The halachic definition of tree vs. vegetables vis-à-vis orlah includes various criteria. An overview of the Talmudic criteria as well as those added by Rishonim and Acharonim. Also, a look at those plants don't fit neatly into a specific category.
Does the orlah count restart when transplanting trees? For grafted trees, does it begin from the time of the graft or from when the rootstock was planted? On partially uprooted trees, rootsuckers, layered branches, and more.
Which fruits could be orlah in the marketplace? What happens when orlah fruit get mixed in with permitted fruit? On buying fruit in the marketplace and ratios of permitted to forbidden fruit required to nullify orlah.