The Collaborative International Dictionary of English v.0.48:

Garden \Gar"den\, v. i. [imp. & p. p. Gardened; p. pr. & vb.
   n. Gardening.]
   To lay out or cultivate a garden; to labor in a garden; to
   practice horticulture.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Gardening \Gar"den*ing\, n.
   The art of occupation of laying out and cultivating gardens;
   horticulture.
   [1913 Webster]

WordNet (r) 3.0 (2006):

gardening
    n 1: the cultivation of plants [syn: gardening,
         horticulture]

Moby Thesaurus II by Grady Ward, 1.0:

25 Moby Thesaurus words for "gardening":
   bloom, blossom, blow, citriculture, floret, floriculture, floscule,
   flower, flower gardening, flower-growing, floweret, fruit-growing,
   groundskeeping, horticulture, hortorium, landscape architecture,
   landscape gardening, market gardening, olericulture, orcharding,
   pomiculture, posy, truck gardening, viniculture, wildflower