Non-herbicidal control of invasive yellow star thistle (Centaurea solstitialis) along seasonal streams in Mediterranean grassland

Background/Question/Methods Yellow star thistle (Centaurea solstitialis) is a pest in Mediterranean grasslands in California, where it degrades rangeland quality, displaces native species, and depletes soil moisture. Seasonal streams exacerbate the spread of yellow star thistle by eroding and redepositing the seed bank, creating an invasion front that moves much faster than wind dispersal. This study tested the feasibility of non-herbicidal methods of removing star thistle patches along seasonal streams. Patches of star thistle were randomly assigned to either control, mowing, solarizing tarps, or topping+removal treatments.

Results/Conclusions All removal methods significantly decreased star thistle biomass. Local eradication (no recurring individuals, no star thistle germination from seedbank) was achieved in some patches of every removal treatment, but not in all patches of any treatment. Star thistle removal significantly decreased downstream colonization of new star thistle patches. Native and perennial species increased in biomass when star thistle declined, but non-native annuals also increased. The most cost-effective method was mowing the star thistle prior to seed set. This study provides support for using mechanical control of star thistle in areas where herbicide use is undesirable.