I bought three peony bushes from a local nursery this spring. They immediately began growing and showed lots of blooms. The blooms opened, one by one. (hurray!) Then, as if someone flipped a switch, they stopped growing in late May and no more blooms appeared. The bushes don't appear to be getting any bigger now. What happened?

I didn't apply fertilizer because the nursery said not to (because they already had).

I did notice some weeds in the flower bed and sprayed them with weed killer (whose label said it could be used around bushes, flowers, etc.) I was careful not to get any on the peonies... could the 'run off' of that chemical have affected the root system?

I initially worried that they may have been planted too deeply but they flourished through April and May.

I have sprayed them (as well as all my other bushes/trees with Sevin for insect treatment.)

Any thoughts would be appreciated! I'm a new southern gardener and want so much for them to grow but I know so little about it!