What +Larry Spiel
said: Make it a constant choice.
As per 2E original rules defiling only harmed plants unless you were a dragon. And that may seem trivial when you're in battle, but considering that EVERYONE hates defilers and defiling wrecks the local ecology (and possibly even food supply), it's not a thing you want to turn to lightly.
So unless you want to brew up some, "I'm a special kind of jerk," feats to make defiling be animal-harmy, I'd go with just plants.
As for how defiling helps you, choose one minor or major effect:
* +5 to attack roll.
* +5 to damage roll.
* Double range.
* Double area.
* Double duration.
* Cast any at encounter spell already cast during this encounter.
* Any one minor effect doubled in effectiveness.
* Any two minor effects used on the same spell.
* Cast any daily spell already cast during the same day.
Minor effects can be cast covertly. They still skill plants, but the damage is more diffuse and it takes a keen eye to spot them: Skill check of: Arcana, Nature or Perception vs. DC 20 - spell level.
Major effects draw a bulls-eye of ash centered on the caster with a radius in yards equal to the spell level. They can not be done covertly.