The Collaborative International Dictionary of English v.0.48:

Fruitful \Fruit"ful\, a.
   Full of fruit; producing fruit abundantly; bearing results;
   prolific; fertile; liberal; bountiful; as, a fruitful tree,
   or season, or soil; a fruitful wife. -- Fruit"ful*ly, adv.
   -- Fruit"ful*ness, n.
   [1913 Webster]

         Be fruitful and multiply and replenish the earth.
                                                  --Gen. i. 28.
   [1913 Webster]

         [Nature] By disburdening grows
         More fruitful.                           --Milton.
   [1913 Webster]

         The great fruitfulness of the poet's fancy. --Addison.

   Syn: Fertile; prolific; productive; fecund; plentiful; rich;
        abundant; plenteous. See Fertile.
        [1913 Webster]

WordNet (r) 3.0 (2006):

fruitfulness
    n 1: the quality of something that causes or assists healthy
         growth [syn: fruitfulness, fecundity] [ant: aridity,
         barrenness, fruitlessness]
    2: the intellectual productivity of a creative imagination [syn:
       fecundity, fruitfulness]