Prickly Sowthistle
Sonchus asper
Aster family (Asteraceae)

Description: This annual plant is about 1–5' tall, branching sparingly in the upper half. The stems are dull green or reddish green, round, and smooth. They have rather conspicuous longitudinal veins and are usually hairless, although occasionally the upper stems and flowering stalks have a few hairs. The alternate leaves are up to 10" long and 3" across, but more commonly they are about half this size or less. On shorter plants, they are rather crowded together on the stems, even where the composite flowers occur. Depending on the local form of the plant, these leaves may be pinnatifid, or they may lack significant lobes along the margins, in which case they are broadly lanceolate or oblanceolate. The margins are conspicuously prickly, while the base of each leaf is auriculate with a pair of large rounded basal lobes that strongly clasp the stem. 

The hairless leaves are glabrous, and they tend to be folded upward along the central vein. However, there are no prickles along the central vein on the underside of each leaf. Both the stems and the leaves contain a milky latex. The upper stems terminate in clusters of 1-5 composite flowers on rather short stalks. Each flower is about 2/3" across when fully open, and consists of numerous yellow ray florets. The base of each flower is covered with dull green bracts and is rather short – only about 1/3" in length. The blooming period can occur from late spring to early fall, and usually lasts about a month for a colony of plants. Each floret is replaced by an achene with a tuft of silky white hairs. The achenes are flat, spindle-shaped, hairless, and have several longitudinal ribs. They are distributed by the wind. The root system consists of a stout taproot. This plant spreads by reseeding itself.

Cultivation: Prickly Sowthistle typically grows in full sun, moist to slightly dry conditions, and different kinds of soil, including loam, clay-loam, and shallow gravelly soil. The size of this plant is highly variable, depending on the moisture and fertility of the soil. It can bolt upward and form flowerheads very quickly during the summer.

Range & Habitat: Prickly Sowthistle is occasional to locally common in Illinois. Apparently, it is uncommon or absent from many areas of NW, central, and southern Illinois, although official records probably underestimate its distribution (see Distribution Map). Habitats include irregularly mowed lawns, edges of yards and driveways, gardens, areas along roads and railroads, vacant lots, barnyards, and waste areas. This species prefers highly disturbed areas, and does not present an invasive threat to natural areas to any significant degree. It is native to Eurasia and North Africa.

Faunal Associations: There is scant information about floral-fauna relationships for this species. The composite flowers probably attract various bees and Syrphid flies. Several aphids suck plant juices from Sonchus spp. (Sowthistles): Capitophorus flaveola, Hyperomyzus lactucae, Pemphigus bursarius (Lettuce Root Aphid), Uroleucon ambrosiae (Brown Ambrosia Aphid), and Uroleucon pseudambrosiae (False Ambrosia Aphid). Among vertebrate animals, the seeds of Sowthistles are a minor source of food to the Eastern Goldfinch. The foliage has a bitter taste because of its milky latex, which may discourage its use as a food source by mammalian herbivores to some extent.

Photographic Location: Along a gravel driveway in Urbana, Illinois. The Prickly Sowthistle in the photographs is the unlobed form of the species.

Comments: Prickly Sow Thistle is somewhat variable in appearance because its leaves may be lobed or unlobed along the margins. There are 3 Sonchus spp. (Sowthistles) in Illinois and they are fairly similar in appearance. One of them, Sonchus arvensis (Perennial Sowthistle), is a perennial plant with spreading rhizomes and composite flowers that are 1" across or more. Prickly Sowthistle and Sonchus oleraceus (Common Sowthistle) have smaller composite flowers that are " across or less. These latter two species can be distinguished from each other by the shape of the basal lobes of the leaves: the basal lobes of Prickly Sowthistle are well-rounded, while the basal lobes of Common Sowthistle are acutely angular.