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Environmental Services

working for clean rivers

Phone: 503-823-7740

Fax: 503-823-6995

1120 SW 5th Avenue, Room 1000, Portland, OR 97204

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Blessed Milk Thistle

What is the species?

  • Silybum marianum (blessed milk thistle)
  • ODA “B” rank

Blessed milk thistle is currently available as an herbal remedy; at the same time, parts of Australia and California are being overrun by it. Milk thistle is not uncommon in southwest Oregon and has been sighted in several locations along the Columbia Slough, as well as the Vancouver, WA area.

Why are we concerned?

Milk thistle is known to form dense patches that begin in open, disturbed areas. Like other thistles, these plants have persistent root systems and produce large amount of windblown seed, making management difficult. Milk thistle infestations can lead to:

  • Reduced plant and animal diversity as other species are displaced
  • Increased health hazards to livestock

How does it spread?

Milk thistle is only spread by seeds. Milk thistle seeds are moved around by:

  • Wind
  • Tires and machinery
  • Wildlife

What does it look like?

Milk thistle can grow to be 6 feet tall, with distinctive white stripes in the leaves. Milk thistle is also very spiny, more so than bull thistle, with flower/seedheads ringed in exceptionally sharp spines.

Are there any lookalikes?

Not really. The white striping in the leaves looks like nothing else in the area, either native or non-native.

How do we deal with it?

  • Manual: Cutting and digging is possible, but not especially effective. These are also particularly spiny plants, making a lot of handling less desirable.
  • Herbicide: Low rates of herbicide appear to keep milk thistle in check.

How can folks help?

If you suspect you’ve found milk thistle, call either the City of Portland or contact the Oregon Department of Agriculture.