When they are low on fertilizer they usually yellow more than turn crispy on the edges. I haven't used miracle grow, I stick with fertilizer tabs, specifically Highland Rim. I can't remember what it is but that white edging makes me think it is something specific, but can't remember.

Check for bugs, often times curl themselves up in a corner or wrap around part of the leaf. There is a white powder you can sprinkle on the leaves, and I am again drawing a blank. Its been a long week. Starts with a "B" I think.

Make sure it is always full of water, they can dry out extremely quickly. I am pretty sure it is a bug though, around here it is often corn bore. Don't know what's native down there though. Full sun all the time or they won't bloom well also.