An Illustrated Guide to ARIZONA WEEDS
The University of Arizona Press
SPINY SOWTHISTLE-Sonchus asper (L.) Hill
DESCRIPTION-A tall, erect fleshy annual or sometimes biennial, with milky juice from a stout taproot; reproducing only by seeds. The large hollow stem is often reddish, unbranched, or few branched, I to 4 or more feet high. The upper stems and flower stalks are sparingly to densely covered with stalked glands the more common growth form. However, glands may be entirely lacking in some plants.
The alternate leaves are numerous, and usually deeply lobed, with the margins soft prickly-toothed. The lower leaves, up to 12 inches long, are definitely stalked. These and the principal stem leaves are divided into 5 to 11 lobes along each side, with the tip-lobe not longer, nor broadly triangular. Only the lowest leaves are stalked. The middle and upper leaves are stalkless, and clasp the stem with a pair of large rounded earlike lobes. The leaves become progressively smaller and fewerlobed above. The margins of the upper leaves are irregularly cut, jagged, and long toothed, but usually not divided into definite lobes. However, in some growth forms, even the lower and middle leaves are scarcely lobed.
The flower heads are numerous, small, 1/2 to 1 inch across, and composed wholly of pale yellow petallike ray flowers, and no disk flowers. The bracts surrounding the flower head and the flower stalks may bear stalked glands (gland-tipped hairs). The achenes are reddish-brown, flattened, and margined with a narrow wing; the edges are thinner than the body. They are 2 to 2 1/2 times as long as broad, with the broadest part through the middle, and about 1/12 to l/8 inch long. There are 3 distinct central ribs (rarely 4 or 5) on each face, with no cross ridging between them, but they may be cross wrinkled near the edges and on the winglike margins. The tuft of fine hairs drops away quickly.
DISTRIBUTION-Spiny sowthistle is a naturalized European weed, widespread throughout the state, but more abundant and serious in the southern part. A pest in grainfields, alfalfa, winter vegetable crops, orchards, lawns, ditchbanks, and waste places. It flowers nearly throughout the year in some places, but principally from November to May; 150 to 8,000 feet elevation.
Copyright (c) 1972 The Arizona Board of Regents