To my eye, it needs more green. The tree openings against the brick wall are big enough to confuse my eyes,not knowing which to focus on. I'd go for a different value on the other green, too - probably darker. It could even be scrappy.
Take a few green scraps and lay them out in the open tree spaces randomly, and you'll focus on the tree and not the wall. You want to stop the horizontal mortar line from being the center of focus. They are so strong that the tree seems in the way.
I'm with the others on introducing an element to the sky, too. It doesn't have to be big.