Of a plant, to sprout and open leaves. Something must be wrong with the tree, because it's nearly May and it hasn't started leafing out yet. Your plant may not leaf out if there is too much direct sunlight on it throughout the day.
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.
[for a plant] to open its leaf buds. Most of the bushes leaf out in mid-April. The trees leafed out early this year.
McGraw-Hill Dictionary of American Idioms and Phrasal Verbs. © 2002 by The McGraw-Hill Companies, Inc.