The Collaborative International Dictionary of English v.0.48:

Leaf \Leaf\, Leaf out \Leaf out\(l[=e]f), v. i. [imp. & p. p.
   Leafed (l[=e]ft); p. pr. & vb. n. Leafing.]
   To shoot out leaves; to produce leaves; to leave; as, the
   trees leaf in May. --Sir T. Browne.
   [1913 Webster]

WordNet (r) 3.0 (2006):

leafing
    n 1: (botany) the process of forming leaves [syn: foliation,
         leafing]