Not very ambiguous at all. Planter as a person is fairly outmoded in English now as it was principally used in colonial times. We don’t map people.

We don’t map movable machinery either.

Pretty much the common term which people use for these things in spoken English is planter. A plant container is not ambiguous either it might be anything from a flower pot to a standard industrial intermodal container (https://en.wikipedia.org/wiki/Intermodal_container) containing machinery (“plant”) (which come to think of it I can think of several locations where such things exist, although I hasten to add I wouldn’t use such tags).

Probably more importantly planter already has more widespread usage on OSM (as both barrier=planter, man_made=planter & planter_box).