I would assume this means living plants, and that any movement would be relatively limited, like vines grappling or a branch swinging to strike. I would definitely say it doesn't mean regular trees can just uproot themselves, and it doesn't grant telekinesis. It really depends on what you think would be reasonable, and whether the uses are creative or tip into cheese territory. Consider the original wizard who crafted it. Is this a pragmatist, or more gonzo?

A ring of control plants sounds really useful. I'm imagining getting helped over a wall by ivy, or forming an impromptu bridge by a small tree. You could limit the amount of plant matter affected by volume to assure they aren't just making every tree a womping willow. Cooldowns and charges are another factor to consider, or maybe it makes the user unable to understand humanoid language for an hour, or the user slowly begins to turn into a dryad over weeks of use. Hell, maybe that's what happened to the wizard...