When trees speak

I have a story where one of my characters is a tree. The tree talks to a man a lot but only in the man's head. This leaves room for doubt as to whether this is actually happening or the man is crazy.

I haven't put the tree's words in dialogue marks, because it's not actually speaking them out loud. Also, to this point, I've had a few people crit the story and nobody has commented about this, probably because it is pretty clear who is talking at any point.

But now, as I am getting ready to end the story out, I'm having a crisis of confidence. Should I just leave it the way it is? I know I could italicise what the tree says, but then there's the risk of email destroying the formatting.

Is there any clear cut rule about this kind of thing? Any advice?

OK, consensus is always good. Crisis of confidence over, I hope.