Annual Sowthistle (Sonchus oleraceusL.) is an annual weed, reproducing by seeds. Sowthistle is native to North Africa, Europe, and western Asia. The stems are erect, 1-4 feet high, hollow, and glabrous. The leaves are alternate with deeply lobed margins on each side. Lower leaves are petiolate while the upper leaves are sessile. Leaf veins may be purple. Flower heads are numerous, yellow, and ¼ to ¾ inches wide. Flowers are produced from July to October. The seeds are flat achenes, approximately 3 mm long; brown, with a pappus of white hair at one end. This weed germinates anytime between spring and fall when conditions are suitable. The seed will germinate with very little water. Most of the seeds can be found within the first centimeter of soil.
Annual sow-thistle thrives in nursery crops, orchards, and disturbed areas resulting from overgrazing, natural processes or cultivation. This weed may compete with other desirable plant species. The seeds are easily disseminated by the wind due to the pappus of white hair at one end.
The over wintering rosette phase may also serve as a reservoir for both vectors and viruses. This weed may also act as an alternate summer host to economically important aphids which are potential vectors of “yellow” virus disease. This weed can indirectly affect the development of field crops because it hosts nematodes, aphids, and viruses.
Many distinguishing characteristics exist when determining annual sowthistle from perennial. Annual sowthistle reproduces only by seed and has deeper lobed leaves. Perennial sowthistle has a larger hairy flower head and is spinier. In particular, perennial sowthistle has a long vertical rhizome-like root system whereas annual sowthistle has a short taproot.
Regular cultivation will stimulate germination. Tillage can be used for control at the emergence phase but may in turn allow for multiple flushes. Any rainfall event can stimulate the germination of annual sowthistle.
Chemical Control: In crops-2,4-D, Clopyralid or other Group 4(synthetic auxin) herbicides can provide adequate control. Quinclorac will control annual sowthistle in turf grass. Many non-selective chemicals can be used for the control of sowthistle in non- crop locations.
More information can be found in the PNW Weed Management Handbook
Use pesticides with care. Apply them only to plants, animals, or sites listed on the label. When mixing and applying pesticides, follow all label precautions to protect yourself and others around you. It is a violation of the law to disregard label directions. Store pesticides in their original containers and keep them out of the reach of children, pets, and livestock.
Biological Control: No biological control agent is currently available for control of annual sowthistle.
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Questions: contact Steve Van Vleet or phone (509) 397 – 6290