What is perennial sowthistle?

Perennial sowthistle is a deep-rooted weed from Europe and Asia that spreads through creeping underground roots as well as wind-blown seed. Good perennial sowthistle identification matters because it looks a lot like its annual cousins, but it’s much harder to get rid of. Cut it or till it, and every root piece can send up a new shoot.

It isn’t on Pennsylvania’s noxious weed list, but it’s a stubborn problem in gardens, crop fields and field edges, where it forms patches that grow wider each year.

The yellow flowers are pretty, and bees visit them, but each one turns into a puff of wind-borne seed. Seed doesn’t usually live long in the soil, so the creeping roots are the real long-term problem.

Perennial sowthistle identification: what to look for

Perennial sowthistle has tall, leafy stems topped with bright yellow flowers in midsummer. Look for these details:

• Stems: upright, hollow and ridged, usually 1½ to 4 feet tall, smooth near the base and branching near the top.
• Leaves: alternate and lance-shaped, with backward-pointing lobes and soft-prickly margins. Upper leaves wrap around the stem with rounded, ear-like lobes at the base.
• Sap: break a leaf or stem and you’ll see bitter, milky white sap.
• Flowers: bright yellow heads about 1½ to 2 inches across that look like large dandelions, blooming July through September.
• Bracts: the green cup under each flower is covered in sticky, gland-tipped hairs. This is one of the best ways to tell it from the annual sowthistles.
• Roots: deep vertical roots plus horizontal creeping roots with buds that sprout new shoots.

Where it grows in Pennsylvania

Perennial sowthistle prefers open, sunny ground with decent moisture. You’ll find it in crop fields, vegetable gardens, roadsides, ditch banks, field edges and old pastures across the state.

In central PA yards, it most often appears where a garden backs up to a farm field, along a fence line or on ground that has been tilled or disturbed. Tilling actually helps it spread by chopping up the roots. It doesn’t hold up well to regular mowing in a thick lawn, so it’s mostly a problem in gardens, beds and rough areas.

Perennial sowthistle look-alikes

Spiny sowthistle is the closest relative. It’s an annual with a simple taproot, smaller flower heads, smooth bracts and much sharper, stiffer leaf spines. Annual sowthistle is similar but has softer leaves and pale yellow flowers, and neither annual has creeping roots.

Prickly lettuce has a line of spines on the underside of the leaf midrib and small, pale flowers. Dandelion has single flowers on leafless stalks, and Canada thistle has the creeping roots but purple flowers and truly spiny leaves.

How to get rid of perennial sowthistle

Plan to wear it down over a season or two. The goal is to stop seed and drain the energy stored in the roots.

Hand and mechanical control. In a garden, pull or dig shoots as soon as they appear in spring, getting as much root as you can. Keep coming back every couple of weeks. Each time you remove the top growth, the roots spend energy regrowing, and over time they weaken. Avoid rototilling an infested patch unless you’re prepared to follow up repeatedly, since tilling spreads root pieces. Repeated mowing of rough areas prevents flowering but won’t kill the roots on its own.

Cultural control. Keep garden soil covered with a thick layer of mulch or a cover crop so new seedlings don’t get established. In lawns and field edges, a dense stand of grass competes well against it.

Chemical control. Broadleaf herbicides such as 2,4-D, dicamba and clopyralid, and the non-selective herbicide glyphosate, are commonly listed for perennial sowthistle. Apply to actively growing plants at the bud stage, before flowers open, or to regrowth in early fall. Repeat treatments within the season improve results. Clopyralid isn’t labeled for home lawns, and glyphosate will kill any plant it touches, so choose a product that fits your site.

• Read and follow the product label. The label is the law, and it tells you whether the product is allowed in gardens, lawns, pastures or near water.
• Applying herbicide for hire in Pennsylvania requires a PA Department of Agriculture pesticide applicator certification, so larger fields and pastures go to a licensed applicator.

Keeping it from coming back

Don’t let plants flower and go to seed. Check patches through summer and cut or pull anything that’s budding. Clean tillers and shovels before moving them from an infested area to a clean one.

In vegetable gardens, combining mulch, frequent shallow weeding and a fall cover crop keeps the pressure on. Expect a few shoots for a couple of years; steady attention is what finally clears it out.

If sowthistle keeps creeping in from a neighboring field, a mowed buffer strip of thick grass along the property line slows it down and gives you one place to watch for new shoots.

Questions we get about perennial sowthistle

Similar weeds

Spiny SowthistleSonchus asperPrickly LettuceLactuca serriolaCanada ThistleCirsium arvense