The pretty flowers of Himalayan or Indian Balsam were welcomed to Britain in 1839. Today, it is recognised as a particularly pesky invasive. Not long after its arrival, it escaped to the wrong side of British garden fences. It became naturalised in Middlesex from 1855. CABI’s Invasives Compendium reports that it has spread at a rate of 645 km2 per year since its 19th century introduction as a pretty ornamental. Efforts at eradication are deemed expensive and unlikely to succeed. It is predicted to become an even greater threat to our native flora due to climate change. Researchers are fighting back with a natural enemy.
Prolific Seed Production
Himalayan Balsam has a high germination rate at around 80% of seeds produced. Scientific research has revealed alarming seed production figures:
- 700-800 seeds per plant with 5.7 seeds per capsule
- 4,000 seeds per plant with 6.4 seeds per capsule.
One European study discovered a rate of 32,000 seeds per square metre. Much is made of the seed pods explosive, even ballistic capabilities. The explosive dispersal can distribute the seeds up to 7 metres from the parent plant. This does not fully explain the plant’s invasive potential. It is the plant’s predilection for British and European riversides that explains its dastardly reputation as a prime invasive. It uses river corridors to transport its seeds. The buoyancy of its dry seeds facilitate even greater distances.
It is hard to predict which plants will become invasive in foreign soils. Himalayan Balsam does not colonise the same habitat in Britain, Europe, Canada, USA and New Zealand as in its natural environment. It thrives at much lower altitudes. Its true home is the Himalayan foothills from north-west Pakistan to northern India and possibly also Nepal. Its native range is far smaller than the unrelenting expansion of its overseas empire. It seems surprising that a plant that lives in harmony with 30-60 other plants in the Himalayas should opt for aggressive world domination.
Other aspects of Himalayan Balsam do not suggest the magnitude of its invasiveness. It does not have a persistent seedbank. Seeds only survive for up to 18 months. It is surprisingly susceptible to frosts for a Himalayan plant. Most importantly, it is an annual that dies back every year. Its root system is far weaker than that other notorious invasive Japanese Knotweed, Reynoutria japonica (formerly known as Fallopia japonica and Polygonum cuspidatum).
Yet, Himalayan Balsam’s annual status is part of its threat. I remember the first time that I came to understand this as I walked along a very muddy riverbank in spring. I noticed a huge expanse of really cute seedlings with a red tinge. Then, it dawned on me who the parent was.
The seedlings grow at a very rapid rate. They outcompete native species, cast them in shade and establish a riverbank monoculture. Their dense stands can alter water flow, increase erosion and cause flooding. They die back quickly in autumn. They leave wide, barren areas exposed to the onslaught of winter rains and fast, heavy river flows.
As if it were not enough that Himalayan Balsam creates havoc on our riverbanks, it should be noted that the plant also establishes dense stands in other habitats. It loves wet, semi-shaded woodland areas too. Here again, it chokes out native species by towering above them.
Forest Regeneration Risk
Most alarmingly of all, its monoculture can prevent tree saplings from getting established. Forest regeneration is a key concern as we face the challenges of climate change.
Himalayan Balsam stands to thrive due to climate change. It is known to react positively to increases in CO2 and temperature. There is a serious risk that it will become an even more aggressive invader. Extreme winds could propel its seeds even further. As it naturally grows at higher altitudes, it is likely to progress upstream and further north. It is not fussy about its soil type. Upland peat will not be a deterrent. Admittedly, it should stay clear of drought-prone areas and densely-shaded woodland. It also needs a winter chill for seed viability. Heavy rainfalls may knock growth back initially, but a damp environment favours a range expansion overall.
At first sight, Himalayan Balsam’s attractiveness to pollinators may seem a major mitigation factor in its favour. Beekeepers traditionally exacerbated the spread of the plant alongside gardeners. My recent riverside walk seemed to demonstrate how popular the flowers were with bees. Unfortunately, they outcompete native species in the production of nectar. Their lure is detrimental. Researchers discovered that their presence resulted in a 50% reduction in pollinator visits to nearby native plants. Himalayan Balsam monocultures are considered to reduce native flora by 25%, and they reduce their future reproduction capabilities too.
