A druid in my game used the Entangle spell, and it was pretty easy to know how a 20' square rendered on a battle mat with a 5' spacing grid. Now he also knows Spike Growth...
The Spike Growth effect manifests in a circle with a 20' radius, which had us debating what happened to the outer corners of the square (this might be a visual exercise for some). Imagine a 20' square on a grid with 5' spacing. Now plant a circle with a 20' diameter over that square. The 4 5x5' corners of the square are barely touched by the circle so I figure that they aren't affected by the spell. Furthering my thought process is that if it did affect these grid squares, then there would be no mechanical difference between a 20' square and a 20' diameter circle, and there would be no need for wizards to word things inconsistently.
So, using a battle mat how does a DM interpret the "corners" of a circle. Are there mechanics for this that I am missing? Failing that, are there any good interpretations anyone has heard of or used that I could try?