In a nature reserve near you, hardy work parties of wardens and volunteers will have ‘bashed’ the dreaded exotic invader Himalayan Balsam (Impatiens glandulifera or HB) all summer. Introduced to Britain in 1839, it is our fastest growing annual plant with beautiful flowers and explosive seed pods loved by children. But according to Natural England and the Environment Agency it:
- Is a highly invasive annual weed, which can reduce biological diversity by outcompeting native plants for space, light and nutrients
Restricts river access, leaves autumn river banks bare of supporting vegetation and prone to erosion and the dead plant material increases flood risk.
- Attracts pollinators away from native species, reducing the genetic diversity of native species and fitness by reducing seed set.
A study by Hulme and Brenner[i] showed that more species are present if you remove Himalayan Balsam (HB), but noted ‘in open and frequently disturbed riparian vegetation, many of the species negatively influenced by Impatiens are widespread ruderal species. Thus while several authors recommend its removal such action may only lead to a compensatory increase in the abundance of other non-native species and thus fail to achieve desired conservation goals.’ This doesn’t suggest that there is much effect on the sort of native plants that we might want to conserve. Tickner[ii] compared the competitive ability of nettle and Balsam; sometimes our native nettle loses out. But nettle only grows where there are a lot of nutrients – the sort of sites that have little diversity anyway.
Chittka and Schürkens[iii] suggested competition for pollinators might reduce the amount of viable seed set. But they only looked at the effects on Stachys palustris, which is pretty common anyway, for just one season. This is a long way from showing that in a real habitat HB affects the viability of even this one species which is of no real conservation interest, let alone other species. We know that HB has high-sugar nectar and flowers for a long time - Bee Keepers love it. Of course it might just support a larger bee population in the medium term leaving the same number of bees available to pollinate other flowers. And finally Tanner[iv] looked at the effect on invertebrates. The evidence was inconclusive - fertile ground for a follow-up study rather than hard conclusions.
I can’t find a scientific paper that looks at the alleged erosion-enhancing effects of HB; all the evidence seems to be anecdotal. HB often inhabits urban river banks where flows are unusually peaked (due to the higher run off because of impermeable surfaces), favouring the sort of erosion that HB is blamed for. It might of course be that HB is just good at colonising river banks that are already prone to erosion.
So there you have it. The studies are short-term and focus on the disturbed, urban and nutrient-rich habitats that HB favours rather than more important plant communities. None of the studies ‘prove’ that HB reduces biological diversity in the long or medium term, or that it is responsible for erosion, or for reducing genetic diversity. Of course, the scientists themselves are honest about pointing these issues out in their papers, and state that more research is needed. But these caveats are never reported in the campaign against HB.
That there is so little hard evidence that HB harms conservation is bad enough. Even worse than this, Wadsworth looked at control strategies and concluded that they are ‘rarely effective in the long term’. Now I’ve ‘bashed Balsam’ myself, enjoyed it, and even met some nice people while doing it. It's certainly true that HB gets in the way and takes up a lot of space. There may be a need to remove the plant for instance where it blocks a path. But in an age of austerity should we really be spending millions of hours and millions of pounds fighting an imaginary enemy?
 Scientific name for plants that tolerate disturbance. Examples include docks, dandelions and thistles.
[i] Hulme and Brenner 2006: Assessing the impact of Impatiens glandulifera on riparian habitats: partitioning diversity components following species removal (Journal of Applied Ecology 2006 43, 43–50)
[ii] Tickner et al 2001: Hydrology as an influence on invasion
[iii] Chittka & Schürkens 2001: Successful invasion of a floral market
[iv] Tanner et Al 2013: Impacts of an Invasive Non-Native Annual Weed, Impatiens glandulifera, on Above- and Below-Ground Invertebrate Communities in the United Kingdom