Scotch Thistle

How to Identify and Remove Scotch Thistle

Scientific name: Onopordum acanthium

Life cycle: Biennial (takes two years to complete its life cycle)

Why it’s a problem:

It’s not native to Utah, and it takes important resources from the ground that native plants need.
It’s like living barbed wire, growing up to 8 ft. tall and 5 ft. wide with painfully sharp, spiked leaves. It keeps people from recreating in certain areas, keeps wildlife away from lands they'd normally use to forage, and can block off wildlife's access to water sources.

Scotch Thistle in several of its stages

Click the arrows on the right and left-hand side of each photo, or swipe left or right on your phone to scroll through the slideshow.

How to tell the difference between Utah's native thistle (Cirsium neomexicanum) and Scotch Thistle

Scotch Thistle

Sage-green in color, but with a more blue-greenish hue
Leaves are massive, and don't have deep indents (known as lobes)
Scotch Thistle is huge - growing 6-8 feet tall and up to 5 feet wide
One plant grows several big stalks that all produce dozens of thistle heads

Scotch Thistle often grows in areas where one of Utah's native thistles grows too. We'd like to keep the native thistles around, so here's how to tell the difference between the two species:

vs.

Notice how much thicker and bulkier the leaves on Scotch thistle are, even when it is first coming up as a rosette, compared to our native thistle.

Utah Thistle

Sage-green in color, but with a powdery whitish hue
Leaves are more narrow, with very deep indents (known as lobes)
This thistle only grows 2-5 feet tall, and is far more narrow
This native thistle typically grows one main stalk that produces a few thistle heads

How to remove Scotch Thistle

Chopping Scotch Thistle at its taproot is very effective

A sharp shovel is your best tool for this
Use the shovel to stab off some of the bigger leaves on one side so that you can get at the taproot.
Use the shovel to stab through the taproot a few inches under the soil, so that what’s left of the root is not exposed to sunlight. This prevents it from growing back.

If the plant hasn’t formed thistle heads, you can leave it on the ground so its nutrients decompose back into the soil. Scotch Thistle is massive and also good at poking holes through trash bags, so if you can leave it behind, do it!

Once the plant has thistle heads, it’s forming seeds that can potentially still grow new plants if left behind.

If you can’t bag the whole plant, try to at least chop off al of the thistle heads and bag them up.
Invasive weed seeds are NOT green waste - they need to go in the garbage.

For more information on Scotch Thistle, check out Salt Lake County's website.

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