Good point. Um...
If the plant has a normal life span less than five years, the plant's children are also affected by this power.
That way you can create a grove of strawberries, but you can't go insane with maple trees.
It's the same language as for druidic wildshape. So, in practice, the bar is very low :-)
What templates leave the creature as an animal?
Constructs are nonnatural, but not really unnatural. I probably need to define this better. I'll have to think about it.
You may be right. What would be a reasonable xp cost? Miracle is 5000. I'm not sure if you could duplicate this effect with a very powerful miracle. It's certainly a lot less flexible, though.
Usually I've found players very reluctant to sacrifice xp, but it might be the best fitting game mechanic.