Wildlife organisations have tried valiantly to eradicate this pesky plant from their land. Large scale, labour-intensive removal campaigns have taken place with volunteer assistance. The aim is to prevent plants from flowering, seed formation and dispersal. Unfortunately, such efforts are hampered by the dispersed nature of land ownership in Britain. There is always some inaccessible pocket that leads to reestablishment.
Himalayan Balsam has been an unwelcome alien for a long time now. Schedule 9 of the Wildlife and Countryside Act (1981) makes it illegal to plant or facilitate the spread of the plant in the wild. It has featured on EPPO’s List of Invasive Aliens since 2004. Even allowing for some scaremongering over invasive plant issue, the scale of the eradication task seems to make success unlikely. The UK’s Environment Agency considered that it would take a prohibitive £150-300 million to rid us of this invasive. The annual bill for invasive species control already stands at £1.7 billion.
Rust to the Rescue?
All may not be lost. Cue Puccinia kamarovii var. glanduliferae. Researchers sought out Himalayan Balsam’s natural enemy from its native environment. They discovered a rust species that infects its stem and leaves. There is now the exciting prospect of two different strains of rust that are host specific to Himalayan Balsam from India and Pakistan respectively. They offer a far cheaper alternative to reduce the species’ aggressive vigour in the UK. After careful research, the rust was released into the wild first in England in 2014 and subsequently in Wales. The results of the targeted attack on this invasive will take a further 5 to 10 years to fully appreciate.
There is now the hope that, where costly labour-intensive eradication efforts failed, this rust species could succeed. Could we see Himalayan Balsam transformed into a pretty, but benign species in the UK in future?
References and Further Reading
- Beerling, David J. & Perrins, James M. (1993): Impatiens Glandulifera Royle (Impatiens Roylei Walp.) Journal of Ecology. Vol. 81, No. 2 June 1993 pp. 367-382. https://www.jstor.org/stable/226150
- Beerling, D. J. & Woodward, F. I. (1994): Climate Change and the British Scene. Journal of Ecology. Vol. 82, No. 2. June 1994. pp. 391-397. https://www.jstor.org/stable/2261306
- Booy, Olaf & Wade, Max & Roy, Helen (2015): Field Guide to Invasive Plants & Animals in Britain. Bloomsbury.
- CABI (?): Impatiens glandulifera (Himalayan Balsam). Invasive Species Compendium.
- Grewcock, Mick (2015): Himalayan Balsam Explosive Seed Pods. Impatiens glandulifera. YouTube. 4 November 2015
- Müller, Karen (2019): The Good, the Bad and the Balsam. Scottish Invasive Species Initiative.
- Pearson, Nicola (2014): Clever enemy could control invasive plant pest. Natural History Museum. 4 June 2014
- Plantlife (2021): Himalayan Balsam, Impatiens glandulifera.
- Pollard, Kate (2021): Biological Control of Himalayan Balsam. CABI
- Pacanoski, Zvonko & Saliji, Alirami (2014): The First Record of the Invasive Impatiens glandulifera Royle (Himalayan Balsam) in the Republic of Macedonia. Fifth International Scientific Agricultural Symposium „Agrosym 2014“. Professional paper 10.7251/AGSY1404814P
- Penrhys-Evans, Tony (2021): Halting the spread of Himalayan balsam: a CABI interview. Invasives Blog and Podcast with Dr Sonal Varia and Kate Pollard. 28 September 2021.
- Poland, John & Clement, Eric (2009): The Vegetative Key to the British Flora. BSBI
- Polunin, Oleg & Stainton, Adam (1997): Flowers of the Himalaya. Oxford India Paperbacks. Oxford University Press
- Rogers, Christopher (?): What about the bees? Himalayan Balsam Wales. Leaflet
- Tanner, Rob et al. (2014): An ecological comparison of Impatiens glandulifera Royle in the native and introduced range. Plant Ecology 215(8): pp. 833-843. August 2014
- Welsh Government (?): Himalayan Balsam. Public information on invasive species in Wales
Karen does not seek or receive any commercial interest or advantage from this blog. She is not promoting any business venture. She simply loves to share fascinating facts about plants. These pages illustrate her love of plants, botany, biodiversity, gardens and creative expression. There is always so much to learn about plant diversity. This blog is designed as a showcase for photography, commentary on plants and wildlife, gardens and other places visited, horticulture and related topics. Viewpoints are her own, not those of her employer.
All above photos © Karen Andrews