Some floras split up the genus into either more or fewer species. A "middle of the road" approach has been used in this key. To identify the varieties of Xanthium strumarium in the Full Key, you must look carefully at the tiny details of the "burs" on involucres of fruiting specimens; flowering plants or ones with immature fruits will give poor results. Reference: Fernald (1946).
This Genus’s Species in New England:
Visit this genus in the Dichotomous Key