Annual Sowthistle (Sonchus oleraceus L.) 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, 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 sow-thistle from perennial. Annual sow-thistle reproduces only by seed and has deeper lobed leaves. Perennial sow-thistle has a larger hairy flower head and is more spiny. In particular perennial sowthistle has a long vertical rhizome-like root system whereas annual sow-thistle has a short taproot.

Control

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.