Armed with spiked leaves, stems, and flower heads, common teasel (Dipsacus fullonum, Dipsacaceae) looks rather formidable. I came across some of these at Side Cut Metropark near Maumee, Ohio last week, and they left an impression. Introduced from Eurasia, this plant is now widespread and is frequently found in disturbed places (Brandenburg 2010).
Common teasel bears a superficial resemblance to thistles (Cirsium spp., Asteraceae) especially when flowering. Thistles, however, have an alternate leaf arrangement while teasels have opposite leaves. Another introduced teasel species, cut-leaf teasel (Dipsacus laciniatus, Dipsacaceae), can be distinguished by its deeply-lobed leaves (Brandenburg 2010).
In addition to its aggressive appearance, there is evidence that common teasel is partially carnivorous. Water often collects near the bases of the leaves, and when dead insects fall into the water the plants grow more seeds (Shaw and Shackleton 2011). This suggests that teasels obtain some nutrients from the decaying insects.
Brandenburg, D.M. 2010. National Wildlife Federation Field Guide to Wildflowers of North America. Andrew Stewart Publishing, Inc., New York, NY.
Shaw, P.J.A. and K. Shackleton. 2011. Carnivory in the Teasel Dipsacus fullonum — The Effect of Experimental Feeding on Growth and Seed Set. PLoS One 6(3): e17935.