The Establishment of Seedlings from Primary and Regrowth Seeds of Ragwort (Senecio Jacobaea)

Abstract

After complete defloration by cinnabar moth caterpillars, ragwort plants are capable of producing regrowth shoots which flower and produce seed in early autumn of the same year. Regrowth seeds are much lighter (0.26 mg dry wt) than the seeds from primary capitula (0.41 mg). The regrowth seeds showed a slight, but statistically significant, reduction in germination rate (78.8%) compared with the primary seeds (86.4%). A field experiment was conducted to determine whether the regrowth seeds were equally viable and produced seedlings that were equally competitive under a range of substratum conditions; 1000 seeds/m2 were sown in each case. In cultivated soil, dug to a fine tilth, both kinds of seeds produced equal numbers of seedlings. In dense, unaltered vegetation, neither kind of seed produced many seedlings. In cut grass, where plant competition was reduced but still substantial, seeds from primary shoots produced just as many seedlings as they did in cultivated, competition-free, soil. In contrast, the smaller seeds from regrowth shoots produced virtually no seedlings when sown in similar cut-grass plots. While the smaller regrowth seeds of ragwort are equally viable, they produce seedlings which are substantially less competitive than those from primary shoots. Regrowth seeds presumably increase the fitness of ragwort plants only in those of places, or in those years, when competition-free microsites are available for germination in early autumn.