| Amblyolepis setigera DC., 1836 |
| Amblyopappus Hook. & Arn., 1841 |
| Amblyopappus pusillus Hook. & Arn., 1841 |
| Ambrosia L., 1753 |
| Ambrosia acanthicarpa Hook., 1834 |
| Ambrosia acuminata (Brandegee) W.W. Payne, 1964 |
| Ambrosia ambrosioides (Cav.) W.W. Payne, 1964 |
| Ambrosia artemisiifolia L., 1753 |
| Ambrosia artemisiifolia var. artemisiifolia L., 1753 |
| Ambrosia artemisiifolia var. diversifolia Piper, 1906 |
| Ambrosia artemisiifolia var. elatior (L.) Descourt., 1821 |
| Ambrosia artemisiifolia var. paniculata (Michx.) Blank., 1907 |
| Ambrosia bryantii (Curran) W.W. Payne, 1962 |
| Ambrosia camphorata (Greene) W.W. Payne, 1964 |
| Ambrosia canescens A. Gray, 1882 |
| Ambrosia carduacea (Greene) W.W. Payne, 1964 |
| Ambrosia chamissonis (Less.) Greene, 1894 |
| Ambrosia cheiranthifolia A. Gray, 1859 |
| Ambrosia chenopodiifolia (Benth.) W.W. Payne, 1964 |
| Ambrosia confertiflora DC., 1836 |