Sow Thistle
Sonchus oleraceus
Photographed in Phoenix, Maricopa Co., Arizona. May 2, 2004
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ANNUAL: Herbaceous annual that may grow low or taller depending on
conditions. Many leaves are basal (at ground level), with progressively smaller leaves higher on
the stems. When broken, stems exude milky sap. Established plants will have a long tap root.
RANGE: A cosmopolitan weed native to Eurasia and now found throughout
North America.
LEAVES: Margins of the leaves have lobes that are pointed but are not as sharp as those of Spiny Sow Thistle (Sonchus asper). The base of the leaf has lobes that nearly encircle the stem, i.e. clasping.
FLOWERS: Small, yellow, dandelion-like flower heads composed entirely of
yellow strap-shaped florets. Flowering mostly in spring.
FRUIT: The achene seeds bear a
pappus of many fine capilary bristles at the top and do not have a
beak at the top.
ARMED: The prickles are sometimes sharp.
Sow Thistle is a very common weed growing in urban areas of the Sonoran Desert in Arizona. In Phoenix, for example, it grows from cracks in pavement, gardens, and vacant lots. Plants germinate from seed in early spring and are blooming by mid March. The plants are quite tolerant of heat and survive through part of the summer. The yellow inflorescence is open briefly for one day and only at dawn. A dandelion-like pappus at the top of each small seed allows them to be readily distributed by the wind. The leaf shape is very similar to the dandelion's as well, but Sow Thistle grows taller and branches freely whereas the dandelion has the leaves all basal and the flowering stalk is unbranched. True dandelions are rare in the hot southwest deserts.