The poem is called "A Poison Tree," so, naturally, vegetation plays an important role. Indeed, the speaker's anger grows until it eventually produces a poisonous apple that, presumably, kills his "foe." Plants, trees, and especially the processes necessary to make them grow (water, sun, care) are our speaker's primary metaphors for how anger develops from a feeling into a destructive action.
Lines 4: The speaker says his wrath grows because he didn't say anything about it. Of course, anger doesn't literally grow, so growth is here a metaphor for the process by which one's anger becomes greater and greater.
Lines 5-6: The speaker waters his anger (with tears and fears), but he doesn't literally water it because it's not a real thing. Watering is here a metaphor for the development and increasing power of one's anger.
Lines 7-8: The speaker gives his anger sunshine, only he doesn't literally give it sunshine. "Sunn[ing]" is here again a metaphor for the things we do to make our anger grow. Does it sound like a pit of snakes in here? Ah. That repetition of the S sound in the beginning of "sunned" and "smiles" is called alliteration.
Line 9: The speaker's anger continues to grow. Anger doesn't literally grow, so growth is here a metaphor for the process by which one's anger becomes greater and greater.
Line 10: The speaker's anger bears an "apple bright." Maybe in Blake's head anger can bear fruit, but we know that the apple is just a symbol of, or metaphor for, the end result of one's potent rage (in this case, a poisonous fruit that will kill a man). The repetition of the "b" in "bore" and "bright" gives us another example of alliteration.
Line 13: The speaker tells us that his foe "stole" into his garden. The garden here seems like a metaphor for the place in our mind where we let our anger fester, grow, develop, and become, essentially, destructive.
Line 16: At last, the speaker's anger has matured in plant terms into a tree. The damage that it causes is also fully developed. The "foe" lies dead beneath the tree, which comes to stand for the giant, destructive thing that unchecked anger can become.