Spotted knapweed

About

Spotted knapweed is a European biennial or short-lived perennial with thistle-like pink flowers that has spread across dry, open ground in much of the country. It crowds out other plants, livestock avoid it, and handling it can irritate the skin.

Also called knapweed

How to identify it

Look for

The first year it makes a low rosette of grayish-green, deeply divided leaves. Later it sends up 1 to 20 wiry, branched stems 2 to 4 feet tall with small, deeply cut leaves. The pink to purple flower heads sit on stiff bracts tipped with black, fringed hairs that look like spots.

Look-alikes

Canada thistle has prickly leaves; spotted knapweed doesn't. Other knapweeds (diffuse, meadow, brown, Russian) look similar, and the bracts under the flower are the main key; the black-tipped fringe marks spotted knapweed.

Where it grows

Dry, sandy or gravelly open ground: roadsides, field edges, gravel pits, pastures, prairies and dunes.

When

It grows from spring, flowers from about late June to September, and drops seed from late summer on; rosettes can wait 1 to 4 years before bolting.

How it spreads

By seed: a plant makes about 1,000 seeds and a big one up to 25,000, which last in soil up to 7 years and hitch rides on vehicles, hay, livestock and rodents.

Most common in

Where it is regulated

Listed as a noxious weed in many states, including Washington (Class B), California and Minnesota (prohibited).

What to do, gentlest first

Protect yourself first. Weed killers are a last step; always read and follow the label.

1. Don't give seeds a ride Prevent

   Avoid driving through patches, wash vehicle undercarriages after being in infested areas, and use weed-free hay. Report new patches to your county weed office or EDDMapS.

2. Dig or pull the taproot before seed By hand

   Pull or dig plants when the soil is wet, getting as much taproot as you can, before seeds form. Mowing from about May into July, before flowering, cuts seed production.

   Wear gloves and long sleeves. Don't mow once seeds are present, and bag flowering plants instead of composting them.

3. Knapweed weevils Natural enemies

   Seedhead weevils (Larinus minutus and L. obtusus) and a root-boring weevil (Cyphocleonus achates) are established biocontrols. Ask your state department of agriculture or Extension about them.

4. Systemic herbicide on rosettes or bolting plants Conventional spray

   Minnesota DNR lists aminopyralid, clopyralid, glyphosate, imazapyr, aminocyclopyrachlor and picloram. Treat rosettes in fall or plants starting to bolt in spring, and expect to repeat for several years.

   Read and follow the label. Many lawn weed killers also kill garden plants, so keep spray and drift off beds and vegetables. Picloram is restricted-use, for certified applicators only.

Uses, wildlife and pets

Pets

Not listed by the ASPCA.