If you’ve ever walked barefoot in a field, chances are you’ve felt a thistle: those spiny-leaved plants that prick your feet and make you jump. Left unchecked, they produce purple or pink flowers at stem ends and plenty of seeds. Milk thistle (Silybum marianum) is no exception. Originally from parts of Europe and Asia, this Class A noxious weed is now invasive in North America, South Africa, Australia, and New Zealand, among other places. Luckily, it’s still pretty rare in King County (and all of Washington), growing mostly in pastures in the Enumclaw area and Pierce County, where it was likely introduced through contaminated out-of-state hay.
But once milk thistle shows up, it’s persistent. Seeds survive in soil an average of 16 years. One King County landowner eradicated a few plants from his pasture more than a decade ago. Last summer, he cleared a blackberry patch on the land. Earlier this spring, Weed Specialist Dan Sorensen found over 50 new plants growing in the cleared area!
An annual or biennial, milk thistle germinates after fall rains, producing a three-foot-wide rosette with spines on its leaf edges and stems. Leaves have a distinct milky-white marbling splashed across them. Plants eventually grow 2–6 feet tall with branching stems. Between April and October, each stem produces a large purple flower head with big, spiny bracts around its base.
This year, some of King County’s milk thistles are already up and blooming:
One milk thistle can produce over 6,000 large seeds. They usually fall near the plant, but vectors like mowers, equipment, animals, or hay can spread them more widely.
Milk thistle is an especially big concern in pastures because it’s toxic to many livestock, including cattle, sheep, goats, alpacas, and llamas. Eating large amounts of the plant can cause nitrate poisoning. Animals are most likely to graze on young thistles, but might eat them at any size.
Digging or hand-pulling (with gloves) can effectively remove milk thistle, especially on sites with fewer, younger plants. Cut the plant with a shovel at about one inch below ground level to prevent re-sprouting. Make sure to bag and throw away all flowering plants because they can go to seed even after you uproot them. Mowing won’t control plants—they can sprout and flower again in the same season. More generally, good pasture management practices can help prevent the milk thistle’s establishment and spread.
For more information on control options, see the following pages:
Be sure to identify milk thistle carefully before removing it. Close-up, milk thistle’s white marbling is a give-away, but at a distance it might be confused with a number of other thistle species. Two similar invasive thistles are bull thistle (Cirsium vulgare) and Canada thistle (Cirsium arvense), both non-regulated Class C noxious weeds.
Like milk thistle, bull thistle reaches 2–6 feet tall with one flower at each stem end, but its lobed, hairy leaves lack white marbling. Its flowers also have smaller spines around their bases, as opposed to the larger, fleshier bracts of milk thistle.
Canada thistle grows to 5 feet tall with narrow leaves and flower heads in clusters at stem ends. Unlike the other two invasive thistles—but like the two native thistles below—this species has spineless stems. It spreads via both seeds and creeping rhizomes, making it extra difficult to control.
Two native thistles also resemble milk thistle. Edible thistle (Cirsium edule) is a 6-foot-tall perennial with short spines on alternate leaves and single pinkish-purple flower heads at stem ends. Tops and bottoms of leaves are lightly hairy.
Short-styled thistle (Cirsium brevistylum) looks similar to edible thistle, but with less-lobed leaves and clustered flower heads. Only bottoms of leaves are slightly fuzzy.
Both native thistles lack spines on their stems, which helps to distinguish them from milk thistle and bull thistle. Be careful, though, because Canada thistle’s stems are spineless, too. Unlike all three invasive thistles, both of these native species have noticeable cobweb-like hairs wrapped around the bracts at the bases of their flower heads.
For more information on native and invasive thistle species, check out Native Thistles: A Conservation Practitioner’s Guide by the Xerces Society for Invertebrate Conservation.
Also, it’s important to note that milk thistle is widely known as a medicinal plant, especially for liver problems. The Washington State Noxious Weed Control Board’s categorization of milk thistle as a Class A noxious weed is based entirely on the plant’s invasive behavior in Washington, and is not related to any medicinal qualities. Local medicinal gardens can get permission to grow this plant from King County if they are careful not to let it spread.