 1 are also presented values 
of Ellenberg-type indicator values for entire flora 
of Ljubljana Marsh.
Table 1: Analysis of Ellenberg-type indicator values for temperature, pH reaction, light, nutrients, and moisture, 
for Himalayan balsam‘s accompanying flora and Ljubljana Marsh flora (Jogan et al., 2001), following 
Chytrý et al. (2018). Corresponding values of Himalayan balsam are highlighted with an asterisk (*). 
Tabela 1: Analiza modificiranih Ellenbergovih indikatorskih vrednosti za temperaturo, pH reakcijo, svetlobo, 
količino hranil in vlago v spremljevalni flori žlezave nedotike na Ljubljanskem barju in za celotno floro 
Ljubljanskega barja (Jogan in sod., 2001), prirejena po Chytrý in sod. (2018). Vrednosti za žlezavo 
nedotiko so v preglednici označene z zvezdico (*). 
Short description of Ellenberg-type indicator 
values for temperature
Himalayan balsam’s accom-
panying flora
Ljubljana Marsh flora
Numerus of 
observations
Percentage (%) Numerus of 
observations
Percentage
(%)
1 – cold indicator (alpine and nival belt) 0 0 0 0
2 – between 1 and 3 0 0 0 0
3 – cool indicator (mostly in subalpine areas) 0 0 5 0.6
4 – between 3 and 5 0 0 55 6.9
5 – moderate heat indicator 41 59.4 321 39.9
6* – between 5 and 7 (lowland and colline 
species)
23 33.3 323 40.2
7 – heat indicator 4 5.8 83 10.3
57Šabić and Jogan: Impatiens glandulifera in Ljubljansko barje 
8 – between 7 and 9 1 1.5 16 2.0
9 – extreme heat indicator 0 0 1 0.1
Average value 5.5 / 5.5 /
Variance 0.45 / 0.74 /
Short description of Ellenberg-type indicator 
values for pH reaction
Himalayan balsam’s accom-
panying flora
Ljubljana Marsh flora
Numerus of 
observations
Percentage (%) Numerus of 
observations
Percentage (%)
1 – indicator of strong acidity 0 0 4 0.5
2 – between 1 and 3 0 0 12 1.5
3 – acidity indicator 0 0 29 3.6
4 – between 3 and 5 1 1.4 48 6.0
5 – indicator of moderate acidity 3 4.4 74 9.2
6 – between 5 and 7 32 46.4 205 25.5
7* – indicator of slightly acidic to slightly basic 
conditions
32 46.4 325 40.4
8 – between 7 and 9 1 1.4 104 12.9
9 – base and lime indicator 0 0 3 0.4
Average value 6,4 / 6.2
Variance 0.45 / 1.9
Short description of Ellenberg-type indicator 
values for light
Himalayan balsam’s 
accompanying flora
Ljubljana Marsh flora
Numerus of 
observations
Percentage (%) Numerus of 
observations
Percentage
1 – deep shade plants 0 0 0 0
2 – between 1 and 3 0 0 2 0.2
3 – shade plant 0 0 29 3.6
4 – between 3 and 5 5 7.3 68 8.5
5 – semi-shade plant 7 10.1 66 8.2
6* – between 5 and 7 19 27.5 151 18.8
7 – half-light plant 32 46.4 296 36.8
8 – light plant 6 8.7 164 20.4
9 – full light plant 0 0 28 3.5
Average value for accomp. flora 6.4 / 6.5
Variance 1.06 / 2.02
58Acta Biologica Slovenica, 2022, 65 (1), 48-69
Short description of Ellenberg-type indicator 
values for nutrients
Himalayan balsam’s accom-
panying flora
Ljubljana Marsh flora
Numerus of 
observations
Percentage (%) Numerus of 
observations
Percentage (%)
1 – occurring on nutrient-poorest sites 0 0 5 0.6
2 – between 1 and 3 0 0 57 7.1
3 – occurring on nutrient-poor sites more 
frequently than at average sites
0 0 112 13.9
4 – between 3 and 5 2 2.9 109 13.6
5 – occurring at moderately nutrient-rich sites 10 14.5 141 17.5
