I've talked with many artists about the language of flowers, but it never occurred to me to look for a dictionary. Until now. And, thanks to Temple Univeristy Library, here are a few...
de la Tour, Charlotte. Le langage des fleur. Paris: Garnier Freres, 1858. (French, illustrated)
Dumont, Henrietta. The Language of Flowers. The floral offering: a token of affection and esteem comprising the language and poetry of flowers. With coloured illustrations from original drawings. Philadelphia: H.C. Peck & Theo Bliss, 1852.
Ingram, John H. Flora Symbolica; or The language and sentiment of flowers. London: F.W. Arne and co., 1869. (link)
Wirt, Elizabeth Washington Gamble. Flora's Dictionary By A Lady. Baltimore: Fielding Lucas, Jr, 1832.