Maxperson
Morkus from Orkus
I don't see any particular problem with it. You don't even get to do CR 1 creatures until you are 8th level, so things like Treants are not going to be on the list of things you can change into. I'd also personally require the druid to be higher level, perhaps 10th since turning into a plant changes physiology. Something akin to the Circle of the Moon's elemental wild shape ability. You could even make it level divided by 3 for plant creature CR and still not have to worry about Treants.I'm adding the plant creature type to druids to go along with beast type for wild shapes.
An additional benefit is: If you remain in plant shape for a short rest, you benefit from a half ration of food and water gained from nutrients in the ground. The DM can rule you cannot benefit from this feature if the terrain is unsuitable.
I've looked at the plant creatures and don't see any problems, myself. Any thoughts or issues with this?