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November 8, 2022

November Weed of the Month: Plumeless Thistle

Maggie Barnick, Minnesota Department of Agriculture

I don’t know about you, but I’m already missing seeing all the purple flowers of July and August! The fall weather is making me nostalgic for watching all the new blooms of native plants each week in the summertime. One purple flower that always catches my eye is that of a thistle. It’s mostly the non-native thistles that catch my eye, though it is always a fun “treat” to stumble upon a native thistle.

Lifecycle of the plumeless thistle.

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In Minnesota, there are several species of thistle that are native to the state. The native thistle species are great pollinator plants and are often overlooked in restoration plantings. The simplest way to distinguish between native and nonnative thistles is by touch: you will be able to touch native thistles without hurting too much! Though, this is not the only distinguishing characteristic, so be sure to do some additional research. Check out the Xerces Society for a guide to native thistles in the United States.

Perhaps more widespread are the nonnative thistles, of which we also have several species in Minnesota. Today our focus is plumeless thistle, Carduus acanthoides.

Plumeless thistle is a biennial species, meaning it grows in two-year lifecycles. Its first year after germination is spent gathering resources as a basal rosette. A basal rosette is a great adaptation for survival, exhibited by many weeds and native plants alike. The outspread leaves shade out competition, grab drops of rain, and absorb the maximum amount of sunlight for a plant to later send up a flower. Plumeless thistle blooms in its second year. It is perhaps the spiniest thistle in the state, and in its second year, has deeply lobed leaves that are up to eight inches long with sharp winged spines all along the leaf margins.

Tear-drop shaped plumeless thistle flower.

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Plumeless thistle is closely related to another non-native thistle, the nodding or musk thistle (C. nutans), though the flowers are a third of the size. Stems of both species differ from native thistle species because they are very spiny as a result of the leaf bases attaching directly to the stems and extending downward, often referred to as “spiny wings”. Touching the stem of both plumeless and nodding thistles without heavy gloves for protection is never a good idea! Flowers of nodding thistle are 2-3 inches in size, and disk-like, while flowers of plumeless thistle are smaller and tear-dropped in shape at 1 inch in size. Flowers of plumeless thistle occur singularly or in clumps of 2-5.

Many people think the stem itself produces spines, but the wings on Carduus are actually remnants of the leaves.

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Plumeless thistle is detrimental for grazing agriculture, can disrupt the native ecosystem, and is listed as a Prohibited Noxious Weed on the Control list in Minnesota. Landowners should actively scout for plumeless thistle on their properties and prevent plants from going to seed and spreading. For more information, and additional identifying characteristics and photos, see the Minnesota Department of Agriculture’s website.

MEDIA: For more information on Weed of the Month, contact Brittany Raveill, MDA Communications, at brittany.raveill@state.mn.us or 651-201-6131