The bed in front of the house has two huge, ugly stumps in it and the soil is very clay-heavy. I picked all the trash out (actual trash; the previous tenant didn't keep the yard clean) and added topsoil. When I planted, I kind of just mounded the topsoil and dug down a little into the clay. The petunias are doing very well (though they turned more pinky-purple; think my soil must be slightly acid), but my delphiniums are looking scraggly.

They have buds, but the leaves are yellowing with brown, burnt looking edges. I am not sure why; it could be the soil. I have been feeding semiweekly with Miracle Gro Bloom Booster formula. I'm attaching a photo of the leaves below.

I plan to dig the delphiniums up and move them to a bed out back in the fall (is that the best time?) but I'd like to keep them healthy. Any advice on why they might look like this is appreciated.