Habitat: riverbanks and areas of damp ground
Description: It can form dense monospecific stands where individual plants can reach 2 – 3 m in height (one of the tallest annual plants in Ireland). The stem of the plant is smooth, hairless and hollow. They grow upright, easily broken and are usually purple in colour with many large oval-shaped pointed leaves bearing teeth around the edges. The flowers of this plant can vary in colour but are usually shades of white, pink or purple. Flowering usually takes place from June to October. Seed capsules arise where the flowers were and when mature and dry, the slightest touch causes these fruits to split open explosively dispersing seeds up to 20 feet from the parent plant. Seeds are capable of further dispersal by water and animal and human aid.
Origin and Distribution: The plant is native to the western Himalayas but is now invasive in many parts of continental Europe. In Britain, Himalayan balsam is regarded as one of the top-ten most wanted species that have caused significant environmental impact.
Impacts: This species grows in thick monospecific stands, shading out native plants such as grasses. From October onwards, the plants die back leaving the soil more exposed to erosion because of the loss of native plants earlier in the year. It has also been shown to produce more nectar in its flowers than native species making the plant more attractive to bumblebees resulting in less pollination of our native species.
How did it get here? Originally brought into Ireland as a garden plant.
Where is it found in Ireland? The species is now found throughout the island of Ireland suitable habitats.
Manual pulling/cutting; commonly known as ‘balsam bashing’ events are now common place
- It is vital to remember that pulling should be performed prior to the formation of the seed pods which explode at the slightest disturbance when ripe
- Himalayan balsam has a very shallow root making uprooting by hand easy.
- The pulling technique must be undertaken so that whole plant is uprooted and normally best done if pulled from low down the plant - If snapping occurs at a node the pulling must be completed to include the roots.
- Uprooted plants can be left to air dry and decompose on a non-permeable membrane. This method is highly suited to dealing with initial outbreaks of the species and in areas where balsam plants are mixed in with sensitive native species.
- Mechanical control, by repeated cutting or mowing, is effective for large stands, but plants can regrow if the lower parts are left intact. The plant must be cut below the lowest node to stop regeneration.
- Strimming and mowing of Himalayan balsam may also be effective but only prior to the seed pods developing.
- Any attempt to cut this plant once the seeds have developed will cause the seed pods to burst, spreading the plant. The seeds of this plant are not very robust and only survive for up to 18 months, therefore a two year control programme can be successful in eradicating this plant provided there is no further infestation from upstream or adjacent sites.
- Grazing by cattle and sheep is effective from April throughout the growing season in some situations. It should be continued until no new growth occurs. Grazing on riverbank habitats can however have negative impacts such as poaching of river banks and the removal of other native vegetation which may act as a buffer zone.
Treatment with Herbicide;
- Where in situ physical removal is not feasible, potentially due to stand density/size or location/inaccessibility, the species can be successfully treated with herbicide.
- Several herbicides have been shown to be effective at killing Himalayan Balsam and often just one application is sufficient. Nevertheless re-application in the same season should be planned for, as new growth from seed is likely.
- As glyphosate is a systemic herbicide, application should be carried out during periods of active growth, before flowering but late enough to ensure that germinating seedlings have grown up sufficiently to be adequately covered by the herbicide (50+ cm would be suitable).
- The initial application should ideally be carried out in May/June with subsequent treatments/monitoring likely being required in July/August and September/October.
- Herbicide application could be used as a follow up to hand pulling, e.g. later in the year to deal with any missed plants or regrowth from seed bank.
- As we discussed, due to Himalayan Balsam’s preference for habitats near water, this limits herbicide selection to products approved for use near water and the operatives applying it must be trained to PA6Aw level.
- The herbicide can be applied as a spot treatment to individual plants, using hand-held equipment, or as an overall spray using machine-mounted spray booms. In the latter instance, total weed control of all vegetation will occur, increasing the requirement for revegetation.
- Where accessibility is problematic, e.g. river banks, a long lance sprayer may be useful.
- Herbicide application will not kill seeds in the seed bank and monitoring with follow-up control must be repeated annually over 2-3 years to eradicate new plants growing in subsequent years, though the numbers decrease significantly from one year to the next.
- N.B. Removal, or herbicide treatment, of plants that have already shed their seeds is pointless, as the plants will die at the end of the growing season regardless. It is likely, particularly in the first year of control, that new plants will sprout following the initial removal/treatment, either because shade suppression will be reduced or due to soil disturbance. As such, several additional visits will likely be required. Three visits, May/June, July/August and September/October should be sufficient to catch all regrowth, although, a cautionary approach is advisable. N.B. Plants that germinate after September/October are very unlikely to have sufficient time to complete their life cycle and produce seeds.
- Import only clean soil from known source
- Ensure all vehicles and equipment are cleaned to avoid cross contamination.
- Be aware of the threat of colonisation from upstream areas washing Japanese knotweed material downstream.
- Promote native species and biodiversity - use alternative, native plants
- Know what you are buying/growing and source native Irish seed and plants
- Do not swap plants and cuttings
- Clean plants before adding to ponds (dispose of water away from water courses)
- Never collect plants from the wild
- Safe disposal of plant material and growing media
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