By skyforbes What to do if you find it
Notify us if you see milk thistle in King County. Our program staff can provide you with site-specific advice on how best to remove it.
We map all known locations of regulated noxious weeds to help locate new infestations in time to control them.
Control methods
We recommend using a combination of methods to control noxious weeds. In areas with few weeds, it is important to act quickly before they become harder to control. Make a long-term plan as it often takes several years to get rid of most weeds. Start in the least infested areas first and then move into more heavily infested areas.
For more in-depth control information, read Best Management Practices (BMP) for milk thistle (615KB)
Manual control
Wear sturdy gloves to avoid getting hurt by sharp spines. Dig up plants ideally before seed heads form. Try to remove as much root as possible. Remove and bag seed and flower heads, if present.
Mechanical control
Mowing works only if done frequently. Only mow if flowers and seed heads are absent. To reduce regrowth, use manual tools (such as brush cutters, machetes, or loppers) to cut plants at or below ground level. Always clean equipment afterwards to avoid spreading seeds.
Cultural control
Maintain healthy, non-invasive, competitive grasses. Sustainable grazing practices support healthier perennial grass populations to compete with weeds. Goats can graze with no ill effects but keep cattle and sheep away. Horses may be injured by the sharp spines.
Chemical control
Stay safe when using herbicide:
- Always read the label before use.
- Wear a long-sleeved shirt, long pants, shoes, and eye protection.
- Follow state and local regulations.
Foliar, broadleaf herbicides are effective on growing plants in the spring and fall. Selective herbicides (such as triclopyr, 2,4-D, aminopyralid) are best. Non-selective herbicides (such as glyphosate) will harm grasses and should only be used where damage to grass is not a concern. Do not cut treated plants until they have died (2 weeks or more after treatment).
Be aware there may be additional herbicide restrictions when pastures are grazed, especially by lactating dairy animals.
For specific chemical recommendations, please refer to the PNW Weed Management Handbook.
Disposal instructions
On site composting is okay if livestock, other than goats, are not present. If livestock are present, you can compost all but the flower and seed heads. Always remove flower and seed heads if present and dispose in garbage. Do not compost. Compost does not get hot enough to kill all the seeds.
Noxious Weed Disposal – Washington State Noxious Weed Control Board