| Ambrosia peruviana var. cumanensis (Kunth) O.E. Schulz, 1911 |
| Ambrosia psilostachya DC., 1836 |
| Ambrosia psilostachya var. psilostachya DC., 1836 |
| Ambrosia pumila (Nutt.) A. Gray, 1882 |
| Ambrosia salsola (Torr. & A. Gray) Strother & B.G. Baldwin, 2002 |
| Ambrosia salsola var. pentalepis (Rydb.) Strother & B.G. Baldwin, 2002 |
| Ambrosia trifida L., 1753 |
| Ambrosia trifida var. aptera (DC.) Kuntze, 1891 |
| Ambrosia x platyspina (Seaman) Strother & B.G. Baldwin, 2002 |
| Ambrosiinae Less., 1830 |
| Amellus L., 1759 |
| Amellus villosus Pursh, 1814 |
| Amolinia R.M. King & H. Rob., 1972 |
| Amolinia heydeana (B.L. Rob.) R.M. King & H. Rob., 1972 |
| Amphiachyris Nutt., 1840 |
| Amphiachyris dracunculoides (DC.) Nutt., 1840 |
| Amphidoxa DC., 1838 |
| Anacyclus L., 1753 |
| Anacyclus australis Sieber ex Spreng., 1826 |
| Anaitis DC., 1836 |