Eupatorium x cordigerum
Distinguishing Features: this name applies to plants that are hybrids of E. perfoliatum and E. rotundifolium - the leaves are broadly ovate as in E. rotundifolium, and the leaf bases are clasping to narrowly connate/perfoliate; the heads do not consistently have 5 flowers, but may have 6-8 flowers per head.
Habitat: open areas, old fields and waste places.
Geographic Distribution: sporadically occurring in eastern U.S., primarily in areas of overlap of its parents, E. perfoliatum and E. rotundifolium.
Similar Species: Eupatorium rotundifolium consistently has 5 flowers per head, and leaves that do not overlap or fuse at the base; E. perfoliatum has consistently at least 7 (and usually more) flowers per head and leaves (at least at the lower nodes) that are broadly perfoliate.
Molecular Data : samples of E. x cordigerum (DNA #s ) are characterized by indel polymorphisms and bp polymorphisms
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