Control where it goes
The best way to do this is to treat plants in the spring, before flowers form and increasing the risk for seeds to blow away.
Control where it grows
- Dig: Cutting and digging is possible, but not especially effective on larger infestations. These are also particularly spiny plants, which discourage casual handling.
- Treat: Low rates of herbicide appear to keep milk thistle under control.
- Check: monitor the site at least annually, and especially years two and three after treatment, as any surviving seed takes its big opportunity to sprout!
If you suspect you’ve found milk thistle, the City of Portland REQUIRES you to report it. Still, because of its prickly nature, you may decide to take advantage of the free management assistance available from the city. You can report suspected patches to Mitch Bixby with the Environmental Services' Early Detection/Rapid Response Program at 503-823-2989 or email@example.com.