Annual Sowthistle

What is annual sowthistle?

Annual sowthistle is a fast-growing, upright weed in the aster family, probably native to the Mediterranean and now found across nearly all of the United States. annual sowthistle identification hinges on a few features you can check in seconds: break a leaf or stem and milky sap appears, the stem is smooth and hollow, and the upper leaves wrap around the stem with sharp, claw-like lobes.

It is a common nuisance in gardens, beds and crop fields because it grows fast and seeds heavily. In warm weather on poor soil it can flower within six weeks of sprouting, and each plant can release thousands of wind-borne seeds that germinate right on the soil surface. It also harbors aphids and nematodes that attack vegetable and fruit crops, some of which can spread plant viruses.

Annual Sowthistle identification: what to look for

Start at the point where an upper leaf meets the stem. That clasping leaf base, together with the milky sap and smooth hollow stem, sets sowthistles apart from most look-alikes, and the shape of the clasping lobes tells you which sowthistle you have.

• Seedlings: A flat rosette at first. Seed leaves are hairless, short-stalked and oval; the first true leaves are sparsely hairy with irregular, backward-pointing teeth, and milky sap shows by the four-leaf stage.
• Leaves: Dull green to blue-green and hairless, cut nearly to the midrib into up to three pairs of lobes with a larger triangular end lobe. Edges are irregularly toothed and may carry soft spines.
• Leaf base: Upper leaves are stalkless and clasp the stem with a pair of pointed, angular lobes.
• Stem: Upright, smooth and hollow, branching near the top, with milky sap when cut.
• Flowers: Pale yellow heads 1/2 to 1 inch wide, one to five per cluster at the branch tips, maturing into white fluffy seed heads.
• Seeds: Brown to olive, flattened, finely wrinkled and ribbed, never winged, with a white pappus.

Where it grows in the United States

Cornell weed scientists report that annual sowthistle occurs throughout most of the United States and southern Canada, and USDA lists it as introduced in every US region, including Alaska, Hawaii and Puerto Rico. In California it is abundant up to about 4,900 feet and grows year-round in the Central Valley and along the coast.

It thrives on disturbed ground with available moisture: tilled gardens, crop fields, orchards, vineyards, vacant lots and roadsides. It tolerates clay to sand, salty and alkaline soils, and low fertility, but not more than light shade. Seeds drift in on their parachutes, ride irrigation water and manure, and cling to fur and clothing, so it appears quickly on any newly bared soil.

Annual Sowthistle look-alikes

A few plants get mixed up with it:

• Spiny sowthistle (Sonchus asper): Glossier, less deeply divided leaves with wavy, much pricklier edges, and large rounded 'ram's-horn' lobes where the leaf clasps the stem. Annual sowthistle's clasping lobes are pointed and its leaves are softer.
• Perennial sowthistle (Sonchus arvensis): Larger flowers, about 1 to 2 inches across, and an extensive system of spreading perennial roots instead of a short taproot, so it returns and spreads from the roots.
• Prickly lettuce (Lactuca serriola): Has a row of prominent spines along the underside of the leaf midrib. Sowthistle midribs lack those spines.

How to get rid of annual sowthistle

Pulling, digging and mowing: Pull or hoe young plants; the short taproot lifts out easily, and the plant cannot regrow from the root alone. Do not leave cut flowering plants lying in the garden, because they can still ripen seed; bag them instead. Shallow cultivation kills seedlings on open soil, and young rosettes lie flat for weeks, which makes them easy to bury or slice off. Mowing nearby field edges and ditches before plants flower limits seed blowing into the yard.

Cultural control matters most, because the right conditions keep it from coming back.

• Mulch beds; sowthistle's long rosette stage makes it easy to suppress with organic mulch.
• Keep soil covered with mulch or dense, shading plantings, since seed needs light and germinates at or near the surface.
• Mow ditches, field edges and other open ground nearby before plants bloom.
• Keep it out of vegetable gardens, where it can host aphids and nematodes.

Herbicides: As an annual, sowthistle should be treated before it flowers. In lawns, Washington State University lists combination products of 2,4-D, MCPP (mecoprop) and dicamba; in landscape beds and on bare ground, glyphosate, used only as a spot treatment. For pasture, rangeland and grass, the High Plains IPM guide lists dicamba with 2,4-D, and clopyralid appears in some grass and grain crop programs. Herbicide response varies among sowthistle species, so confirm which one you have before spraying. Use only products labeled for your site, and follow the label exactly. The label is the law. If you hire someone to spray, make sure they hold your state’s pesticide applicator license; the rules for each state are in our pesticide applicator license guide.

Keeping it from coming back

Do not let it flower. Annual sowthistle reproduces only by seed, and the seed bank is fairly short-lived, with most seed in stirred soil dying within a few years, so a few seasons of removing every plant before bloom makes a big dent.

Keep soil covered. Seeds blow in on the wind and germinate best on the surface in light, so mulch, dense plantings and quick replanting of bare spots keep new seedlings from getting started.

Questions we get about annual sowthistle

Similar weeds

Spiny SowthistleSonchus asperPerennial SowthistleSonchus arvensisPrickly LettuceLactuca serriola