In 1838, he began with Asa Gray The Flora of North America, which was issued in numbers irregularly until 1843, when they had completed the Compositae, but new botanical material accumulated at such a rapid rate that it was deemed best to discontinue it.
I 1838 begyndte han i samarbejde med Asa Gray arbejdet på The Flora of North America, der udkom i uregelmæssige antal numre indtil1843, hvor de var færdige med Compositae, men nyt botanisk materiale blev indsamlet i så hastigt et tempo, at man mente det bedst at standse udgivelsen.