6 – between 5 and 7 20 29.0 180 22.4
7* – occurring at nutrient-rich sites more often 
than on average sites
25 36.2 141 17.5
8 – pronounced nutrient indicator 11 15.9 55 6.8
9 – concentrated at very nutrient-rich sites 1 1.5 4 0.5
Average value for accomp. flora 6.5 / 5.1
Variance 1.13 / 3.03
Short description of Ellenberg-type indicator 
values for moisture
Himalayan balsam’s accom-
panying flora
Ljubljana Marsh flora
Numerus of 
observations
Percentage (%) Numerus of 
observations
Percentage (%)
1 – strong drought indicator 0 0 1 0.1
2 – between 1 and 3 0 0 7 0.9
3 – missing on damp soil 1 1.45 64 8.0
4 – between 3 and 5 6 8.7 165 20.5
5 – indicator of average moisture and fresh soils 25 36.2 225 28.0
6 – between 5 and 7 16 23.2 105 13.0
7 – humidity indicator 5 7.3 53 6.6
8* – between 7 and 9 13 18.8 69 8.6
9 – wetness indicator 2 2.9 64 8.0
10 – aquatic plant which survives long periods 
without flooding
1 1.45 25 3.1
11 – aquatic plant rooted under water 0 0 10 1.2
12 – (almost) permanently submerged aquatic plant 0 0 16 2.0
Average value for accomp. flora 6.0 / 5.8
Variance 2.15 / 4.6
Values for Himalayan balsam’s accompanying flora from Tab. 1 are summarized graphically on 
box and whisker plot, seen on Fig. 3.
59Šabić and Jogan: Impatiens glandulifera in Ljubljansko barje 
Figure 3: Ellenberg-type indicator values of Himalayan balsam‘s accompanying flora on Ljubljana Marsh, shown 
with outliers for each ecological condition. x represents average value.
Slika 3: Ellenbergove indikatorske vrednosti za spremljevalno floro žlezave nedotike na Ljubljanskem barju, s 
prikazom odstopajojočih vrednosti. x predstavlja povprečno vrednost.
Similarly, for accompanying flora, variance 
across Ljubljana Marsh flora dataset was the 
highest for moisture, but the difference between 
average values between accompanying flora and 
entire dataset was the highest for nutrients (Tab. 
1 above). Comparisons between observed values 
of Ellenberg-type indicator factors for accompa-
nying flora and Ljubljana Marsh flora are shown 
graphically on box and whisker plot below, on 
Fig. 4., showing that the species primarily fell into 
middle, mesophilic range for many conditions on 
large-scale area, except regarding nutrients.
60Acta Biologica Slovenica, 2022, 65 (1), 48-69
Figure 4: Box and whisker plots with comparisons between Ellenberg-type indicator values for temperature, pH 
reaction, light, nutrients and moisture between Himalayan balsam‘s accompanying flora and Ljubljana 
Marsh flora. x represents average value.
Slika 4: Grafična primerjava Ellenbergovih modificiranih indikatorskih vrednosti za temperaturo, pH, svetlobo, 
nutriente in vlago med spremljevalno floro žlezave nedotike in floro Ljubljanskega barja.
Analysis of species’ potential future spread-
ing, based on existing data on distribution of its 
accompanying flora has shown that it is highly 
likely for Himalayan balsam to continue spreading 
across temperate regions of Slovenia. The only 
parts which are ecologically unsuitable are Alps, 
Dinarides (both with higher altitudes) and Sub-
Mediterranean region (with higher temperatures and 
longer summer drought period) (below on Fig. 5).
