Growth of Plants: This spell causes normal brush or woods to become thickly overgrown and entangled with creepers, vines, thorns, briars and so on, so as to make the area virtually impassable. It will effect an area of up to 30 square inches, the dimensions decided by the caster of the spell. Duration: until the spell is negated by a Dispell Magic. Range: 12".Plant Growth is related to the Polymorph spells, but as written, it is a magical change of state, rather than an extreme transformation. It causes the plants in an area to become overgrown, but the spell says nothing about making plants abnormally huge (turning a daisy into an umbrella, for example.) That effect is probably reserved for Polymorph Others.
The intent of the spell thus seems to be defensive, which may be why it has a range in keeping with pre-combat spells. We could easily substitute Line of Sight for the range. The spell lists impeding movement as one effect, but natural concealment of objects or sealing of portals would make sense as well. Also, we could include ripening or maturing of plants, to replace damaged crops as a service to a community or to grow fruit to replenish a group's food supply.
I think I'll change the area of effect to the 30 feet per three levels rule I've been using, so the spell now affects a 60-foot diameter circle: a somewhat larger area than described in the original, but not imbalancing.