PEONIES DO NOT BLOOM ALL AT THE SAME TIME!
You can extend the flowering period of your peony gardens up to seven weeks in selecting cultivars/species with different blooming time. All peonies don't bloom at the same time. Some botanical species as P. tenuifolia and P. Mlokosewitschii bloom very early in the season whereas others, can bloom one month later. Generally, peony catalogues and nurseries use a code to give you an indication of the blooming time:very early, early, mid-season and late season.
In order to clarify this, I suggest you a wonderful tool. This is a brand new blooming <<blooming calendar >> produced by members from the Canadian Peony Society. Using these data, you can select peonies from different blooming period in a way to obtain almost 7 weeks of blooms. For more details, look at the whole calendar at << http://bloomdate.paeonia.com >> .
The number that appears in the parenthesis indicates the number of days that this peony bloom in comparaison of the peony Red Charm, who serves as the stallion with 0 as the blooming period.
It is important to note that for some peonies, particularly the intersectional hybrids, blooming period is longer and stretch from midseason to late season. Use this list when selecting your next purchases of peonies and buy those that will bloom at different periods than the ones you already grow.