If a plant is botanically a perennial and identical to a fruit tree, but it grows within a few inches of the ground, many authorities say its fruit warrants a borei pri ha'adama, as if it were a vegetable. Others ignore the height issue, and say the difference between ha'etz and ha'adama is botanical only.
According to the former opinion, could you say Birkat Ha'Ilanot on, say, a wild blueberry bush that's only 20 inches tall? Is it a "fruit tree" for this purpose?
We find that a caper bush is halachically considered a tree, and one says ha'eitz on its berries (Shulchan Aruch, Orach Chaim 202:6, from Berachos 36a). And for purposes of the berachos on spices, the definition of "tree" is even more elastic: plants such as rosemary and jasmine get the berachah borei atzei besamim, even though their stems are only semi-woody (ibid. 216:7; Seder Birchos Hanehenin of R' Shneur Zalman of Liadi, 11:2).
So it would seem likely, though I don't have any solid proof for this, that fruit-bearing bushes ought indeed to be considered trees for purposes of birkas ha'ilanos.
As the Bracha is called Birhas Ha'Ilonos I would say it can only be said on a tree, not on a shrub.
