Milk thistle, a plant that was introduced to North America in colonial times, has been historically used as a medicinal plant. However, it has escaped cultivation and is found in many places throughout the United States. Milk thistle stands out from other thistles by the white marbling of its large, dark green, hairless leaves. The plant is native to the Mediterranean and is particularly fond of growing in rocky, dry locations. Milk thistle is considered invasive because like other invasive plants, it grows in dense patches, outcompeting native plants and depriving them of water, sunlight, and nutrients.
|Common Name||Milk thistle|
|Botanical Name||Silybum marianum|
|Plant Type||Herbaceous, biennial|
|Mature Size||2-6 ft, tall, 1-3 ft. wide|
|Soil Type||Loamy, sandy, clay, silt|
|Hardiness Zones||5-9 (USDA)|
Milk Thistle Invasiveness
Milk thistle is considered an invasive plant, or at the very least, listed as a noxious weed by several states. In some locations, such as in Washington State, selling or buying milk thistle is actually illegal and existing plants must be removed.
Milk thistle can grow in a wide range of habitats. The plant is very cold hardy and difficult to eradicate once it's established thanks to its deep taproots and dense foliage. Milk thistle will grow easily in a variety of soil types, including sandy varieties and heavy clay dirt. This high adaptability has allowed it to spread in many areas of the United States and destroy ecosystems. Milk thistle can quickly overtake any other vegetation nearby and absorb all of the space and nutrients. Milk thistle infestations are also problematic for farmers as milk thistle is toxic to livestock.
One plant produces thousands seeds that remain viable in the soil for at least nine years.
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
Regardless of what removal method you use, the most important thing is that you get rid of the plant in the earliest stages, when it is still a low-growing rosette in the spring. If you miss that point in time, you should definitely remove the milk thistles before they flower. If you remove the plants when they have already developed flower buds or are flowering, carefully 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. The plant has a long taproot so if the soil is compacted, you might need a shovel or another tool to fully remove it.
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.
Is milk thistle edible?
Although milk thistle, specifically the active ingredient in the plant called silymarin, has been used a herbal remedy for 2,000 years, is it not recommended that you consume or use milk thistle plants in any form.
Are there native thistles that I can plant instead of milk thistle?
One of the most diverse native thistles is the Cirsium genus with more than 62 species. To determine which native thistle is best suited to your area, contact the native plant society in your state, or a local native plant nursery.
How are the seeds of milk thistle spread?
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.