Impatiens glandulifera Royle, Indian Balsam
Introduction, neophyte, an invasive, well-naturalised garden escape, sometimes locally dominant.
1946; MCM & D; roadside at Ballagh Crossroads.
May to November.

Growth form and preferred habitats: Growing up to 2.5 m tall with thick, hollow, translucent, succulent, ribbed stems and with buttress roots to keep it erect, I. glandulifera regularly forms dense pure stands in moist sites, often along the banks of waterways, but also in open areas of damp woods, especially where there has been moderate disturbance, plus in flushes and bogs. It is the tallest annual in the British flora and is branched above, swollen at the nodes, with lanceolate to ovate, serrately toothed leaves, 10-20 cm long, held opposite or in whorls of about three, borne on 8 cm petioles (Grime et al. 1988, 2007; Sell & Murrell 2009).
Introduced to these islands from India as a garden ornamental around 1839, I. glandulifera seeds abundantly and very soon escaped from cultivation. Valentine (1978) says it was first recorded in the wild in 1848, only nine years after its first garden introduction, although another report puts the first record in the wild as being 1855 in Middlesex (Kent 1975). It has since then become naturalised in moist soils in many parts of lowland Britain and, to a lesser extent, throughout Ireland (Beerling & Perrins 1993; R.M. Burton, in: Preston et al. 2002). The spread was so rapid and the plant so dominant that it had been given weed status as early as 1898 (Perrins et al. 1993). This is remarkable considering that, unlike most serious weeds, it is neither a perennial nor a weed of agricultural importance.
A summer annual therophyte with no powers of vegetative reproduction, the plant does however produce abundant seed. These require chilling to break their dormancy, germination being timed to occur synchronously in the spring. On suitably damp soils growth is very rapid and produces tall, dense stands typically of 20 or more plants per square metre. It is tolerant of a wide range of soil texture, structure and chemistry, ranging from free-draining mineral soils to maritime shingle and peat bogs (Beerling & Perrins 1993).
The 'competitive-ruderal', or perhaps better, 'ruderal dominant' growth strategy of I. glandulifera (Grime et al. 1988, 2007), allied at the end of the season with a heavy accumulation of persistent litter, enables the species to shade out competitors, including previously established perennials. The accumulating litter then suppresses other seedlings. In this way, I. glandulifera holds its ground and greatly reduces plant species diversity in all forms of invaded habitat (Grime 1977; Grime et al. 1988, 2007).

Flowering reproduction: Plants flower continuously from June to October, a period of about 12 weeks. The large, bisexual flowers are borne in a diffuse cyme of 3-12 in the axils of upper branches. They are strongly zygomorphic (irregular) in shape and are pollinated by bumble and other types of bees that crawl right inside the large open corolla tube to reach the stout nectar spur. Wasps and moths also frequently visit the flowers and, presumably, they can carry out pollination (Beerling & Perrins 1993). It is unusual for an alien insect-pollinated species to be successful in B & I without having to resort to self-pollination, as most introductions do not manage to attract sufficient suitable insect visitors (Valentine 1978). Having said this, I. glandulifera is self-compatible and can set seed to a lesser extent without insect involvement (Valentine 1971). Seed is set from mid-July onwards. Flower colour varies from purple, through shades of pink, to white, indicating some degree of genetic variation. After fertilisation, the fruit capsule begins to develop and the whole corolla, including the nectar spur, falls off the plant (Beerling & Perrins 1993).

Seed production and dispersal: Seed production per plant varies with the density of plants in a stand. Medium-sized plants growing at a density of 20 per square metre produce between 700 and 800 seeds per plant (Salisbury 1964). I. glandulifera generates a peak seed rain of around 1,200 per square metre measured at 2 m from the parent plant. Short-distance dispersal is dependent on explosive dehiscence as the 15-35 mm long capsule dries and ruptures along linear sutures (lines of weakness), firing the contained 1-6 seeds a distance of up to 5 m from the parent plant (Salisbury 1964; Sell & Murrell 2009). The dispersal distance achieved depends on the height of the capsule above the ground, the direction and strength of the prevailing wind at the relevant moment, and the degree of crowding by adjacent stems. Waterside seed may also spread rapidly with the flow. Upstream movement is slower but does occur; apart from the wind, it may involve seed catching by small rodents or human intervention.

