Flora of North America species comparison

Varieties 2 (2 in the flora).

At the beginning of the twentieth century, E. L. Greene described over a dozen species belonging to the Geum triflorum complex based on differences in leaf form and indument, the relative length and shape of the epicalyx bractlets and sepals, and petal length and shape. Most of these species were quickly reduced to synonymy by other botanists. When specimens are examined from across the continent, most of the characters used to separate species in the G. triflorum complex show nearly continuous variation. It seems best to treat these variants as belonging to one species. Whether and how to classify the variation within the species will remain controversial. Some character expressions correlate reasonably well with dividing the species into two varieties, as was first proposed by N. C. Fassett (1928). Variety triflorum occurs east of the Rocky Mountains and is typical of the grasslands covering the northern Great Plains; it also is the variety found in the mountains of Arizona and New Mexico. Variety ciliatum is common throughout the rest of the Rocky Mountains, the Sierra Nevada, and Cascade Range. Intermediate specimens occur here and there, particularly in Alberta and British Columbia.

(Discussion copyrighted by Flora of North America; reprinted with permission.)

Specimens from Chicago, Illinois, and Cambridge, Massachusetts, indicate that Geum urbanum has been present in North America for at least 100 years; herbarium specimens are scarce. The species seems to be spreading in ruderal habitats of the northeastern United States and adjacent Canada. The plants have the habit and foliage of G. canadense and G. virginianum, with stipules as large as or even larger than those of the latter; the petals are bright yellow versus white or cream, and the distal segments of the styles are glabrous or nearly so versus partially pilose.

Geum urbanum is often found growing near G. canadense, and the two occasionally form hybrids; G. ×catlingii J.-P. Bernard & R. Gauthier is known from New Brunswick, Ontario, Quebec, and Michigan. Less common is G. ×macneillii J.-P. Bernard & R. Gauthier [G. laciniatum × G. urbanum], known from Quebec. Both hybrids closely resemble G. urbanum in vegetative form, with little hint of their other parent; it is likely that hybrid plants often go undetected. Most herbarium specimens of these hybrids were collected along with their parental species at the same locality. The hybrids are often robust, apparently showing hybrid vigor. Unlike G. urbanum, they have cream petals and heads with mostly sterile ovaries.

(Discussion copyrighted by Flora of North America; reprinted with permission.)