Herbaceous and tree peonies (Paeonia spp.) are native to Europe, eastern Asia or northwestern North American depending on species. A deep, fertile soil that is rich in organic matter promotes best growth and flowering; the soil must be moist and well-draining, too. Tall-stemmed herbaceous peony stems may need staking to prevent their big, heavy blossoms from flopping to the ground, and tree peonies are best planted out of windy locations.
Peonies need a cool to cold winter dormancy in order to produce flowers and overall persist as perennial plants. Herbaceous peonies, those that grow on non-woody stems that rejuvenate each spring, are best grown in U.S. Department of Agriculture hardiness zones 3 through 8, where winter temperatures drop to anywhere from minus 40 to 10 degrees F. Tree peonies, those that grow with lasting woody shrub-like stems, are better is zones 4 through 8. An insufficient winter dormancy with cold doesn't allow plants to go inactive and thus are weakened when springtime's growing season starts.
Once the threat of frost in springtime passes, the soil and air temperatures allow the roots of herbaceous peonies or the branches on tree peonies to break dormancy and reveal their first leaves. The foliage expands and reaches mature size by mid-spring, providing nourishment to the plants in preparation for flowering.
Depending on species, the range of flowering displays on peonies varies from mid-spring to early summer. Generally speaking, tree peonies produce their blossoms in mid- to late spring while herbaceous peonies bloom more from late spring to early summer.
As the flowers wane from the plants, the black pod-like fruit with two to five lobes mature over the course of the summer. The fruit pods eventually break open to release the seeds to the ground, and will not germinate until the next spring after first being exposed to the winter cold. The leaves remain and provide nourishment for the roots, allowing them to enlarge, produce off-shoots or store starches for the winter dormancy. Interestingly, just like peonies need a cold winter, they also grow best in areas where summers are not too long and hot. According to the American Horticulture Society's plant heat-zone maps as discussed in the "A-Z Encyclopedia of Plants," peonies are best suited to regions that experience no more than 120 days of temperatures above 86 degrees F. during the growing season.
The healthier the roots are over winter, the more readily the peonies produce lush new leaves and stems next spring, leading to a good flowering display. The first killing frost or freeze (temperatures at or below 32 degrees F.) blackens and kills the foliage of herbaceous and tree peonies.