Walnuts ripen in late August to mid September depending on the type and your climate. They will fall from the tree when ripe. The wind could blow some of the nuts to the ground early but you can tell when the outer green husks start to split that the walnut is ready. The husks should be removed from the ripe walnuts as leaving the green husks on the nuts can give the nut meat a bad taste. The walnut shell is inside the outer green husk so that is why you see green walnuts on the tree now. Keep growing, Marianne Binetti