Sonchus arvensis glabrescens
Aster family (Asteraceae)
Description: This adventive perennial plant is 2-4' tall, branching occasionally in the upper half. The dull green stems are hairless. The alternate leaves are up to 12" long and 3½" across, becoming smaller as they ascend the stems. Most of these leaves are located in the lower half of the plant. They are lanceolate, oblanceolate, or ovate in overall shape, but usually have about 2-5 lobes with pointed tips on each side (pinnatifid). Some of the upper leaves may be unlobed, and occasionally there are plants without significant lobes on the leaves. The margins are dentate-prickly. The upper surface of each leaf is glabrous, while the lower mid-rib is without prickles. At the base of each leaf, there is a pair of small basal lobes that clasp the stem. These basal lobes are usually rather small and well-rounded.
The upper stems terminate in flower stalks of composite flowers. These stalks are quite long and have few leaves. These stalks branch sparingly and produce clusters of composite flowers. Each composite flower is about 1–1½" across, consisting of numerous yellow ray florets. The base of the flower is covered with overlapping green bracts – it is about ¾" in length. In variety glabrescens, both the flowering stalks and bracts are hairless. However, the typical variety has flowering stalks and bracts with glandular hairs. The blooming period occurs during the summer and lasts about 1-2 months. There is a pleasant floral scent. Each ray floret is replaced by a rather flat achene that is tapered on both ends and has several longitudinal ribs along its length. The achenes have tufts of fine white hairs and are distributed by the wind. The root system consists of a stout taproot that can run 9 ft. into the ground, and it produces long spreading rhizomes. This plant frequently forms clonal colonies.
Cultivation: The species prefers full sun, mesic conditions, and rich loamy soil. It is an aggressive spreader that can be hard to get rid off because even small pieces of the extensive root system can regenerate new plants. Seeds can remain viable in the ground for several years.
Range & Habitat: The glabrous variety of Perennial Sowthistle is common in northern Illinois, occasional in central Illinois, and rare or absent in southern Illinois (see Distribution Map). It is more common than the typical variety in the state. Both varieties of this species are adventive from Eurasia. Habitats include cropland, weedy meadows, edges of gardens, areas along roads, and miscellaneous waste areas. This species prefers disturbed areas. So far, it has not invaded high quality natural areas to any major extent.
Faunal Associations: According to Müller (1873/1883) in Germany, the nectar and pollen of the flowerheads attract long-tongued bees, short-tongued bees, drone flies (Eristalis spp.), skippers, and beetles. Among the bees, are such floral visitors as bumblebees, honeybees, dagger bees, Halictid bees, cuckoo bees (Nomada spp.), mason bees (Osmia spp.), and leaf-cutting bees (Megachile spp.). Several aphids feed on the plant sap of Perennial Sowthistle and other sowthistles (Sonchus spp.). These species include Hyperomyzus lactucae (Currant-Sowthistle Aphid), Pemphigus bursarius (Poplar-Lettuce Aphid), and Uroleucon sonchi (Large Sowthistle Aphid); see Blackman & Eastop (2013). The larvae of Aulacidea tumida (Lettuce Tumor Gall Wasp) form galls along the stems of these plants, while Melanoplus bivittatus (Two-striped Grasshopper) feeds on the foliage or flowerheads (Felt, 1917; Wyoming Agr. Exp. Station, 1994). Among vertebrate animals, the Franklin Ground Squirrel eats the leaves, buds, and flowerheads of sowthistles; the Ruffed Grouse browses on the leaves; the Eastern Goldfinch eats the seeds; and the White-tailed Deer browses on the foliage and mature flowerheads, spreading the seeds into new locations (Ostroff & Finck, 2003; Bennetts, 1900; Martin et al., 1951/1961; Myers et al., 2004).
Photographic Location: An abandoned vegetable garden in Urbana, Illinois.
Comments: Perennial Sowthistle produces attractive dandelion-like flowers on long stalks. It is the only Sonchus sp. (Sowthistle) within the state that is perennial in habit, and its flowerheads are larger in size (at least 1" across) than the others. The flowerheads of Sonchus asper (Prickly Sowthistle) and Sonchus oleraceus (Annual Sowthistle) are ¾" across or less. The basal lobes on the leaves of Perennial Sowthistle are usually well-rounded and small, while the basal lobes of Prickly Sowthistle are well-rounded, but large, and the basal lobes of Annual Sowthistle have sharp angles. The typical variety of the Perennial Sowthistle is a hairier plant than var. glabrescens. Because the latter is sometimes considered a distinct species, it is occasionally referred to as Sonchus uliginosus.