The speaker's perserveration over the perceived slight is what serves as the water and sunlight for the anger plant.
And I watered it in fears
Night and morning with my tears,
And I sunned it with smiles
And with soft deceitful wiles.
"Fears" in this case are his fear of confronting the foe; for the speaker, it was easier to confront his friend and thereby end their feud. He seems too cowardly to confront an enemy, however, and instead assumes a disengenuous facade ("smiles" and "deceitful wiles"). He never shows his enemy that he is angry or afraid; instead, he maintains a "sun[ny]" disposition and pretends to be amicable toward the foe.