Yellow starthistle

About

A spiny Eurasian weed that covers millions of acres in the West, especially California, Oregon, Washington and Idaho. It crowds out pasture and wildflowers, and it causes a deadly brain disease in horses.

Also called yellow star-thistle, St. Barnaby's thistle, Barnaby's thistle

How to identify it

Look for

Grayish, cottony plants from 6 inches to over 6 feet tall, with stiff stems that look winged where the leaf bases run down them. Rosette leaves are deeply lobed. Bright yellow flower heads sit on a ring of long, stiff, sharp spines.

Look-alikes

Malta starthistle and Sicilian starthistle look similar and are also weedy; the long spines under bright yellow flower heads are the key clue.

Where it grows

Dry, sunny grasslands, pastures, roadsides, vacant lots, parks and open hillsides.

When

Sprouts after the first fall rains, grows as a rosette over winter, bolts in late spring and flowers from about June into fall.

How it spreads

Spreads only by seed, often huge numbers, carried on vehicles, boots, animals, hay and soil; most seeds sprout within a year, but some last up to 10.

Most common in

What to do, gentlest first

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

1. Stop seed from coming in Prevent

   Clean boots, tires and equipment after visiting infested areas, and buy weed-free hay. Keep horses off infested ground.

2. Mow at first bloom Prevent

   Mow just as the first yellow flowers open; repeat if plants regrow and rebloom.

   Wear eye protection and long sleeves.

3. Hoe seedlings and pull bolting plants By hand

   Hoe seedlings, or pull plants when they bolt in late spring while the soil is still moist.

   Wear heavy gloves against the spines, and bag flowering plants.

4. Solarize small patches Barrier

   Covering moist soil with clear plastic during hot summer weather kills seeds and seedlings.

5. Seed-feeding insects, grazing and competitive plants Natural enemies

   Released insects such as the hairy weevil and false peacock fly cut seed production. Sheep, goats and cattle graze it before spines form, and dense lawns, groundcovers or perennial grasses crowd it out.

   Never let horses graze it.

6. Pre-emergent before fall rains Conventional spray

   A prodiamine or dithiopyr pre-emergent, applied just before the rainy season, stops seedlings in landscaped areas.

   Read and follow the label. Many lawn and brush herbicides also kill garden plants, so keep spray and drift off anything you want to keep.

7. Clopyralid, aminopyralid, triclopyr or glyphosate Conventional spray

   Treat seedlings and rosettes from about December to March with a selective product like aminopyralid or clopyralid; glyphosate works through early flowering. Expect three or more years of work.

   Read and follow the label. Many lawn and brush herbicides also kill garden plants, so keep spray and drift off anything you want to keep.

Uses, wildlife and pets

Pets

Toxic to horses (ASPCA), causing 'chewing disease'; the ASPCA lists it as non-toxic to dogs and cats.