61Šabić and Jogan: Impatiens glandulifera in Ljubljansko barje 
Figure 5: Simplified model of possible Himalayan balsam‘s distribution in Slovenia, based on distribution of its 
accompanying flora on Ljubljana Marsh. Color code: blue, area of Ljubljana Marsh; orange, predicted 
distribution.
Slika 5: Poenostavljen model predvidenega areala žlezave nedotike v Sloveniji, narejen na podlagi razširjenosti 
spremljevalne flore vrste na Ljubljanskem barju. Barvna shema: modro, območje Ljubljanskega barja; 
oranžno, predviden ustrezen areal vrste v prihodnosti. 
Eradication expenses
According to our calculations (Tab. 2), ex-
penses for eradication on the entire area would 
run around 20.000 EUR per year. Eradication on 
accessible localities costs less since they would 
– theoretically - require only machine work. In 
our case, that cost was estimated approximately 
to 3.300 EUR, evaluated as number of accessible 
sites (330), multiplied by hypothetical wage of 10 
EUR/h. Larger effort and budget will be needed 
for eradication of the inaccessible populations 
hidden in deep drainage ditches, on steep banks, 
in thickets, below hedgerows and on private prop-
erties, which would require manual processing 
and therefore. Final costs depend on stands’ sizes 
and density - number of individuals that need to 
be pulled out, cut down and properly handled. 
Combining different possibilities of sizes (mini-
mum, minimum and maximum and just maximum 
sizes), expected prices range from ~ 5.000 EUR 
to ~ 50.000 EUR, which are both extremes with 
smallest possible and highest possible number 
of individuals included. Intermediate value, 
with the highest probability of combination of 
smaller and bigger populations (as observed on 
field), comes to a price point of 16.000 EUR, 
which combined with mechanical work, gives a 
final estimate of ~ 20.000 EUR per year. If the 
first-year eradication approach was proven as 
effective, repetition of the same approach in the 
consecutive years would be less expensive and 
after about 6 years, when the soil seedbank of 
existing populations would be exhausted, only 
regular monitoring of sites and hand pulling of 
individual plants (when needed) would suffice.
62Acta Biologica Slovenica, 2022, 65 (1), 48-69
Table 2: Himalayan balsam‘s eradication expenses‘ estimation for Ljubljana Marsh,  calculated on the basis of field 
results from 2019, by methodology explained in Methods.
Tabela 2: Ocena stroškov odstranjevanja žlezave nedotike na Ljubljanskem barju, narejena na podlagi rezultatov 
terenske študiji po metodologiji opisani v zgornjem delu besedila, v poglavju Metod.
Stands classification by size classes and accessibility
Stand size 
classes
Number of acces-
sible stands 
Number of partially 
accessible stands 
Number of inacces-
sible stands 
∑ of stands by 
size classes
1-10 97 9 46 152
10-100 151 13 59 223
100-1000 58 34 30 122
Over 1000 27 14 10 51
333 70 145 548
x10 EUR/h 3330
∑ partially 
accessible and 
inaccessible stands
Minimum sizes of 
stands considered in 
calculations
Min. and max. stand 
sizes combined in 
calculations
Min. and max. 
stand sizes 
combined (2) in 
calculations 
Maximum 
stand sizes 
considered in 
calculations
1-10 55 550 550 550 550
10-100 72 720 7200 7200 720
100-1000 64 6400 6400 64000 64000
Over 1000 24 24000 24000 24000 240000
∑ 215 31670 38150 95750 311750
In hours 527.83 635.83 1595.83 5195.83
x10 EUR/h 5278.33 6358.33 15958.3 51958.3
Total estimated cost 
in EUR (+ acc.)
