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Invasive Plant: Milk Thistle

Photo shows a mass of green plants with spiky leaves and brush-like flowers.
Milk thistle poses a health risk for grazing animals making it a dangerous invasive for Oregon. Grown for the herbal supplement market, milk thistle has escaped cultivation. Locally, it has been found along the Columbia Slough in Portland, throughout Sauvie Island, and in Clark County, Washington.
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At a Glance

  • Photo shows flowerhead with purple brush flower with green spikes around it.
    Although not common in Portland, milk thistle is on the Required Eradication list. All sightings must be reported. 
    Scientific name: Silybum marianum
  • Annual, perennial, or biennial? Biennial
  • Conditions it likes best: Open, sunny fields and along railways and roads
  • How it spreads: These plants have long-lasting root systems and make wind-blown seed.
  • How it causes damage: Milk thistle forms dense patches that push out other species. On farm and ranch lands, milk thistle is a danger to grazing livestock.
  • Best time and way to manage: If there only a few plants, cutting the stems and digging the roots in March or April is very effective. If there are a lot of plants, herbicide may be necessary.
  • Oregon Department of Agriculture noxious weed rank: B
  • Report patches of this plant? Yes. City code lists this as a Required Eradication species. Report all sightings to Portland’s Environmental Services (contact information on this page).  Management of this plant is required, but free help is available.

How to Identify Milk Thistle

Photo shows plant with spiky green leaves with white striping.
Milk thistle can grow to be 6 feet tall with distinctive white stripes in the leaves.

Milk thistle can be 6 feet tall and has very obvious white stripes in the leaves. Milk thistle is also very spiny, even more than most other local thistles. Flowers and seed heads have a ring of very sharp spines at the base. Plants bloom from April through July with reddish-purple flowers.

Fall–Winter. Seedlings and rosettes (flat circles of leaves) can start appearing in October and throughout the winter months.

Spring–Summer. Rosettes will start to bolt (grow a stem and get taller) in March and April, and start to flower in May and June. Seeds are already ripe in the flowers, which change into fluffy seedheads by mid-June and July.

How and When to Remove Milk Thistle

Photo shows a mass of green plants with spiky leaves and brush-like flowers.
Milk thistle is very spiny, more so than bull thistle, with flowers and seedheads ringed by exceptionally sharp spines.

Control or treat this plant in the spring, before flowers form. This keeps seed from blowing around in early summer.

  • Manual: Cutting and digging up patches is possible, if there aren’t many plants. These are spiny plants though, so most people don’t like doing this.
  • Herbicide: Low rates of certain herbicides appear to keep milk thistle under control.
  • Check. Check the site for new plants at least once a year, and especially for the first 2-3 years after removal .
  • Replant. After a year or two of managing this plant, consider planting native or less aggressive non-native plants.

Prevention is Best Practice

Clean your boots, pets, and maybe even your tires when you finish a hike or trail ride in Pacific Northwest natural areas, or if you have invasive plants on your own property. Cleaning boots and pets keeps invasive plants from spreading to new places. Be careful when trading plants with neighbors and fellow gardeners to make sure you’re not trading this or any other plant of concern.

Be Aware of Look-alikes

Milk thistle doesn’t have any look-alikes. Its white-striped leaves look like nothing else, native or non-native.

Properly Dispose of Invasive Plants

To prevent spreading invasive plants:

  • Never place plants or seeds in your compost or yard debris.
  • Never throw pulled plants on the ground or into the street or sidewalk.
  • Put all pulled plants, bulbs, or seeds into a plastic bag and put the bag in the trash.
  • Wash all garden tools and gloves.