in Thoreau’s Journal:  

Within the last week or so the grass & leaves have grown many shades darker––& if we had leaped from last wednesday to this we should have been startled by the change—the dark bluish green of the rank grass especially.  How rapidly the young twigs shoot—the plants trees—shrubs—no sooner leaf out than they shoot forward surprisingly as if they had acquired a head by being repressed so long.

They do not grow nearly so rapidly at any season. Many do most of the growing for the year in a week or two at this season. They shoot—they spring & the rest of the year they harden & mature.  & perhaps have a 2nd spring in the later part of the summer or in the fall.