8608.33 9688.33 19288.33 55288.33
Discussion
Spreading dynamic and species’ ecology
Scattered distribution of Himalayan balsam 
in the given area is very likely a result of many 
circumstances, such as area’s geographical and 
hydrological structure, complex conservational 
system, intensive agriculture, and several other 
human activities. Dominance of point-type stands 
is not surprising, since those included an array of 
different sized stands, including either handful of 
individuals or up to 1000. Those smaller stands were 
found in a range of different habitats - predominantly 
dispersed alongside the roads, but were also located 
in fields, meadows, by the forest edges or along the 
waters. Linear and polygon-type stands were mostly 
located on larger, often less accessible areas, such as 
alongside drainage ditches, where linear stands were 
predominant, or on abandoned fields or properties, 
dominated by polygon-type stands with over 1000 
individuals. Those are likely older stands, which 
successfully maintain their size by local spreading. 
Between the two seasons, Himalayan balsam has 
spread across the Ljubljana Marsh only moderately 
and to relatively short distances: mostly up to 150 
meters away of existing populations, what confirms 
its hypothesised successful spreading capabilities by 
water only partially. Based on our results and our 
field observations, we assume that water bodies of 
Ljubljana Marsh are not the main vector of species’ 
spreading on chosen area. Firstly, there were only few 
populations located near the biggest water course - 
river Ljubljanica, while the rest were mostly found 
63Šabić and Jogan: Impatiens glandulifera in Ljubljansko barje 
by the streams or along ditches. Secondly, their main 
hydrological characteristic: slow water flow due to 
minimum elevation differences, could not carry out 
long-distance spreading as seen in some other studies 
(Perrins et al. 1993), which report on spreading dy-
namics in British Isles of 38 km/year. We did confirm 
successful short distance spreading in the nearby area 
of the parent plants, which reaches up to 7 meters by 
the ballistochory mechanism (Balogh 2008). The main 
spreading mechanism in these instances was more 
effective than just ballistochory, but situation is not 
clear-cut. We can assume that various anthropogenic 
factors on Ljubljana Marsh have had a significant 
role in species’ spreading, along with limited hydro-
chory. Thus, we presume that the primary role of 
water bodies in drainage ditch network of Ljubljana 
Marsh might be maintenance of local populations, 
by enabling favourable moist conditions, regular 
opening of bare soils due to ditch maintenance and 
reduced accessibility for the agricultural activities 
(mowing, pasturing, trampling). Čuda et al. (2014) 
hypothesized that existence of Himalayan balsam in 
riparian zones represents an evolutionary adaptation 
which would reinforce the hydrochory. Our results 
cannot confirm that either. Another note was the ab-
sence of typical winter floods in given period. Floods 
were reportedly mentioned as a contributing factor 
in invasiveness of the species (Čuda et al. 2017a). 
For additional insight into spreading dynamic, we 
checked Flood Warning map of Ljubljana Marsh 
(data from Slovenian INSPIRE metadata system, 
2022). The southwest part of Ljubljana Marsh is the 
one primarily exposed to regular flooding (Fig. 6), 
Himalayan balsam is mostly absent there, as seen in 
Fig. 1, except for parts of Podplešivica and some parts 
of Notranje Gorice and Vnanje Gorice. Hereof, we 
conclude that hydrochory should not be considered 
as the main pathway in spreading of given species 
on Ljubljana Marsh, especially in such short time 
interval. Further research, which would incorporate 
the possible impact of flooding, is therefore highly 
appreciated. 
Figure 6: Flooding areas of Ljubljana Marsh (bordered with black line), which are divided into areas of  extremely 
rare flooding (lightest blue on map), which include floods with return period of 50 years and more, areas 
of rare flooding (medium blue on map), which include floods with return period between 10 and 20 years, 
and areas of frequent flooding (darkest blue on map), which include floods with return period of 2 to 5 
years. GIS layers and correspoding legend descriptions are found at publicly available server of Slovenian 
INSPIRE metadata system, as Flood warning map (2022). Red points represent clusters of Himalayan 
balsam‘s populations found at Ljubljana Marsh, grouped by built-in function Point Cluster in QGIS 3.18.1.
