Noxious Weeds are non-native plants introduced into an ecosystem. They spread quickly and can be difficult to control. They invade croplands, rangeland, forests, prairies, rivers, lakes, wetlands, and estuaries, causing both ecological and economical damage that affects us all.
Purple Star Thistle is a thorny issue for us (sorry about the pun). We found it growing on several coastal properties a few years ago, and noticed that it was quickly multiplying. It effectively chokes rangeland and displaces native species. With no natural predators, small patches become entire fields within a couple of years.
Similar in some respects to yellow star thistle, this is an aggressive weed that is a major problem on annual rangelands in the San Francisco Bay area.
Purple star thistle is native to Asia Minor from a region between the Black and Caspian seas. It was first introduced into California in the early 20th century. It was reported from Ellensburg, Washington in 1929. However, no additional reports were made for Washington until 1989, when purple star thistle sites were found in Asotin and Island counties.
The weed has made its way to the North and Bay counties of California, probably unknowingly transported with hay.
Grubbing or digging can be effective for small infestations of purple star thistle, however this is completely impractical on rangeland. Mowing is not effective.
No biological control program is currently being developed for purple star thistle. Reportedly, biotypes of Bangasternus, a seed head weevil, utilize purple star thistle in Europe.
At this point, the only remedy to stop this foreign invader are specifically targeted herbicides. These are most effective when applied in the spring when the plant is in its sensitive rosette stage.
Only through diligent land management, repeated follow-up, and an active integrated pest management program can one control this type of noxious weed.