The plant grew both from the speaker's inner feelings and from his outward behavior. What caused the inner feelings and outward behavior?

In both cases we don't really know what caused the speaker to be angry. He simply tells us that he was angry with his friend, and then later on, his foe. The difference lies in how the speaker then goes on to deal with his anger. In relation to his friend, he openly expresses his feelings. Doing so makes his anger go away. But with his foe, however, he remains completely silent, bottling up his emotions inside, which only makes his anger grow. The anger that the speaker harbored towards his friend quickly subsided, but the anger that he feels towards his enemy has grown like a poisoned apple tree.
The biblical reference here to Adam and Eve and the Tree of Knowledge is inescapable. The foe has tasted anger's bitter fruit, and it has destroyed him. Although the speaker expresses satisfaction at the death of his bitter enemy, the abrupt ending serves to emphasize the sheer destructiveness of anger and hate. The poem has gone full circle, and our attention is drawn once more to the opening lines:

"I was angry with my friend;
I told my wrath, my wrath did end.
I was angry with my foe:
I told it not, my wrath did grow."

These words provide us with a salutary moral lesson. Communication is the key to dispelling the anger that often eats us away inside and can all too easily lead to disaster. But this will only happen if we want it to, if we choose to talk things through no matter how angry we might be. The question that the poem forces us to ask ourselves is: are we really prepared to do this?
https://www.poetryfoundation.org/poems/45952/a-poison-tree