What varieties of peonies in red, pink and white have shown the best results in drying and should I spray the dried heads with something to make them last and hold up better?
The easiest results come from bomb or fully double red and pink ones, and some of our most dependable plants were bought simply as "red" some a small nursery and a pink variety passed on, without a name, from a neighbor. Of named varieties, we've had good luck with Mons. Jules Elie and Bowl of Cream, among others. But really, just about any peony will work, including singles, although for some you have to dry them by sticking their stems through hardware cloth and supporting the petals with twists of tissue paper until they're dry.
Red peonies tend to turn a bit purplish as they dry, just like roses. Whites turn a little creamy. We haven't tried any tree peonies or intersectionals, so I don't know what happens with yellows or strong corals.
We normally don't spray peonies with anything, although a brief mist of unscented hairspray probably wouldn't hurt.
The big concern with peonies is that they're very attractive to Indian meal moths. We monitor them with pheremone traps and protect the blooms as best we can with paper bags or tissue-paper tents while they dry and store them in closed plastic containers with packets of silica gel and a mothball or two. If we have the time, we freeze dried blooms in a frost-free freezer for several days.