An Illustrated Guide to ARIZONA WEEDS

The University of Arizona Press




MILK THISTLE-Silybum marianum (L.) Gaertn.

DESCRIPTION-Stout spiny annuals or biennials, 2 to 5 feet high, from a thick taproot, reproducing by seed. The leaves are large, the lower deeply-lobed, 1 to 2 feet long and stalked; the upper progressively shorter, fewer-lobed and stalkless, clasping the stem at the base with a pair of earlike lobes. The leaf margins are edged with many yellow spines, 1/8 to 1/2 inch long. The main veins on the upper leaf surfaces are more or less outlines with white, often there are also scattered, white-blotched patches. The thistlelike purple flower heads are often nodding, globeshaped, 1 to 2 inches broad, and solitary at the end of long stalks. There are no outer petallike ray flowers. The heads are composed of many red-purple central tubular flowers with 5 long narrow lobes. The leathery bracts surrounding each flower head are in several overlapping series, spine-margined, and tipped by stiff spines 1/2 to 1 1/2 inches long. The achenes are smooth, shiny, mottled with buff and dark brown, and about l/4 inch long. They bear a tuft of white scaly bristles at the tip, which are deciduous in a ring.

DISTRIBUTION-Milk thistle is a native of the Mediterranean region. It is occasional in most areas in southern Arizona, but common in the Salt River Valley. It grows along roadsides, irrigation ditches, pastures, and waste places in Maricopa, Gila, Pinal, Pima, and Cochise counties; mostly from 1,000 to 3,500 feet elevation; flowering March to August, but most abundantly from April to June.

POISONOUS PROPERTIES-Although milk thistle may contain nitrates at a potentially toxic level when growing on fertile soil, it is not common enough in Arizona to cause poisoning. Heavy losses of cattle and sheep from this weed have occurred in Australia, and some in California.

Copyright (c) 1972 The Arizona Board of Regents

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