Plant taxonomy classifies tree peonies, primarily, as Paeonia suffruticosa. Various cultivars are available, including 'Kinkaku,' which has a lot of yellow in its flowers. These plants, despite their common name, are classified as deciduous sub-shrubs. Thus, while they share similarities with the better-known perennial peony plants, the two are also different in important respects. The latter, whose Latin name is Paeonia lactiflora, are herbaceous perennials. But P. suffruticosa sticks around above-ground for the winter, being a woody plant. Both are remarkably long-lived.
Tree peonies may reach up to ten feet tall or more at maturity, but they're very slow-growers. Growing conditions also affect size. So it's quite possible that your plant will remain relatively small for many years.
While their leaves are attractive, it's all about the flowers with these plants, which, really are not "trees" at all, but rather shrubs that bloom in late spring. But that is not the only misnomer you may encounter in growing these beautiful plants. Some gardeners buy a type called P. suffruticosa 'Red' (which is shown in the picture), thinking that they'll be growing a plant with red flowers. The label at the garden center may even show a picture of a red flower. But as you can see from the photo, the blossom, while gorgeous, is a pink flower and not at all a true red in color. You will encounter the same discrepancy with another supposedly red cultivar: P. suffruticosa 'Zi Qiao,' also known as "Luoyang Red."
Kinkaku's flowers are bicolored, sporting a pale yellow color with pink on the edges. Fortunately (if you're looking to stagger sequence of bloom), these two tree peonies do not bloom at the same time, effectively distributing the color display they furnish your landscape. One year, for example, P. suffruticosa 'Red' may bloom the first week in May (in zone 5), while P. suffruticosa 'Kinkaku' may bloom the last week in May.
Some types of tree peonies have fragrant flowers, and some do not. The two cultivars mentioned above do not (or at least the smell is nothing like the fragrance that many herbaceous forms have). Both among the tree and herbaceous types, there will be differences in aroma based on cultivar selection.
Planting Zones, Sun and Soil Requirements
Tree peonies can be successfully grown in planting zones 4 through 8. They are not quite as hardy as P. lactiflora, which is listed as being cold-hardy as far north as zone 2. But like P. lactiflora, they are not a plant that Americans in the Deep South have much success growing. Both have chilling requirements, so they grow best in regions that experience cold temperatures during the winter.
Grow tree peonies in full sun to dappled shade. The further south you go in their range, the more sense it may make to grow them in dappled shade. The plants like to grow in ground that is well-drained, with a soil pH that is neutral or slightly alkaline. Plants should thrive in a loamy soil fortified with compost.
Care for Tree Peonies
The tree peonies you buy at the nursery are typically the result of grafting. The scion is a tree-peony cutting, which has been grafted onto an herbaceous rootstock.
In the context of plant care, why is it important to be aware of the graft? Well, for one thing, you should plant the graft union at least 2 inches below the soil surface (some recommend more; follow the instructions on the label, if any, that come with the plant). This will promote the establishment of the scion's root system.
Knowing you are dealing with a grafted plant also comes into play in pruning. Pruning is rarely necessary on tree peonies. But there are two exceptions:
- Cutting off dead branches.
- Removing branches that are the result of suckering from the herbaceous rootstock.
When you do prune, wait till early spring to perform the operation; avoid pruning in fall, as above-ground growth helps shelter the root system in winter. For fall care, mulch to provide additional winter protection if you live in an area where the plants are only borderline-hardy.
Like the herbaceous form, tree peonies do not like to be transplanted. So find a suitable home in which to establish them when planting and consider it their final resting place.
Uses in Landscaping
You can use tree peonies as specimens, edging plants or even in short, ornamental hedges. The beauty—including the size—of their flowers is their most outstanding quality. The flower pictured measured 7 1/2 inches across. But if you're a gardener who has always had extremely fragrant types of herbaceous peonies, these two cultivars of P. suffruticosa may disappoint you, as they will suffer by comparison in terms of smell. Fragrance aside, when tree peonies are considered simply on their own merits, you will likely judge them to be superb plants. In fact, they may be among the best plants to grow that you have never heard of. Another positive attribute to tree peonies is that they are deer-resistant shrubs.
Tree Peony Types
A very popular species of tree peony is P. delavayii. The lutea variety has yellow flowers. But even within the suffruticosa species, there is plenty of variety, including different floral colors. For example, P. suffruticosa 'Kinshi is also called the "Golden Bird," because it bears huge flowers in a golden-yellow color. While many tree peonies have double flowers, the white P. suffruticosa 'Qing Xiang Bai' produces single blooms.
The differences between tree peonies and herbaceous forms have been explained above. But yet another major class is known as "intersectional" peonies, also called "Itoh Hybrids." Think of them as the intersection where the two other forms meet, as they are a cross between the other two types.
Paeonia, the genus name, derives from the name, Paeon, who, in Greek mythology, was a disciple of the god of medicine, Asclepius (the Roman Aesculapius). The origin of the specific epithet, suffruticosa, may not boast the same pedigree, but it is more useful, being descriptive in nature.
Think of suffruticosa as being composed of two parts:
- Fruticosa means shrub-like
- Suf- is really the prefix, sub (meaning "under" or, in this case, "less than"); the "b" mutates into an "f" when it precedes an "f," as in the English word "sufficient"
Thus suffruticosa literally means "sub-shrub," which is how this plant is classified. Because tree peonies are plants from China (and surrounding areas), you will sometimes see them referred to as "Chinese tree peonies."The Itoh Hybrids are named after Toichi Itoh, the Japanese grower who originated the intersectional class.