Eupatorium serotinum
Distinguishing Features: leaves are petiolate, heads with 7-12 flowers. A common name for E. serotinum is "Late-flowering Thoroughwort", and it usually comes into bloom relatively later than most other species.
Habitat: old fields, waste areas, often forming large populations.
Geographic Distribution : widespread in eastern U.S.
Similar Species: Eupatorium mikanioides also has petiolate leaves, but has only 5 flowers per head; E. serotinum hybridizes with many other species, and hybrids or hybrid derivatives can often be recognized by a combination of somewhat petiolate leaves and heads that have variably 5 or 6 flowers.
Molecular Data : samples of E. serotinum (DNA #s ) differ in the ITS sequence from all other species by a minimum of 10 bp changes and at least one distinctive 1 bp indel in ITS-2
Images
Eupatorium serotinum, habit (left); and closeup of heads with a skipper (right)