64Acta Biologica Slovenica, 2022, 65 (1), 48-69
Slika 6: Območja poplavljanja na Ljubljanskem barju (omejeno s črno linijo), razdeljena v območja zelo redkih 
poplav (svetlo modra na karti), ki vključujejo poplave s povratno dobo 50 ali več let, območja redkih 
poplav (srednje temno modra na karti), ki vključujejo poplave s povratno dobo od 10 do 20 let, ter 
območja pogostih poplav, ki vključujejo poplave s povratno dobo od 2 do 5 let. Uporabljene GIS da-
toteke in njihovi opisi so dostopni na spletni strani Slovenskega INSPIRE metapodatkovnega sistema, v 
zavihku Opozorilna karta poplav (2022). Rdeče pike na karti predstavljajo grupirane populacije žlezave 
nedotike, narejene s pomočjo sistemske funkcije Point Cluster v programu QGIS 3.18.1.
Accompanying flora was similar to the litera-
ture reports (Hulme and Bremner 2006, Balogh 
2008, Diekmann et al. 2016, Kiełtyk and Delimat 
2019), simoultaneously very heterogenous and 
reflective of Ljubljana Marsh’s habitat diversity 
and its degradation. Even though over half (285) 
of our localities had at least one other alien plant 
or other elements of neophytic vegetation, on 263 
localities Himalayan balsam was the only one. 
Those were usually the ditches with native riparian 
vegetation, most commonly willow thickets and 
occasional alder communities. Adequate ecologi-
cal conditions allowed the Himalayan balsam to 
competitively exclude and supress native riparian 
forest vegetation, since black poplar, white willow 
and crack willow appeared to be ecologically most 
similar species to Himalayan balsam (see Chytrý et 
al. 2018). Those ecosystems have a crucial role as 
pionneers in the progressive succession of riparian 
forests (Garófano-Gómez et al. 2017), but due to 
their heavy invasion by alien species on entirety 
of Ljubljana Marsh, those natural processes seem 
to be significantly disturbed. Our observations and 
results therefore put focus on disturbance of riparian 
zones as the potentially biggest negative impact 
caused by Himalayan balsam on Ljubljana Marsh. 
Based on the results of robust ecological analysis 
of indicator values, we suggest that Himalayan 
balsam should be observed as a species with 
somewhat broader ecological niche than the one 
usually ascribed to it. Most surprising variation 
from its ecological optimum was for moisture, 
with accompanying flora’s average resulting in 
6.0. If we look at values of entire Ljubljana Marsh 
area and compare them with accompanying flora 
of Himalayan balsam, we see that Himalayan 
balsam falls into average ecological values of 
area, with a possibility of realized ecological 
niche profiling into its alleged nitrophily, since 
micro-sites where Himalayan balsam grew were 
by the results of indicator values analysis more 
suitable for nitrophilous species (averages 6.5 and 
5.1, respectively). Those results were probably 
influenced by large presence of Urtica dioica, the 
second most common species in accompanying 
flora and only true nitrophilous species, with EV 9. 
Even though its accompanying flora didn’t reflect 
hidrophily per se (with average value of 6.1 and 
theoretical optimum of 8), there were small differ-
ences between its stands and entire area (averages 
6.0 and 5.8), hinting that it grows at areas slightly 
more hydrophilous than the entire area.
 Species is probably more tolerant to moderate 
variations of ecological conditions and capable of 
surviving in different habitats, co-existing with a 
range of different species, what Bieberich et al. 
