Hmm.
Mechanically, viewing this from the point of view of a plant creature being either the caster or a meaningful combatant, it surpasses fire shield (a sorcerer/wizard 4 spell) in many ways--it lasts ten times as long and affects a large area rather than just the caster. In trade, it no longer provides protection from heat/fire or cold, which is relatively minor--resist energy and protection from energy work better for that purpose.
Thematically, it'd a tad odd that growing spiky foliage and brush results in no effect other than retaliatory damage. I'd expect it to produce difficult terrain. Ostensibly, this spell also provides plants in a wide area for entangle and its many friends to exploit even in areas devoid of a single blade of grass, something usually difficult to accomplish (wall of thorns can do it, as can an expensive evergreen seed pouch).
Is the important part of this spell the theme of protecting a natural area or the mechanic of providing defensive damage to creatures? I'd work the spell more around whichever is most important. If it's the theme, there are many possible ways to provide defense to lots of creatures. If it's the mechanic, bump the level up some or dial back the duration and/or area.
It's easy to look at the "only works on plant creatures" clause as a restriction that should make the spell lower level than otherwise, but it isn't really--in the hands of a leshy, fey, or plant-summoning/plant companion character, it's a considerable bonus and should be more difficult to cast.