The speaker talks about cultivating anger as if it were a plant. So, naturally he must "water" it. Only, he doesn't water it with water, but rather with his tears. Sadness, anger, and other negative feelings become the life-giving liquid that causes the anger to grow. It is arguable that, because the speaker waters his anger with negative things like tears and fears, it eventually bears a poisonous apple.
Line 5: The speaker waters his anger with "fears." He doesn't literally water it because it is not a plant, so "watered" is here a metaphor for the cultivation of anger, for adding fuel to the fire as we might say
Line 6: The speaker also waters his anger with "tears." While you could probably water your plants with your tears (they do have water in them, after all!), it seems more likely that the speaker is again speaking metaphorically about the cultivation of anger. It seems that pain and sadness is an important element in the cultivation of anger. Do you see them related in this way, too?