Plants that out-compete other more desirable plants or simply invade half the garden are classed as weeds and require control. First, consider whether this can be done using non-chemical means such as pulling or digging out, or suppressing with mulch. If this can't be achieved, consider using chemical methods.
The main method of non-chemical control, and usually the most appropriate, is pulling or cutting the plants before they flower and set seed. Conservation authorities regularly organise ‘balsam bashing’ work parties to clear the weed from marshland and riverbanks.
Where non-chemical control methods are not feasible, chemical controls may need to be used. Choose a weedkiller that is most appropriate for the purpose by reading the label carefully before buying or using. Contact weedkillers and glyphosate* have low persistence in the soil, being virtually inactivated on soil contact. Residual weedkillers persist in the soil for several weeks or months and can move deeper or sideways in the soil, leading to possible damage of underlying plant roots.
Before using weedkillers alongside waterways it is necessary to contact the Environment Agency (see telephone directory for your local office). It can advise on suitably qualified contractors, as can the National Association of Agricultural and Amenity Contractors.
Take care when applying weedkillers near ornamental plants. Cover them with plastic sheeting while spraying, and only remove it once the spray has dried on the weed foliage.
It may take a couple of seasons to obtain good control of Himalayan balsam, as additional weed seedlings germinate after the parent plants are killed off.
Diquat, fatty acids, acetic acid and pelargonic acid
The non-selective contact weedkillers acetic acid (Scotts Weedol Fast Acting Gun!), fatty acids (Bayer Garden Ultrafast) or pelargonic acid (Doff 24/7 Superfast Weedkiller) can be applied before flowering.
Himalayan balsam can be controlled with a weedkiller based on glyphosate (e.g. Scotts Roundup, Scotts Tumbleweed, Bayer Garden Super Strength Weedkiller or Doff Maxi Strength Glyphosate Weedkiller). Glyphosate is a non-selective, systemic weedkiller that is applied to the foliage. It is inactivated on contact with the soil, so there is no risk of damage to the roots of nearby ornamentals, but care must be taken that the spray doesn't drift onto their foliage. Glyphosate is most effective when weed growth is vigorous. Treat Himalayan balsam at early flowering stage to ensure the weed is knocked back before it has chance to self-seed.
*A note on glyphosate May 2016: After reviewing glyphosate, the European Parliament has given the go ahead to relicense it but proposes disallowing certain uses such as public open space and restricting it to professional use only. The final resolution will be decided by national authorities this summer which may mean the withdrawal of glyphosate-based weedkillers for home gardeners in the UK. Check the RHS website for further updates.
Weedkillers for gardeners (Adobe Acrobat pdf document outlining weedkillers available to gardeners; see sections 3 and 4)
Chemicals: using a sprayer
Chemicals: using safely and effectively
Chemicals: using spot and broad-scale weedkillers
Weeds: non-chemical control