Thistle, Sow


Perennial Sow-thistle (Sonchus arvensis) and Annual Sowthistle (Sonchus oleraceus). Perennial sow-thistle is a creeping rooted perennial growing 0.4 to 1.5 meters tall; stems branch near the top; leaves with weak marginal prickles clasp the stem; plants contain a bitter milky juice; upper stalks and flower bracts usually covered in gland-tipped hairs; yellow flowers up to 3.8 cm broad often confused with annual sow-thistle (Sonchus oleraceus) (Provincial Noxious) which is taprooted and has much smaller flowers (less than 2.5 cm across) or spiny annual sow thistle (Sonchus asper) which has sharp, spiny leaves and smaller flowers. Perennial sow thistle is an aggressive, creeping weed that can severely reduce yields in cultivated fields. It is an alternate host for several viral diseases. Sow thistle can become a serious problem on marshes, ponds, and riverbank areas. Chemicals from the roots and decaying residue from old growth inhibit seed germination of other species. A common weed of cultivated crops, annual sow thistle acts as a host to aphids, several viral diseases, and nematodes. Rapid germination combined with wind dispersal of seeds allows annual sow thistle to invade both native plant communities and disturbed sites quickly. It can also be a problem in marshes, ponds, and riverbanks as it tolerates saturated soils.

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