in Thoreau’s Journal:

How rapidly the young twigs shoot — the herbs, 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 other season. Many do most of their growing for the year in a week or two at this season. They shoot — they spring — and the rest of the year they harden and mature, and perhaps have a second spring in the latter part of summer or in the fall.