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Invasive Plant: Scotch Thistle

Photo shows countryside filled with plants and a few houses in the background.
Scotch thistle causes problems mostly in rangeland and pine woodlands. Thistle thickets take the place of plants that large animals, like cattle and elk, eat. The plant costs a lot to remove, and treatment must be repeated for several years. Scotch thistle has been found in Clackamas County.
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At a Glance

  • Scientific name: Onopordum acanthium
  • Annual, perennial, or biennial? Biennial
  • Conditions it likes best: Sun and well-drained soils
  • How it spreads: It makes a lot of seed, which then stick to people and animals.
  • How it causes damage: Scotch thistle hurts Oregon cattle farmers and rangelands by taking the nutrients and water from the grasses that cattle, elk, and other large animals eat.
  • Best time and way to manage: Scotch thistle is not currently found in Portland.
  • 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 Scotch Thistle

Photo shows plants with stems spikes and fluffy purple flowers.
Scotch thistle is causing problems mostly in eastern and southern Oregon rangelands and pine woodlands. Photos from the Oregon Department of Agriculture.

Scotch thistle lives for two years and are known for leaves that run up the sides of the stems, looking like wings.

Fall–Winter. In their first year, rosettes (circles of leaves) can be 2 to 4 feet tall.  

Spring–Summer. In their second year, rosettes will start to bolt (grow a stem and get taller) in March and April, sometimes getting 8 feet tall. Plants grow large purple flowers in May and June. Seeds are already ripe in the flowers, which change into fluffy seedheads by mid-June and mid-July.

How and When to Remove Scotch Thistle

  • Requires professional management. Scotch thistle is a high priority plant and would be managed by professionals from the City or the State. This plant is on Oregon’s and Portland’s Required Eradication List, so it is important to report any plants found so they can be removed.
  • 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

There are no look-alikes for this very tall thistle, although bull thistle can get up to 4 feet tall. Any thistle 6 feet tall or taller should be reported.

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.