Invasive plants: asters
Asters are perennial herbaceous plants belonging to the Asteraceae (or Compositae) family. Their name derives from the Greek aster, meaning star or flower.
They are ornamental plants widely cultivated in gardens, with more than 250 species. Most originate from North America, some were introduced from Asia, while others are native to Europe.
While these flowers have a highly decorative effect in flowerbeds, several species of the genus Aster can become invasive in Switzerland.
The garden aster, or New York aster (Aster novi-belgii), is the most widespread species in this country. However, other species and hybrids are still sold in horticultural trade and may cause problems in the future. These plants produce large quantities of seeds which, dispersed by wind, can escape from ornamental gardens and invade surrounding areas. As they favour nutrient-rich, moist soils, they are capable of displacing native flora in wetlands, which are often protected habitats.
In addition to the garden aster mentioned above, the lance-leaved aster (Aster lanceolatus) is also considered problematic. Although it is still rare in Valais, its presence should be reported and the plants removed; cultivation should be avoided.
Most asters grow to a height of 70 to 150 cm and bear numerous flowers at the ends of their branched stems. These flowers may be bluish violet, mauve or pink, but also white, and bloom from spring through summer, sometimes into autumn. Very frost-hardy (down to −30 °C), asters prefer fresh, rather calcareous soils but also tolerate dry periods and acidic soils. All species are highly melliferous and are frequently visited by bees, which find abundant nectar and, above all, pollen there until November.
Beekeepers are encouraged to learn to recognise aster flowers and, despite their attractiveness, not to hesitate to remove them and replace them with other native, pollinator-friendly plants.
Sources
Infoflora.ch
J. Piquée, Les plantes mellifères mois par mois, 2014
T. Silberfeld, C. Reeb, Les plantes mellifères, 2016