This is the second half of the famous poem The School Boy written by Willaim Blake. This poem is included in the volume of Songs of Innocence and of Experience and focuses on how a child's growth is hindered by the so-called education system.
NOTE: In my explanation, I've mistakenly interpreted: if the trees are dry before spring. In fact, it'll be: if the trees dry out in spring, a season when trees get new leaves and form. I regret the unconscious mistake. Thank you!