Milk Thistle – March 2020 Weed of the Month

milk thistle growing on a fence
Milk thistle in King County is mostly limited to a cluster of properties near Enumclaw including some dairies.

Milk thistle is a perfect example of how plants can have both positive and negative impacts. On the one hand, milk thistle is a Class A noxious weed and quarantined species in Washington because of its negative impacts on pastures and livestock, the potential for rapid spread, and the difficulty to eradicate it once it establishes. But on the other hand, milk thistle seeds are widely used as an herbal medicine and are commonly sold even here in Washington (which is legal if the seeds have been steam-treated or otherwise made non-viable).

Milk thistle seeds are used for medicine and can be found for sale in some local stores. Milk thistle is on the quarantine list for Washington but as long as the seeds are treated so they are non-viable, WSDA allows their sale as an herbal supplement.

Also called blessed milkthistle, its seeds have been used as medicine for thousands of years, starting in Europe and Asia where the species originated. Milk thistle is used for a wide range of reasons but primarily to improve liver health and to increase breast milk production. The plant is also edible to people – just be sure to remove the incredibly sharp spines!

Milk thistle seed heads with Mount Rainier in the background
Even noxious weeds can be scenic: milk thistle seed heads looking ominous with Mount Rainier in the background.

On the dark side, however, milk thistle plants are toxic to cattle and sheep (and other ruminants) because the species is a nitrate accumulator. Nitrate poisoning reduces the animal’s ability to get oxygen. The nitrate is transformed to nitrite, which then reacts with hemoglobin to form a new compound that doesn’t release oxygen in the bloodstream. The symptoms of acute nitrate poisoning are trembling, staggering, rapid breathing, and sometimes death. Chronic poisoning may result in poor growth, poor milk production and abortions. Milk thistle can also cause injury to grazing animals due to the sharp spines.

Milk thistle flowerheads are stunning but armed with sharp spines.
Milk thistle flowerheads are stunning but armed with sharp spines.

Milk thistle was used as an ornamental species in Seattle-area gardens before it was added to the state noxious weed list. The plants are quite large, with shiny leaves displaying white marbling patterns and spines at leaf edges. They have artichoke-like flower heads with round, purple flowers surrounded by fleshy, spine-tipped bracts. Unfortunately, milk thistle spreads easily and abundantly by seed so it doesn’t stay where it’s planted!

milk thistle in a garden planting
Milk thistle has been used as an ornamental plant due to its striking milky leaves and large pink flowers.
milk thistle escaping a garden
Milk thistle spreads readily by seed and doesn’t stay where it’s planted.
milk thistle seedlings
Milk thistle leaves always have the characteristic milky veins and spiny edges, even on the first true leaves of the seedlings.

Milk thistle thrives in disturbed areas such as pastures, roadsides, ditches, and fencerows. It is a winter annual or biennial growing 2-6 feet tall and blooms from April to October. Each plant can produce around 6,000 seeds that persist in the soil for over 9 years.

milk thistle on edge of pasture
Milk thistle thrives in disturbed areas such as fence lines and high traffic zones. These large rosettes are surrounded by countless tiny seedlings that would be new plants if left alone.
milk thistle rosette with seedlings
In the fall and spring, milk thistle rosettes and seedlings can be found in openings and disturbed areas around the known infestation. Seeds often fall near the parent plants and germinate in large numbers.

In fields and natural areas, milk thistle grows so densely and abundantly that it can overwhelm pasture grasses and native meadow species. A large field of milk thistle looks like a blackberry patch from a distance, with large dense mounds of spiny plants completely covering the ground. California reports milk thistle stands of up to 4 tons per acre in heavily infested areas. In addition to the risk of nitrate poisoning, milk thistle dramatically reduces forage productivity and disrupts farm practices.

milk thistle infestation going to seed
Milk thistle grows in dense patches that exclude other vegetation.

As you can imagine, removing large infestations of milk thistle is labor-intensive and the sharp spines make it a painful undertaking. The seed bank is also very long-lived, requiring many years of follow-up control to keep the plants from returning. Even controlling the plants with an herbicide requires spring and fall treatments combined with summer monitoring visits to remove any skipped flowering plants. Basically, once you get milk thistle in a field, expect to have it for many years even with diligent control work!

milk thistle in field before removal
Milk thistle in a pasture in Enumclaw before we dug it up.
milk thistle dug up
Digging up milk thistle is effective but no small task.
milk thistle in seed
Sometimes one escapes! Even with spring and fall treatments, summer monitoring is important to catch the occasional survivor before it goes to seed.

Fortunately, milk thistle is not widespread in King County and is limited primarily to a cluster of properties in the Enumclaw plateau and a few residential gardens. Because of this, eradication is our management goal, and early detection and rapid response is of the highest priority for this noxious weed.

The first milk thistle was found on the Enumclaw plateau in 2001, and by 2008 infestations had been found on over 40 properties in the area, spread by seed from field to field. Farmers had tried to manage the milk thistle by mowing it regularly but that increased the spread and didn’t control the plants.

Milk thistle is a tall, spiny thistle with large flowerheads.
Controlling milk thistle by cutting the flowerheads is very labor-intensive and mowing can spread the plant to new sites because seeds get carried on equipment.

In order to keep the milk thistle from spreading further, our program helps farmers control the milk thistle using an integrated management plan that includes manual, chemical, and cultural control, and monitoring and prevention of new infestations. This integrated approach allows us to exhaust the seed bank in the soil, prevent seed dispersal, and support competitive pasture grasses.

In 2019, we worked with 39 farmers and other landowners to help control milk thistle. Although many sites continue to have plants germinate from the seed bank, we were able to mark 6 sites dormant, meaning no new weeds have been found in over 3 years. Public education, strong partnerships with property owners, and control assistance from our program has reduced the amount of milk thistle infested area by 95 percent.

milk thistle in King County 1998-2019
After discovering the majority of the large infestations by 2008, our IPM program for milk thistle has reduced the infested area significantly. Note: In 1998-2003 less than 100 square feet was found.
milk thistle sites in King County 1998-2019
The number of milk thistle sites has evened out as we have contained the spread. As we deplete the seed bank, the number of eradicated sites will continue to go up, but it is a slow process!

More information about milk thistle:

Milk Thistle – May 2018 Weed of the Month (so important we are featuring it again!)

King County Noxious Weed Program

Washington State Noxious Weed Control Board

In King County, help us find milk thistle by reporting it with the mobile app King County Connect or on our online reporting form.

Elsewhere in Washington, report noxious weeds to your local county weed board or with the WA Invasives app.

milk thistle with shovel
Milk thistle is a daunting noxious weed but it can be tackled with hard work and diligence.