Answer: Peony flowers last longest when the weather stays cool and it doesn't rain and knock down the plants. In my experience we either get a serious rain or a mini heat wave the week the peonies bloom. So the answer to your first question is well, it depends. But they are so lovely we keep on growing them. Every once in a while they last for a week or ten days!
Healthy peony blooms and stems should not turn black, however. Peonies are subject to a number of diseases and a wet spring encourages problems. The best method of limiting disease from year to year is to cut off, remove and destroy all peony foliage every fall after frost. You may also wish to cut off, remove and destroy all the blackened plant parts now; dip your tool into a disinfectant 1 part household bleach and 1 part water solution as you cut to avoid spreading any disease to uninfected parts. You might also want to take a sample to your County Extension to identify exactly what is affecting your plant and verify suggested controls, if any. Their phone number is 696-3500.
As for planting depth, the rule of thumb is to set the "eye" at about 2 inches below the soil surface. If you have dug a huge deep richly amended hole prior to planting, the soil in that hole will settle over time and your eye will drift lower along with it. If you suspect this has happened, you could lift and replant this fall. However, if they are of different types, it may just be that the second variety is "shy" about blooming and may take one or even two more years before it starts to bloom for you. (If they are of the same variety but in different areas, it may also be that one spot is better than the other.)
If you decide to relocate (yes, they do get big -- allow at least three feet or possibly even four) fall is the best time to do that. I would trim the foliage by about a third and move in late Agust if the weather is cool, early September otherwise. Then follow the original planting instructions as to transplant care.
Good luck with your peonies!
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