As with most garden flowers, the size of peony (Paeonia spp.) blooms depends on several things, some cultural and some botanical. Cultural influences include the amount of sunlight and water the peony plant receives, the composition of the soil and the type of fertilizer used. Botanically, flower size is relative to which of the three types of peony you are growing, as well as the particular cultivar.
Except for a handful of native and wild peonies that grow in partial shade, peonies prefer full sun. Those that receive less than optimal light may not produce the large blooms you anticipated. Blooms also tend to be smaller and fewer if the peony is planted too deeply, becomes overcrowded from lack of division, must compete for water and nutrients with other plants or receives too much nitrogen fertilizer.
There are three kinds of peonies: herbaceous (Paeonia lactiflora), also called garden peony; tree peony (Paeonia suffruticosa), a woody shrub; and a cross between the two, the intersectional, or Itoh, peony, named after grower Toichi Itoh, who made the first successful cross. Herbaceous peonies are perennial plants that grow in spring, bloom in April and May and disappear in fall. Tree peonies are deciduous shrubs with woody branches that lose their leaves in fall, although their woody structure remains. Itoh peonies have characteristics of both kinds, with stronger stems and larger flowers from their tree peony parent yet with a smaller size and a perennial growing cycle from their herbaceous parent.
Tree Peony Flowers
Tree peonies bear the largest flowers of the three types, up to 12 inches across, and usually bloom about two weeks earlier. “Redon” (Paeonia suffruticosa “Redon”) has peach-colored blossoms that reach 10 inches, while the plant itself stays a compact 3 feet tall. “Daedalus” (Paeonia suffruticosa “Daedalus”) bears deep, rich red-burgundy flowers with bright yellow anthers that reach up to 11 inches. Tree peonies thrive in U.S. Department of Agriculture plant hardiness zones 4 through 9.
Itoh Peony Flowers
Most Itoh peony flowers grow 6 to 10 inches across. They are double or multipetaled, come in pastel colors and usually have a contrasting center color. “Keiko” (Paeonia (Intersectional hybrid) x “Pink Double Dandy”), has 6-inch pale pink blooms, and the yellow flowers of “Bartzella” (Paeonia x “Bartzella”) reach 9 inches. Itoh peonies are still new on the market, even though they were originally hybridized in 1940, so larger flowers may be on the way as breeders continue to create new cultivars. They thrive in USDA zones 4 through 9.
Herbaceous Peony Flowers
The blooms on herbaceous peonies can reach about 10 inches across, but most grow to about 6 inches. Herbaceous peonies must be staked or supported, because the heavy blooms, especially when wet, topple over on their thin stems. “Ann Cousins” (Paeonia lacticflora “Ann Cousins”), with white double blooms, and “Bowl of Beauty” (Paeonia lactiflora “Bowl of Beauty”), with single pink blooms with frilly yellow anthers, both reach 10 inches across. Herbaceous peonies need a period of winter chill, so they grow best in U.S. Department of Agriculture plant hardiness zones 2 through 9.
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