Smooth perennial sow-thistle, Sonchus arvensis L. var. glabrescens Guenth, Grab. & Wimm., [SONAU, laiteron des champs glabre, Creeping sow-thistle, Field sow-thistle, Perennial sow-thistle, Sonchus uliginosus Bieb ] Virtually identical to Perennial sow-thistle in seedling and juvenile stages, being distinguishable only after the flower heads begin to form and open. In Smooth perennial sow thistle the stems and involucral bracts around the flower heads are smooth, completely lacking the dark glandular hairs characteristic of Perennial sow-thistle. Smooth perennial sow-thistle occupies similar habitats to those of its sister variety.

It is common only in extreme southwestern Ontario and in the northern and northwestern parts of the province, although it occurs sporadically elsewhere.

(Source: Ontario Ministry of Agriculture and Food, Publication 505, Ontario Weeds)