All three of our species are native in Europe but are now widespread as weeds around the world. Leaf shape is excessively variable in the genus, ranging from unlobed to deeply pinnatifid.
The sow-thistles are not always easy to distinguish, especially using dry specimens. Many collections lack indication of underground parts, the sure feature to recognize the deep-rooted perennial S. arvensis, which one cannot pull easily from the ground. Plants with the biggest heads and longest involucres are this species. The achenes are similar to those of S. oleraceus, with a prominently papillate-tuberculate surface, but they seem often not to develop in S. arvensis. The ligules are paler yellow in S. oleraceus than in the deep (even orange-) yellow heads of S. arvensis. A giant species 2–4 m tall, S. palustris L., is known from adjacent Essex Co. Ontario and may well be found in Michigan.
1. Flowering heads ca. 2.5–4.5 (–5) cm broad; largest mature involucres mostly (13–) 14–18 mm long; deep-rooted perennial, spreading by horizontal roots.
1. Flowering heads less than 2.5 cm broad; largest mature involucres 8–13 mm long; tap-rooted annual (or winter-annual).
2. Leaf bases with conspicuously rounded lobes, often so large and curved around as to suggest a nautilus shape; mature achenes essentially smooth except for 3 (–5) prominent ribs on each face, very strongly flattened.
2. Leaf bases with ± acute lobes; mature achenes densely papillate-tuberculate, slightly flattened.
All species found in Sonchus
MICHIGAN FLORA ONLINE. A. A. Reznicek, E. G. Voss, & B. S. Walters. February 2011. University of Michigan. Web. December 12, 2017. http://www.michiganflora.net/genus.aspx?id=Sonchus.