Description:
Milk thistle is described as an annual, winter annual, and biennial herb. It is an annual in its native range. The main stem is stout, ridged, and branching, and the overall plant size can range from two to six feet tall. A distinguishing characteristic of milk thistle is the white patches, or marbling found along the veins of the dark green leaves. The broad leaves are deeply lobed, and basal leaves can be 20 inches long and 10 inches wide. The leaf margins are yellow and tipped with woody spines 1/8" to 1/2" long. The leaves are alternate, and clasping to the stem. The stem leaves are smaller and not quite as lobed. Each stem ends in a solitary composite flower head, about two inches in diameter, consisting of purple disc flowers. The flower head of milk thistle differs from other thistles with the presence of broad leathery bracts that are also tipped with stiff spines 3⁄4 inch to two inches long. The seeds are heavy, 1⁄4 inch long, flat, smooth, and shiny and the color ranges from black to brown mottled. The seeds do have a tuft of minutely barbed bristles, which is deciduous, and falls off in a ring when the seeds mature.

• Mechanical: Mowing may prolong plant survival for another year producing plants more resistant to chemical control (Sindel 1991).
• Biological: The European weevil Rhinocyllus conicus was released in Canada in 1968, and released in southern California in 1971 for milk thistle control. This weevil has an annual life cycle, and attacks thistles in the genera Carduus, Onopordum, Silybum and Cirsium. (Goeden and Ricker 1974). The larvae of R. conicus do not always attack the seed tissue of milk thistle, even though they are often found in the seed head. (Coombs et al. 1995). Septoria silybi is a fungus, causing leaf lesions, interfering with photosynthesis (Roche’ 1991).
• Herbicide: Annual thistles are most susceptible to chemical control in the fall. Biennial thistles are most susceptible to control in the early spring, in the rosette stage, although the rosettes may cover some seedlings (Hodge 1970 and cited in Sindel 1991). For specific chemical control recommendations, refer to the Pacific Northwest Weed Control Handbook, updated annually (William et al. 1998). Refer to the State Noxious Weed Control Board site

For More Information:
Detailed information about Milk Thistle is available at the Washington State Noxious Weed Control Board Web Site.