from SRD:
Spread: Some effects, notably clouds and fogs, spread out from a point of origin, which must be a grid intersection. The effect can extend around corners and into areas that you can’t see. Figure distance by actual distance traveled, taking into account turns the spell effect takes. When determining distance for spread effects, count around walls, not through them. As with movement, do not trace diagonals across corners. You must designate the point of origin for such an effect, but you need not have line of effect (see below) to all portions of the effect.
In terms of AoE it will still stay the about the same (at least 40') as the effect is going to need to touch the ground to effect the plants....unless you want to give a bonus to reflex saves for partial AoE. The Sprite Magister in my group would often cast Sorcerous Blast to end at medium height, just above his tiny size.