Peonies are among the most versatile of all plants in cultivation and are remarkably well-suited to our relatively cold climate. As early as the 1920’s they were the plant of choice due to their hardiness and long life in cooler regions; it’s not unusual to have an individual plant survive and bloom for more than 50 years.
This article is an overview of the different peony structural forms, species evolution, and bloom types. It’s the first in a short series of articles intended to provide an introduction to the realm of peonies. Over the next few weeks, subsequent articles will describe general growing conditions for peonies as well some basic history and notable breeders.
TYPES OF PEONIES BASED ON STRUCTURE
There are three types of peonies based on overall plant morphology or form: herbaceous, intersectional, and shrub (woody). The herbaceous form, like ‘Mackinac Grand’ shown below, is deciduous and dies back to the ground each fall, then sends up entirely new stems in the spring.
Intersectional peonies (sometimes called “Itoh” peonies from Japanese breeder Toichi Itoh) are a cross between the herbaceous form and the woody form. They possess features that are the best of both forms in that their stems are shorter and stronger than herbaceous varieties, but they produce a greater abundance of blooms compared to their woody cousins. Like the herbaceous form, intersectional peonies also die back to the ground each fall.
The woody form has stiff, partially woody stems and grows like a bush rather than a tree. There is some ‘die-back’ of the newer green stems but the majority of the above-ground portion of the plant remains from year to year.
A NOTE ON SPECIES DEVELOPMENT
Although there is no complete agreement among botanists, approximately 30 distinct species of peonies exist. To describe the diversity among these various species, well-known breeder Bill Siedel created the diagram below, which is an interesting visual method to demonstrate peony classification and distribution (click the image for a full-scale version).
TYPES OF PEONIES BASED ON FLOWER FORM
Single Flower Form: One or more rows of large petals surround a band or row of natural, pollen-bearing stamens, centered by a cluster of seed-bearing structures (carpels), usually topped by the natural, pollen-receiving stigma.
Japanese Flower Form: This form is similar to the single form above, except all of the stamens are transformed, also called “all-over transformation”, into staminodes (petal-like segments).
Anemone Flower Form: Similar to single and Japanese forms, except the stamen segments are further transformed into slender petals called petalodes, often nearly the same hue as the guard petals.
Bomb (Bomb Double): Technically, the anatomy is the same as the singles, except the center petals, while smaller than the larger outer petals, have the same texture and color density. Ideally, the center segments form a neatly tailored ball or mound (“bomb”).
Double (Full Double): This is the classic flower-in-flower peony form, sometimes described as “two-stage double”, some actually triple-stage, resulting in the most massive of peony flowers. Some of these also have the “all-over” stamen transformation of the bombs type, where no stamens can be found in the depths of the flower.
Semi-Double: Also a flower-in-flower form, particularly in the Lactifloras, always having progressive stamen transformation, having a lesser quantity of inner petals and greater proportion of natural, pollen-bearing stamens. This results in a sufficiently large component of yellow stamens that their yellow color is a prominent factor in the flower color