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Weeds and Plant Identification
Perennial sowthistle - Sonchus arvensis

Flower

Milky sap

Upper leaf

Lower leaf

Plant
 
 

 
Perennial sowthistle Sonchus arvensis L.
Asteraceae (Aster family)
 
Life cycle
Erect, patch-forming perennial.
 
Leaves
Seedling leaves initially develop from a basal rosette from a deep taproot. Lower leaves are alternate, blue-green, 4 to 12 inches long with two to five lobes with prickly teeth on each side, gradually becoming smaller towards the top with a clasping base. Leaves have a thickened midvein and exude a milky sap when damaged. Leaf midvein is not prickly underneath.
 
Stems
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 single-seeded, narrow, oval, reddish brown, ribbed, wind-disseminated fruit.
 
Reproduction
Seeds and extensive creeping roots.
 
Similar weeds
Annual sowthistle (S. oleraceus L.)
Differs by having an annual life cycle and short taproot, less prickly leaf margins, and pale yellow, 0.5- 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 0.5 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 0.5 to 1 inch wide. Leaf midvein is not prickly underneath.

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