Sonchus arvensis L. Asteraceae (Aster family)
Erect, patchforming perennial.
Perennial sowthistle plant.
Seedling leaves initially develop from a basal rosette from a deep taproot. Lower leaves are alternate, bluegreen, 4 to 12 inches long with two to five lobes with prickly teeth on each side, gradually becoming smaller toward the top with a clasping base. Leaves have a thickened midvein and exude a milky sap when damaged. Leaf midvein is not prickly underneath.
Erect, herbaceous stem up to 5 feet tall that branches only near the flowers. Stems are hollow and smooth with ridges and exude a milky sap when damaged. Upper stems may have gland-tipped hairs.
Flowers and fruit
Bright yellow ray flowers are 1 to 2 inches wide and found in clustered heads. The seed is enclosed in a singleseeded, narrow, oval, reddish brown, ribbed, wind-disseminated fruit.
Seeds and extensive creeping roots.
- Annual sowthistle (S. oleraceus L.) Differs by having an annual life cycle and a short taproot, less prickly leaf margins, and pale yellow, .5- to 1-inch-wide flower heads. Leaf midvein is not prickly underneath.
- Prickly lettuce (Lactuca serriola L.) Differs by having an annual or biennial life cycle; fine prickles on leaf margins and sharp prickles on the underside of the leaf midvein; and a many branched, spreading panicle with numerous pale yellow flower heads less than .5 inch wide.
- Spiny sowthistle (S. asper [L.] Hill) Differs by having an annual life cycle and short taproot, leaf margins with many sharp prickles, earlike lobes where the leaf attaches to the stem, and flower heads .5 to 1 inch wide. Leaf midvein is not prickly underneath.
Sharp prickles on the underside of the leaf midvein of prickly lettuce (left). Prickly lettuce rosette (middle). Spiny sowthistle leaf (right).
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