So, let’s say I’m an evil dragon (rawr) who hates plants and wants all plants to die (messy break-up with Lalai-Puy; it’s a whole thing).
If I made a 10th level spell that powered further defiling with defiling (i.e. the power drawn by defiling just goes to power more defiling, not to power a spell), how big do you think I could get my swath of death?
Hmm, probably pretty huge. A sparse vegetation spell, the reversed form of prolific vegetation (Dragon Kings p. 107-108), can kill up to a 25 mile radius of vegetation if cast by a 30th level dragon, though it doesn’t fully defile the land and takes time. So maybe a little smaller than that to get the full defiling effect… 10 mile radius or so for a 30th level dragon?