Ah, William Blake! One of the most fascinating Romantic Era poets...he is a master of the English language in more ways than one.
As you know, diction is word choice, and for poets, diction is especially important! Words are carefully chosen.
In " A Poison Tree," the references to a garden are everywhere in words like "grow," "water'd," "sunned," and "grow," among others. Also, because of these references to a garden, we can also compare this poem to a famous bible "story": Adam and Even in the Garden of Eden. Some critics have also pointed out that the poem might be a reference to God being angry with mankind (hence the flood that destroyed the Earth, etc.), as well.
The use of couplets throughout the poem (with perfect end rhymes) is also indicative of careful word choice.