Fermanagh occurrence: From its first recorded occurrence on a roadside in 1946, in recent years, I. glandulifera has spread along the banks of the Colebrooke, Kesh and Ballinamallard rivers and around the eastern shores of Lough Erne. It has also appeared along a few damp grassy roadsides, forest paths, in urban and rural waste ground, field corners and even into much drier conditions in a neglected area of a demesne yard. It has been recorded in a total of 37 Fermanagh tetrads (7%), 34 of them with post-1975 dates and it is definitely actively spreading in the lowlands E of Lough Erne. The fact that it is beginning to appear in drier ground, sometimes quite distant from water bodies, suggests that, like Heracleum mantegazzianum (Giant Hogweed), it may be entering a phase of secondary colonisation, during which it may spread into the general countryside.

Weed control: Man has sometimes created favourable conditions for the spread of the species through the destruction of natural riverbank communities and unwittingly has facilitated long-range dispersal by trade and transport of seed-contaminated soil. Fortunately for present and future control of the species, there is little or no persistent seed bank (Grime et al. 1988, 2007; Perrins et al. 1993). Like another eastern alien, Fallopia japonica (Japanese Knotweed), I. glandulifera is sensitive to late spring and early autumn frosts and it cannot survive prolonged drought. Unlike Japanese Knotweed, I. glandulifera can tolerate moderate shade, and it can flower and seed in open woodland and amongst tall reeds (Grime et al. 1988, 2007). In Fermanagh, I. glandulifera has near ideal conditions as the climate is hyper-oceanic, with typically mild temperatures and evenly spread precipitation right throughout the growing season.
Weed control of I. glandulifera generally requires at least two applications of herbicide (2, 4-D-amine), as the canopy of taller plants may protect stunted individuals. Spraying must be carried out prior to flowering and seeding, since plants sprayed while flowering can still set seed (Beerling & Perrins 1993).

Herbivory: While birds do not appear interested in the seed, sheep and cattle graze and trample all the aerial parts of the plant and a limited range of insects feed and lay eggs on the plants (Beerling & Dawah 1993). Otherwise, I. glandulifera appears immune to attack from both parasites and diseases.

British and Irish occurrence: Common and widespread throughout lowland Britain and most frequent on the banks of waterways. It is still spreading and has reached more outlying areas including the Isles of Scilly, Orkney and Shetland. As the New Atlas hectad map indicates, it becomes confined to coastal or near coastal sites further north into Scotland (Beerling & Perrins 1993; New Atlas). In Ireland it is very much more thinly and widely scattered although it is quite a lot more frequently recorded in NI than in the RoI. Even here, the occurrence thins out westwards and southwards, although the mild, damp, temperate oceanic climate should be very suitable for its growth (New Atlas).

European and world occurrence: I. glandulifera is widespread throughout 18 countries in temperate Europe between 30°N and 64°N. Large populations have been reported from S Sweden, especially along watercourses and it is widespread and spreading in both Czechoslovakia and Switzerland. It is also widespread in the Pyrenees, the Alps and around the Mediterranean basin. In the north, it is widespread throughout the Baltic and S & SW parts of Russia (Beerling & Perrins 1993). It has also been introduced and become naturalised on both islands of New Zealand (Webb et al. 1988).

Threats: I. glandulifera is still spreading and poses a threat to native species in a variety of damp habitats.

Grime, J.P., Hodgson, J.G. and Hunt, R. (1988 & 2007); Grime, J.P. (1977); Beerling, D.J. and Dawah, H.A. (1993); Beerling, D.J. and Perrins, J.M. (1993); Sell & Murrell (2009); Salisbury (1964); Valentine (1971); Valentine (1978); Preston et al. (2002); Kent, D.H. (1975); Webb et al. (1988); Perrins et al (1993)

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