Invasive plants: balsams (Himalayan balsam)

Himalayan balsam or glandular balsam (Impatiens glandulifera), of Asian origin, is a typical example of a plant that divides beekeepers, gardeners, and ecologists: highly nectar-producing and very ornamental, it nevertheless poses an ecological threat, as its invasive nature has a substantial impact on local biodiversity.

It has escaped from the ornamental gardens for which it was originally intended and has colonized all fresh, slightly moist habitats and sites, such as riverbanks, ditches, and marshes. This distribution favors its expansion, as the flow of streams and rivers facilitates the dispersal of its seeds.

It is an annual herbaceous plant belonging to the family Balsaminaceae; it grows very rapidly and reaches a height of 1 to 2.5 meters. The flowers, whose petals form a helmet-like shape, are purple, pink, or white.

Because the maturation of the male and female organs is staggered, these flowers require the intervention of insect pollinators to be fertilized: initially, the stamens (male organs) conceal the stigmas (female organs), and bees, attracted by the nectar located at the base of the corolla, become coated with the pollen produced by the stamens before carrying it to another flower; somewhat later, the stamens wither and expose the stigmas, onto which pollen is deposited during the visit of another bee, thus ensuring pollination. When the fruit reaches maturity, it swells until the pressure is such that, at the slightest contact or breath of air, it explodes, projecting the seeds several meters through the air. Owing to its excessive growth, the balsam quickly becomes smothering and eliminates local species.

Bees are strongly attracted to this species and, in areas where it has established itself, it can result in a significant honey flow. The honey produced is light-colored and remains liquid for a long time due to its high fructose content; its taste is mild and pleasant.

Himalayan balsam is present in Valais mainly in the Rhône valley. Even if it has not (yet) invaded the territory—unlike in other regions of Switzerland—it is being actively controlled. Control consists of uprooting isolated plants 3 to 4 times per year (from May to October), before seed maturation, while larger areas must be mown several times during the same period. These measures must be continued for at least 5 years.

While the bicolored balsam (Impatiens balfourii), a closely related species with pink and white bicolored flowers, also needs to be controlled, there is a European balsam, the touch-me-not balsam (Impatiens noli-tangere), which is likewise highly nectar-producing. Smaller than its foreign counterpart, it is also distinguished by its yellow flowers with a long spur.

Sources

2302_18 Cantonal concept for the control of neophytes.pdf

Infoflora.ch

J. Piquée, Melliferous plants month by month, 2014

T. Silberfeld, C. Reeb, Melliferous plants, 2016