How to Identify and Remove Milk Thistle

Milk thistle is an invasive plant in the U.S. known for its distinctive white-marbled, dark green leaves. Originally from the Mediterranean, this plant was introduced to North America for its medicinal properties. Now, it thrives in rocky, dry areas, forming thick patches that overpower native plants by hogging essential resources like water, sunlight, and nutrients. Its ability to spread quickly makes it similar to other invasive plants, threatening local ecosystems.

Milk Thistle Invasiveness

Milk thistle’s adaptability to different habitats and wide range of soil types, including sandy and clay, makes it cold hardy and difficult to eradicate once established. Milk thistle can quickly overtake any other vegetation nearby and absorb all of the space and nutrients. Milk thistle infestations are problematic for farmers because they are toxic to livestock.

One plant produces thousands of seeds that remain viable in the soil for at least nine years. Most of the seeds drop near the plant but they can also be moved by erosion, wildlife, pets, rain, and human activity, such as on the wheels of a lawn mower.

What Does Milk Thistle Look Like?

Identifying a particular thistle species can be tricky because many thistles look so much alike. Milk thistle is an exception so identifying it isn’t difficult. What gives this plant away is the distinctive white marbling of the dark green leaves. It is also what gave the plant the “milk” moniker. The leaves are large, up to 20 inches long, lobed, and hairless. The leaf edges have spines that can be up to ½ inch long.

The life cycle of the milk thistle spans over two years. The plant is also classified as a winter annual, a plant whose seeds germinate in the fall and winter and that starts actively growing in the spring. In the spring, milk thistle grows a large rosette, up to 3 feet in diameter. From this basal rosette, it grows a tall, sturdy stem with branches that can reach a height of 6 feet or more. At the top of the stem, a solitary large purple flower emerges between April and October. The flower is about 2 inches in diameter and spiny. After setting seeds, which are dark brown and about ½ inch long, the plant dies.

Milk thistle has a deep taproot, which allows it to grow even in dry soil.

How to Get Rid of Milk Thistle

Remove milk thistle early, when it forms a low-growing rosette. If flowering, cut off the flowerheads, bag them, and dispose of them in the garbage. Do not throw them on your compost pile, as the flowers can still produce seeds even after being removed.

If you have a small patch, hand pulling can be effective but watch out for those spines and always wear protective gloves. Its long taproot may require a shovel for removal from compacted soil.

If you have a large infestation that cannot be managed by hand, mowing the milk thistles is not a good way to get rid of them, as they can resprout and still flower. Large heavy infestations can be treated with herbicide, either a broadleaf herbicide (2,4-D or clopyralid), or a broad-spectrum herbicide. Either one should be used in the spring before the flower stems appear. Carefully follow the label instructions for amounts and proper timing.

Treating the area with a herbicide is no guarantee that milk thistle won’t grow again from seeds still in the soil so keep an eye out for new seedlings and either hand-pull or spray them as needed.

How to Prevent Milk Thistle From Spreading

After you have removed the milk thistle, it is important to replant the area promptly. Choose a plant that is well-adapted to the growing conditions and grows densely. A native perennial grass such as wood grass is ideal. If the infestation was heavy, it is likely that viable seeds still remain in the soil. By filling every bare spot of soil quickly with other plants, you deny milk thistle the chance to reestablish itself.