Milk Thistle Weeds

Milk thistle grows as an invasive weed in most states.

Milk thistle weed, also referred to as blessed milk thistle, grows in most of the Western and Southern United States. The weedy plant also grows in some Midwest and Eastern states along with several provinces in Canada. Milk thistle weed grows as an annual, reproducing by seeds capable of staying in the ground for long periods of time until they germinate. In some states, such as Washington, milk thistle remains illegal to sell or buy for use in the garden.

Description

The spine-laden milk thistle plant can grow up to 6.5 feet tall. Its lower leaves can reach up to 28 inches long, although the leaves get shorter toward the top of the plant. Purplish-pink flowers bloom on long stalks with spines up to 2 inches long beneath the flower head. After the flower fades, one-seeded fruits with a tuft of bristles appear. The plant reproduces via seed, relying on the wind to carry the tufts and seeds to new locations. The seeds also cling to birds and wildlife that unknowingly help spread the seeds.

Background

The plants grow as a native species in the Mediterranean region. Milk thistle was referred to by its name, silybum marianum, as early as the first century. The plant showed up in several states in the 1800s. Milk thistle competes and lives with a variety of thistle plants that grow as native plants in the U.S. To biologically control the spread of milk thistle, a seed-eating weevil was released in southern California in 1971. While the insects do eat some milk thistle seeds, they also eat the seeds of native thistles, so the weevil's effect on the invasive weed is minimal at best.

Problem

When milk thistle becomes established, it forms dense clumps that crowd out other species of plants. The thistle takes over pastures, eliminating more desirable foraging plants. If livestock eat milk thistle, they may die due to the high amount of nitrate in the plant. Milk thistle also grows in ditches along the road, in disturbed areas and in combination with other type of thistle plants, including Italian and slender-flower varieties.

Medical Uses

While gardeners may consider milk thistle a weed, a substance in the plant known as silymarin may offer a remedy for liver and gall bladder problems. Some people believe that milk thistle works as a treatment for alcohol-related hepatitis and cirrhosis, but studies give mixed results, according to the University of Maryland. Studies show that milk thistle works as an emergency antidote against the effects of deathcap mushroom when used within 24 hours. Other studies have shown that silymarin may stop cancer cells from reproducing.

References

• University of California Statewide Integrated Pest Management Program: Blessed Milkthistle
• Oregon Department of Agriculture Plant Division, Noxious Weed Control: Milk Thistle
• University of Maryland Medical Center: Milk Thistle
• USDA Natural Resources Conservation Service: Silybum marianum (L.) Gaertn.
• King County Noxious Weeds: Milk Thistle

Resources

• Washington State Noxious Weed Control Board: ...Class A Weed