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Milk thistle flower. Photo Credit: Jan Samanek, Phytosanitary Administration, Bugwood.org -
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Milk thistle plant. Photo Credit: Eric Coombs, Oregon Department of Agriculture, Bugwood.org -
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Also known as: blessed milkthistle, Saint Mary’s thistle, lady’s thistle, Mediterranean thistle, spotted thistle, variegated thistle
With its large, multi-colored leaves and bright purple flowers, milk thistle is a standout in pastures and gardens. It has been known for its medical properties, but it can cause havoc in poorly managed pastures and fields.
Description:
| Life Cycle: | Biennial (life cycle lasts two years) |
| Early Detection and Rapid Response species: | Yes |
| Height: | 6 feet (2 meters) |
| Leaf Description: | During its first year, milk thistle forms a rosette with dark green and white leaves with sharp spines along their edges. During its second year, it has similar leaves that grow alternately along the flowering stem. |
| Fruit & Flower Description: | Round, bright purple flowers grow at the end of the stem. Each flowerhead is surrounded by spine-tipped bractsSmall leaf-like structures that are just below a plant’s flower. at their bases. A mature plant can produce more than 6,000 seeds annually that can remain viable in the soil for many years. |
| Bloom Time: | April to July |
Habitat:
- Milk thistle is commonly found in poorly managed pastures, agricultural fields, and along roadsides.
- It prefers full sun or part shade and grows best in fertile soils.
Impact:
- Milk thistle is particularly troublesome for ranchers since it is toxic to livestock. It accumulates nitrates in its leaves which can poison grazing cattle and sheep.
- It forms dense stands that outcompete forage and native plants for nutrients and light.
- Its spiny leaves can cause injury to people and livestock.
What we’re doing about it:
- Milk thistle is a priority species for the Tualatin SWCD. As such, our Invasive Species Program has been actively monitoring and treating it throughout the watershed.
- If identified within Washington County, a specially trained crew can come out survey for milk thistle. If found, the crew will treat the infestation for free.
What you can do about it:
- If you think you’ve found milk thistle anywhere in Washington County, please report it to the Oregon Invasive Species Hotline.
- Mowing is not recommended, as this can produce more plants in the future.
- For smaller patches, hand-pulling individual plants can prevent further spread. Digging is easiest in the spring or fall before the plant flowers.
- For flowering plants, remove any flowers or seeds and place them in a plastic bag before you dig the plant up. Make sure to throw the plastic bag away in the trash.
Look-alikes:
Italian thistle (Carduus pycnocephalus) is another invasive species that is often confused with milk thistle. Italian thistle’s leaves have hairs on their underside whereas milk thistle’s leaves are hairless.
Noxious Weed Listing:
| State of Oregon: | Class B |
| State of Washington: | Class A |
More Resources:
Download the Milk Thistle Best Management Practices Factsheet
- Pacific Northwest Weed Management Handbook: Milk thistle
- University of California, Weed Research & Information Center: Blessed milkthistle