Blake opens with stark moral clarity: expressed anger toward a friend dissolves, but unexpressed anger toward an enemy multiplies. The couplet's simple parallel structure makes the lesson seem obvious, almost childlike. Yet the poem's genius lies in what follows—showing how unexpressed wrath doesn't merely persist but transforms into something cultivated and deadly. Suppression becomes not passive containment but active nurture.

The second and third stanzas detail the poison tree's growth through paradoxical care. The speaker "waters" anger with fears and tears, "suns" it with smiles and deceit. These aren't contradictions but complementary poisons: inner anxiety feeds the root while outer hypocrisy provides false warmth. The wrath becomes a project, tended deliberately until it produces fruit—an "apple bright" that recalls Eden's forbidden fruit. Blake suggests that nursed grievances acquire their own terrible beauty, becoming objects of pride ("he knew that it was mine") that attract both cultivator and victim.

The closing image—the dead foe "outstretched beneath the tree"—completes the allegory with chilling satisfaction. The speaker's gladness reveals how thoroughly anger has corrupted: what began as withheld emotion has become murder via passive-aggressive cultivation. Blake leaves ambiguous whether the apple literally poisoned the foe or whether this entire scenario represents the psychic effect of sustained enmity. Either reading condemns the same sin: choosing to nurture rather than release anger. The poem invites readers to examine their own poison trees—what resentments they water in secret, what grievances they sun with false smiles, and whether they've mistaken this cultivation for strength when it's actually corruption.