how can I make it so that my decal dont blend additively?

i’m making a game where you’re a wizard and one of the element you control is fire. I’ve made a grass burning decal and niagara system. The thing is, when the grass burn, I don’t want it to be possible for it to burn again, so I would need every new decal I create to be ‘‘under’’ the ones already in the world if that makes sense. Any idea how I could go about doing this?