Flora of North America species comparison

Gamochaeta pensylvanica is recognized by its obovate-spatulate, loosely tomentose and concolor or weakly bicolor basal and proximal cauline leaves, and similarly shaped spreading bracts among the heads. Occasional plants appear intermediate between G. pensylvanica and G. antillana. The latter differs in its more erect stems, linear to oblanceolate basal and proximal cauline leaves, and more nearly continuous arrays of heads with linear to narrowly oblanceolate bracts.

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

Gamochaeta purpurea apparently is native to North America and adventive elsewhere.

Basal cells of hairs on adaxial faces of leaves are expanded and glassy (versus hairs filiform to bases in most other species) and are diagnostic for Gamochaeta purpurea. From Maryland northward, plants of G. purpurea produce relatively small basal rosettes and relatively shallow fibrous roots or a filiform taproot; southward and southwestward, the basal rosettes often are larger and the fibrous roots are denser.

Gamochaeta purpurea apparently occurs widely through the world as a weed; it is fairly clearly native to eastern North America, where it is the least weedy of the gamochaetas. Plants of G. purpurea in southern Arizona along perennial streams at the base of the Santa Catalina Mountains were first collected in 1903 (G. L. Nesom 2004) and were, perhaps, accidentally established through visitation; the same sites are heavily infested by other, more aggressive, nonnative species. Collections of G. purpurea also have been made at higher elevations in the Santa Catalina, Rincon, and Chiricahua mountains, where the species is less likely to have been introduced by human activity. It also seems unlikely that plants in scattered Mexican localities were introduced there by human activity.

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