Russian knapweed

About

Russian knapweed is a creeping perennial from Central Asia that forms dense patches in western fields, ditches and rangeland. Its deep, spreading roots make it very hard to kill, and it is poisonous to horses.

Also called hardheads, Turkestan thistle

How to identify it

Look for

Upright, branched stems grow 1 1/2 to 3 feet tall, whitish and woolly when young. Lower leaves are gray-green and lobed, upper leaves small and smooth-edged. Small urn-shaped pink to lavender flower heads sit on bracts with broad, papery tips, and the roots are black or dark brown and scaly.

Look-alikes

Spotted knapweed has black-fringed bracts and a taproot. Russian knapweed's papery-tipped bracts and dark, creeping roots set it apart.

Where it grows

Pastures, hayfields, orchards, vineyards, irrigation ditches, roadsides and disturbed ground across the West.

When

Shoots come up in early spring, bolt in May or June and flower through summer into fall; in late summer the roots set buds for next year.

How it spreads

Mostly by creeping roots, which can cover about 12 square yards in two seasons and reach over 8 feet deep; root pieces regrow after tilling, and it spreads less often by seed.

Most common in

Where it is regulated

Listed as a Class B noxious weed in Washington and California, and prohibited in Wisconsin.

What to do, gentlest first

Start at the top. Weed killers are a last step; always read and follow the label.

1. Reseed with tough perennial grasses Prevent

   After any treatment, seed competitive perennial grasses so it can't take back the ground. Keep horses away from patches.

2. Pull or mow before seed, again and again By hand

   Pull young plants before seed set and mow repeatedly at the bud stage. This only suppresses it; it regrows strongly from roots.

   Don't till; root pieces start new plants.

3. Grazing and a gall-forming nematode Natural enemies

   Sheep or goats can graze it down, and a nematode (Subanguina picridis) forms galls on it. Ask Extension what's available locally.

   Don't let horses graze it.

4. Fall systemic herbicide Conventional spray

   Colorado State lists clopyralid (alone or with 2,4-D), aminopyralid, picloram and chlorsulfuron, at bud-to-flower stage or in late fall when roots are storing food. Glyphosate failed in their trials.

   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. It is poisonous to horses and can cause 'chewing disease,' a brain disorder (Washington State Noxious Weed Control Board; UC IPM).