(2020) list as one of crucial traits which determine 
the success of invader. As an example, we point 
out Himalayan balsam’s road populations. Even 
though those stands were usually smaller in size, 
species is obviously capable of existence in sites 
which are by default more open, bright and dry 
than the expected species’ ecological optimum (see 
Chytrý et al. 2018). Accompanying flora at those 
sites consisted of different ruderal grasses, such as 
Lolium perenne L., Setaria pumila (Poir.) Roem. & 
Schult., Setaria viridis (L.) PB., Panicum capillare 
L. and Digitaria sanguinalis (L.) Scop., or ruderals 
like Cichorium intybus L. On the other hand, light 
and/or dryness weren’t shown as possible limiting 
factors when stands were found in an optimum 
regarding water and/or nutrients, for example along 
ditches on an open field, where Himalayan balsam 
might have compensated exposure to open light 
with sufficient amount of water from ditches. On 
arable land, Himalayan balsam’s stands probably 
have a dependable source of nutrients, coming from 
both water and soil. Thus, our results – at least 
partially - refute findings of Čuda et al. (2014), 
which indicate that species avoids fully open sites, 
but agree with the later study (Čuda et al. 2017a), 
whose results hint that species successfully grows 
65Šabić and Jogan: Impatiens glandulifera in Ljubljansko barje 
in open areas, when appropriately supplied with 
water. Optimal conditions for Himalayan balsam 
(see Chytrý et al. 2018) can therefore be achieved 
in micro-ecological sites and niches, what is not 
unusual for Himalayan balsam (see Bieberich et 
al. 2020 – different approach with a similar final 
result). In those (relatively) optimal conditions, 
negative impact of species could come to its peak. 
Eradication suggestions
For successful supression of Himalayan bal-
sam’s further invasion, we suggest regular and 
proper eradication, followed by continuous moni-
toring. Proper eradication methodology is mostly 
dependent on knowledge of species’ phenology, so 
it is important to choose the right moment for the 
first summer eradication. In this case, it would be 
recommended to start in July. If we start priorly, 
we risk the re-establishment of the population from 
seeds or by resprouting. If we start later - towards 
the fall season, we might assist in the seed spread-
ing from ripe capsules whilst handling the plants. 
Therefore, we suggest one removal in July and one 
later in the season, to remove potentially regener-
ated individuals (Hartmann et al. 1995). Skalová 
et al. (2019) have confirmed the viability of seed 
bank lasting up to 4 years, so we highly suggest 
the continuation of proper monitoring and prospec-
tive repeated removal in 4-year period at shortest, 
preferrably longer, since there are also reports on 
seed viability after 6 years (Schuldes 1995). With 
regular removal and control, eradication costs 
(proposed in Results) will probably decrease in 
the consecutive years. To terminate the invasion 
process, we must also prevent further importation 
and re-introduction, which in case of Himalayan 
balsam would be prohibition of horticultural and 
beekeeping usage. 
Conclusions
This study has confirmed the importance of 
context-dependence in research field of invasion 
ecology. Despite great number of studies on 
Himalayan balsam (Impatiens glandulifera), spe-
cies is still somehow overlooked and trivialized. 
On Ljubljana Marsh, Himalayan balsam has shown 
more ecological plasticity than usually considered, 
surviving in wider range of mesophilic conditions 
which deviate from its theoretical optimum – 
moist, fertile and shady places. As long as any 
of species’ critically impactful ecological condi-
tions – light, moisture and nutrients – are at least 
near its theoretical optimum, deviations of other 
ecological conditions don’t pose as the limiting 
factor in species’ persistance, especially regard-
ing moisture. We have found that species doesn’t 
spread as drastically by water bodies on Ljubljana 
Marsh in two consecutive seasons as mentioned 
in other literary sources, meaning that specific 
hydrology of Ljubljana Marsh cannot contribute 
to its invasiveness, as other factors – presumably 
anthropogenic – might. Hydrology of Ljubljana 
Marsh therefore probably has a role in maintenance 
of already existing populations and spreading 
dynamics is not the basis of species’ invasiveness 
in given circumstances. We propose competitive 
exclusion of native riparian vegetation as the 
main mechanism of species invasiveness, along 
with flexibility and adaptability to wide range of 
ecological conditions. As species of Union concern, 
Himalayan balsam should be regularly eradicated 
and monitored at adequate period, dependent on its 
phenology. At chosen site, in approximately five-
year period, species can be successfully controlled 
with moderate expenses, although prevention of 
further introductions is crucial.
