Invasive plant: Cape ragwort

Cape ragwort (Senecio inaequidens) is a perennial herbaceous plant belonging to the Asteraceae (or Compositae) family, native to South Africa, from where it was accidentally introduced through the wool trade. Its branched stem reaches a height of 40 to 60 cm and bears yellow flowers that bloom over a long period, from early summer through autumn, attracting bees mainly for their yellow pollen.

The plant produces seedlings as early as the first year, with numbers increasing exponentially in subsequent years. Dispersed by wind and by animals (clinging to their fur), the seeds retain their ability to germinate for several decades. The growing season is further extended by increasingly mild winters.

Cape ragwort occurs mainly along transport corridors (railways, roads, paths) and on uncultivated land. Although its presence in agricultural fields is still limited, it nevertheless represents a serious problem for agriculture and a major threat to native flora. Its spread is very rapid due to its high reproductive capacity and the absence of diseases and parasites that would control its expansion in its region of origin.

In Valais, this invasive plant, which grows equally well on acidic and calcareous soils, is common in the Rhône valley plain and on slopes up to 1,000 m above sea level. It may be confused with other Asteraceae, in particular common ragwort (Senecio jacobaea) and hoary ragwort (Senecio erucifolia), native species that are also highly melliferous but are considered weeds because of their vigorous spread.

All three ragwort species contain significant quantities of pyrrolizidine alkaloids in all their parts, making them highly toxic to many mammals and birds. These substances are also present in the pollen and nectar collected by bees, thereby contaminating honey, which could cause intoxication in humans. Even though the concentration of pyrrolizidine alkaloids in Swiss honeys is generally not high enough to pose a risk to consumers, this is an additional reason to combat the spread of Cape ragwort.

Sources

J. Piquée, Les plantes mellifères mois par mois, 2014

T. Silberfeld, C. Reeb, Les plantes mellifères, 2016

SAR journal, January–February 2018

2302_18 Cantonal concept for the control of invasive neophytes.pdf