Sonchus asper is an annual, or winter annual herb, to 1.5 m tall from a taproot, sap milky, plant often waxy pale green.
Leaves initially a basal rosette that may overwinter, alternate, to 20 x 10 cm, tip pointed, base clasping stem with large, spiny, rounded lobes, margin undulate, densely spiny, surface green glossy, reddish in open habitats, lower leaves widest above middle, tapering toward base, upper leaves lobed, with winged stalk.
Flower heads to 2.5 cm wide, yellow, 80-250 per head, flowers all rays, petal tube longer than ray, July-Oct.; inflorescence branched, stem often yellowish to purplish, glandular-hairy. Although flowers are self-compatible, insect visitors include solitary bees and syrphid flies (Hutchinson et al. 1984).
Fruit dry achenes tan, margins winged, flattened, 3-ribs per side, with white plume, wind dispersed. Seeds eaten by some birds. Seeds apparently live only a few years in soil seed bank. Plants germinating in spring form flower buds in nine weeks.
Wetland status: FAC.
Frequency in NYC: Infrequent and ephemeral
Origin: Europe and western Asia.
Habitat: Open areas, vacant lots, fill, roadsides.
Notes: Annual sow thistles are attacked by a number of insects including: the aphids (Aphididae) Aphis fabae, Hyperomyzus lactucae, H. pallidus, and Uroleucon sonchi; The aphid predator Aphidoletes aphidomyza, a fly larva, was originally described from S. asper. Other insects attacking annual sow thistles include the leaf miners (Agromyzidae) Ophiomyia pulicaria, O.coniceps, O. pulicaria, Phytomyza syngenesiae, Phytoliriomyza arctica and Calycomyza sonchi ; and the moth (Tortricidae) Onephasia longana. Nematodes infesting sow thistle include: Meloidogyne spp. and Pratylenchus penetrans. Fungal infections include: Bremia lactuae, Sphaerotheca fuliginea (a powdery mildew) and Alternaria sonchi (Deuteromycotina), probably a leaf-spot. Viruses that attack annual sow-thistles are often diseases of crop plants. These include lettuce necrotic yellows, beet yellow stunt and sow-thistle yellow vein (Hutchinson et al. 1984).