Impact of the Invasive South African Ragwort on Germination and Growth of Radish

The South African ragwort Senecio inaequidens is among the most invasive plant species in Europe, especially in mountainous and Mediterranean regions, and is suspected to exert allelopathic effects (release of chemical inhibitors) on native vegetation. This study investigated the allelopathic potential of South African ragwort using radish Raphanus sativus as a model species. In laboratory experiments, I treated radish seeds with ragwort leaf extracts at varying concentrations. After three days, the extract reduced radish seed germination by up to 40% in a concentration-dependent manner. High extract concentrations significantly inhibited radish seedling root length, whereas lower concentrations rather stimulated root growth. Measurements of oxidative stress-related parameters, including lipid peroxidation and total antioxidative capacity, showed no changes in treated roots. In a complementary pot experiment, I exposed radish seeds for 36 days to pulverised ragwort leaves mixed with vermiculite. This treatment delayed, but did not suppress seed germination. Subsequent plant growth and physiological status remained unaffected. The results suggest that South African ragwort exerts moderate allelopathic effects, primarily influencing seedlings, whereas mature plants appear less sensitive to its allelochemicals.