Johnson County Secondary Roads Perennial Sow Thistle Weed

Perennial Sow Thistle (Sonchus arvensis)

Description: Perennial sow thistle is a perennial reproducing by seed and creeping rhizomes. It has been present in Iowa for many years but is not a serious problem. The roots are deep penetrating, wide spreading horizontally and producing new shoots from frequent buds on fleshy rhizomes. The erect stems are smooth or glandular, hairy, leafy, hollow, and branched at the top. The stem and leaves are milky and covered with a whitish bloom. The leaves are variable. The basal leaves narrow at the stem to produce short petioles, are deeply lobed and have soft spines on the irregular margins. The stem leaves are alternate, clasping with round lobes, deeply cut, irregularly toothed, and have curving blades. The numerous flower heads are on the end of terminal branches and similar to a dandelion. They are a deep yellow and 1 to 1 ½ inches in diameter. The flower bracts are dark green or lead colored. The seeds are a dark reddish-brown, 1/8 inch long, slightly flattened, and have a tuft of white hair at the end. Perennial sow thistle produces flowers in June and July and matures seeds in July and August.

Distribution: Perennial sow thistle is found in northern and northeastern Iowa, and scattered throughout Johnson County. It is seldom found in pastures, but frequently along roadsides and in waste areas.

What’s The Problem?: Perennial sow thistle can cause economic losses in crop fields due to reduced crop yields, increased cultivation and herbicide expenses, and land depreciation. It also competes with native plants for sunlight, nutrients, and living space.

Management Methods: Use of mowing, hand pulling or cutting, pasturing of sheep and cattle, cultivation, seeding of competitive species, and careful use of broadleaf herbicides can all be used to reduce infestations and prevent establishment of perennial sow thistle. Be sure to always read and follow all herbicide label directions. Various research has been done by scientists to evaluate using insects, nematodes, and microorganisms as possible future Management Methods.

Above photos courtesy of Washington State Noxious Weed Control Board

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