The currently correct basic citation for Flora of North America as a whole is:

Flora of North America Editorial Committee, eds.  1993+.  Flora of North America North of Mexico.  12+ vols.  New York and Oxford.

The currently correct expanded citation for Flora of North America as a whole is:

Flora of North America Editorial Committee, eds.  1993+.  Flora of North America North of Mexico.  12+ vols.  New York and Oxford.  Vol. 1, 1993; vol. 2, 1993; vol. 3, 1997; vol. 4, 2003; vol. 5, 2005; vol. 19, 2006; vol. 20, 2006; vol. 21, 2006; vol. 22, 2000; vol. 23, 2002; vol. 25, 2003; vol. 26, 2002.

To cite a particular part of one of the published volumes, use the following model:

Boufford, D. E.  1997.  Fumaria.  In: Flora of North America Editorial Committee, eds.  1993+.  Flora of North America North of Mexico.  12+  vols.  New York and Oxford.  Vol. 3, pp. 356-357.

To cite a particular part of a volume not yet published, use the following model:

Strother, J. L.  In prep.  Muntingiaceae.  For: Flora of North America Editorial Committee, eds.  1993+.  Flora of North America North of Mexico.  12+ vols.  New York and Oxford.  Vol. 6.