Perennial sowthistle (Sonchus arvensis L.)
Perennial sowthistle is an herbaceous perennial that grows from basal rosettes and produces erect stems reaching 2–6 feet tall. Stems are stout, smooth, hollow, and generally unbranched except near the top, where flowering branches form.
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Leaves are light to medium green and arranged alternately along the stem. They clasp the stem at the base and vary in shape from oblong to lance-shaped. Margins may be lobed or entire but typically have small spines or teeth. Leaves are somewhat variable in appearance. When broken, all parts of the plant exude a thick, milky sap; however, the sap is not toxic.
Flowering occurs from early to late summer. Flowers are bright yellow and resemble flat dandelion flowers. They occur in small clusters at the ends of stems and branches. Each flower head is composed of numerous yellow ray flowers (composite flower structure typical of the daisy family).
Perennial sowthistle reproduces both by seed and by an extensive rhizomatous root system that may extend up to 10 feet deep. Seeds are small, reddish-brown, and equipped with tufts of fine hairs that allow for wind dispersal.
Quick ID Guide
Life Cycle:
• Herbaceous perennial
• Extensive rhizomatous roots (up to 10 feet deep)
Height:
• 2–6 feet tall
Stems:
• Erect, stout, smooth
• Hollow
• Unbranched except at the top
Leaves:
• Alternate arrangement
• Clasping stem
• Variable shape (oblong to lance-shaped)
• Slightly toothed or lobed
• Small marginal spines
Flowers (Early–Late Summer):
• Bright yellow
• Dandelion-like
• In clusters at stem tips
Sap:
• Thick, milky latex
• Not toxic
Seeds:
• Small, reddish-brown
• Tufted hairs for wind dispersal
Management Options
Prevention and cultural control strategies should be emphasized.
Controlled grazing with sheep or cattle may help prevent seed production and gradually deplete root reserves. There are currently no biological control agents available for perennial sowthistle in Sublette County.
Hand pulling or digging is generally not recommended unless conducted repeatedly throughout the growing season, as rhizome fragments readily resprout. Mowing at regular 3–4 week intervals can effectively reduce seed production and weaken the root system. Do not mow once seeds are present.
Tillage may provide control if rhizomes are buried at least 12 inches deep, though this may not always be practical.
Larger infestations may require herbicide applications. Early to mid-summer treatments before bloom can be effective, and fall applications of systemic herbicides are often successful in targeting the rootstock. Always read and follow the herbicide label before application. Contact Sublette County Weed & Pest for assistance with product selection, rates, and application timing.