Povzetek
Biološke invazije so, zaradi negativnega vpliva 
na biotsko pestrost in ekosistemske storitve eden 
izmed največjih izzivov, s katerimi se soočajo 
sodobno naravovarstvo, družba in gospodarstvo. 
Zaradi velike raznolikosti tujerodnih vrst, povzro-
čajo tudi različne vplive v različnih ekosistemih, 
kar je odvisno od njihovih bioloških lastnosti, 
vitalnosti in odpornosti avtohtonih združb. V tem 
članku so predstavljeni rezultati terenske študije z 
Ljubljanskega barja iz leta 2019, ko smo primer-
jali stanje populacij žlezave nedotike (Impatiens 
glandulifera) med letoma 2018 in 2019. Glede na 
to, da se v literaturi pogosto poudarja sposobnost 
hitrega širjenja te vrste z vodami, smo predvsem 
vzdolž vodotokov pričakovali  velike razlike 
stanja populacij med dvema sezonama. Izkazalo 
66Acta Biologica Slovenica, 2022, 65 (1), 48-69
pa se je, da hidrološke in poplavne razmere na 
Ljubljanskem barju ne omogočajo hitrega širjenja 
žlezave nedotike, ker med dvema sezonama ni bilo 
izrazitih razlik. Rezultatov tujih študij razširjanja 
žlezave nedotike ob hitro tekočih vodah torej ne 
moremo preprosto uporabiti pri napovedovanju 
širjenja ob počasi tekočih ali skoraj stoječih voda 
Barja. Za te se je v študiji pokazalo, da imajo 
predvsem pomen v vzdrževanju primernih razmer 
za obstoj populacij nedotike. Ta študija nam je 
pokazala, da je vrsta ekološko bolj plastična, kot si 
jo morebiti predstavljamo iz posplošenih navedb iz 
literaturnih virov, ki jo praviloma obravnavajo kot 
vlagoljubno, sencoljubno in nitrofilno. Vse dokler 
je vsaj eden izmed kritičnih okoljskih dejavnikov 
(svetloba, hranila in vlaga) zunaj pesimuma ali vsaj 
blizu optimuma, drugi okoljski dejavniki za vrsto 
očitno niso izrazito omejujoči. Zaradi velikega 
števila spremljevalnih drugih tujerodnih vrst na 
rastiščih nedotike in splošnega degradiranega 
stanja Ljubljanskega barja tudi ni bilo mogoče 
govoriti o konkretnem vplivu žlezave nedotike 
na ta ekosistem, vendar naši rezultati kažejo, da 
vrsta vrhunec svoje invazivnosti kaže v obrežnih 
gozdovih vrbe in jelše, kjer kompetitivno izklju-
čuje avtohtono obrežno floro in moti naravni tok 
sukcesije. Ker so rezultati naše in drugih študij o 
žlezavi nedotiki dokaj raznoliki, poudarjamo po-
men vsake individualne in primerjalne raziskave. 
Le tako bomo lažje razumeli biologijo teh vrst v 
širšem kontekstu, kar je izjemno pomembno pri 
raziskavah invazivnih tujerodnih vrst v raznolikem 
in zanje novem okolju, kar nam bo kot biologom 
omogočilo zaznati ključne momente biologije 
posamezne invazivne vrste, na katere bomo lahko 
oprli naravovarstvene ukrepe. 
Aknowledgments
We would like to thank to all kind people in 
administration of Nature park Ljubljana Marsh 
for allowing us to do our research in the area 
and for providing us with all the data we needed, 
primarily field data. Research was financed by 
project of Krajinski park Ljubljansko barje 430-
074/2019-2 and defended as MSc Thesis of the 
first author at the Biotechnical Faculty, University 
of Ljubljana. First author is now affiliated with 
Agricultural Institute of Slovenia.
We are also thankful to anonymous reviewers 
for their useful suggestions and commentaries.
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