| Ageratum purpureum Sessé & Moc., 1889 |
| Ageratum rhytidophyllum B.L. Rob., 1901 |
| Ageratum rugosum J.M. Coult., 1895 |
| Ageratum sessilifolium S. Schauer, 1847 |
| Ageratum solisii B.L. Turner, 2005 |
| Ageratum sordidum S.F. Blake, 1924 |
| Ageratum stachyofolium B.L. Rob., 1901 |
| Ageratum tehuacanum R.M. King & H. Rob., 1990 |
| Ageratum tomentosum (Benth.) Hemsl., 1881 |
| Ageratum wrightii Torr. & A. Gray, 1846 |
| Agiabampoa Rose ex O. Hoffm. & Prantl, 1894 |
| Agiabampoa congesta Rose ex Hoffman, 1894 |
| Agnorhiza (Jeps.) W.A. Weber, 1999 |
| Agnorhiza ovata (Torr. & A. Gray) W.A. Weber, 1999 |
| Agoseris Raf., 1817 |
| Agoseris heterophylla (Nutt.) Greene, 1891 |
| Agoseris retrorsa (Benth.) Greene, 1891 |
| Aiolotheca DC., 1836 |
| Alarconia DC., 1836 |
| Alcina Cav., 1791 |