Flora of North America species comparison

Plants with narrow leaves, generally in the southwest part of the range of Ageratina altissima, have been recognized as var. angustata and were so mapped by A. F. Clewell and J. W. Wooten (1971), who indicated that all var. angustata occurs west of the Mississippi River and that this taxon was completely congruent in distribution with var. altissima. The present treatment confirms the westward tendency toward size reduction and observes that narrow-leaved plants occur widely through the southeast United States (including Arkansas, Kentucky, Louisiana, North Carolina, Oklahoma, Tennessee, and Texas). The transition is gradual and the region of intergradation is wide. In Texas, where the leaves mostly are narrow, plants with broad, cordate leaves are scattered through the range.

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