How to Grow Milk Thistle

Milk thistle with purple flower and marbled leaves

The Spruce / Randi Rhoades

Though it's sometimes considered a weed due to its invasive properties and ability to spread quickly, the milk thistle plant (Silybum marianum) has an array of medicinal benefits.

The plant is an antioxidant that has been shown to protect cells from damage, and it also contains the active ingredient silymarin, which is used to treat liver and gallbladder issues. Silymarin can be found in the plant's seeds, which are often used to make extracts or tinctures that can be used medicinally. Milk thistle is also a known antidote for certain poisonous mushrooms.

All parts of these tall and thorny biennial flowering plants can be eaten. The seeds can be harvested and you can cook and consume both the leaves and flower heads.

The flowers are most commonly harvested for their seeds. The flower heads can be cut with scissors when they're young, and then either boiled or steamed until they're tender enough to eat. However, these plants can be toxic to many animals, so, if it ends up in pastures, it can poison livestock.

The milk thistle's purple flowers sit on top of spiky heads, and it is also known for its bright green foliage with white marbling (which is what give the plant it's milky moniker).

The Mediterranean native plant is particularly fond of growing in rocky, dry regions.

In some locations, growing milk thistle is actually illegal because of its ability to spread and destroy ecosystems. They can also grow to be rather large (up to three feet tall and four feet wide), and the flower stalks themselves produce multiple flowers and can reach heights up to five feet. The milk thistle also happens to be very cold hardy and difficult to eradicate once its established thanks to its deep taproots and dense foliage.

There are some other species of thistle that are not as invasive, but you'll know it's a milk thistle because of the white markings on its foliage that other native species do not have.

  Botanical Name  Silybum marianum
  Common Name  Milk thistle
Plant Type   Biennial
  Mature Size  Up to 3 feet
  Sun Exposure  Full sun, Part shade 
  Soil Type  All varieties
  Soil pH  6 - 6.5 
  Bloom Time  Summer
  Flower Color  Purple
  Hardiness Zones  5 - 9, USA
Native Area   Europe

Milk Thistle Plant Care

Milk thistle is considered very easy to grow and can tolerate most conditions, but this means it is also extremely invasive. These plants behave and spread much like weeds, which means they can quickly overtake any other plants nearby and absorb all of the space and nutrients.

You'll need to check if you can grow it in your region legally and weigh up the plants pros and cons before deciding to grow it.

Milk thistle with purple flower closeup

The Spruce / Randi Rhoades

Milk thistle seeds on rocky ground

The Spruce / Randi Rhoades

Milk thistle marbled leaves

The Spruce / Randi Rhoades

Light

The milk thistle plant will grow best in either a sunny or part shade location.

Water

Although milk thistle is a drought-resistant plant, it will still require water in completely dry conditions, at least until it's established.

Soil

Milk thistle will grow easily in a variety of soil types, including sandy varieties and heavy clay dirt.

Temperature and Humidity

The hardy milk thistle plant isn't picky when it comes to the type of weather it will grow in. Their seeds can germinate in temperatures ranging anywhere from 32 to 86 degrees Fahrenheit, and the plant can remain viable for up to ten years.

Fertilizer

Many growers of milk thistle will fertilize the soil with both nitrogen and potassium in order to increase the yield of seeds.

Pruning

If you've planted milk thistle to add a little bit of color to your landscape, their top stems can be pruned as soon as its flowers start to die off. This will not only enhance the overall appearance of the plant but will also help prevent the dispersal of seeds via the wind.

Propagating Milk Thistle

These weed-like biennials can produce thousands of seeds from just one plant. Unlike many other edible plants, the milk thistle's seeds can remain viable for many years--even up to a decade after harvesting.

Growing Milk Thistle From Seeds

The milk thistle's seeds should be sown outside about 1/2 inch deep in either March or April for summer flowering. Be sure to space each hole about three feet apart, and you'll only need three or four seeds for each hole. The weakest plants can be thinned out after they germinate, which should take about 10 to 20 days.

Growing in Containers

When growing milk thistle for herbal or medicinal purposes, many gardeners will plant them indoors. This also helps prevent the plant from taking over the entire garden. The milk thistle's seeds can be planted in seed trays filled with potting soil, and then covered with a light layer of soil. The tray will need to remain in an area that remains at 60 degrees Fahrenheit and then watered daily until germination occurs.