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Herbaceous Peonies: Home

Paeonia lactifolora 'Chiffon Clouds'; photo by Marlon Co
Paeonia lactifolora 'Chiffon Clouds'; photo by Marlon Co
 

One of the most joyful and anticipated events in the NYBG seasonal succession of blooming plants is the flowering of the Matelich Peony Collection, when more than 150 herbaceous peonies reach their peak in mid- to late-May. Ranging in form from single to fully double, with petals in white, pink, coral and red, the flowers perfume the air.

Herbaceous peonies are the most common peonies found in home gardens and have stems that die back to the ground in the winter. Classic herbaceous peonies are cultivars of Paeonia lactiflora, a native of China, Mongolia and southeastern Russia, and are often referred to as Chinese peonies. They typically bloom late in the peony season with multiple blooms per stem and a sweet fragrance. Newer hybrid herbaceous peonies with more than one species in their parentage have the same cultural needs as the Chinese peonies but often just one bloom per stem and more vibrant color.

There are six standard forms of herbaceous peony flower recognized by the American Peony Society, the International Cultivar Registration Authority for peonies: Single, Japanese, Anemone, Bomb, Semi-double and Full Double. The characteristics of each form are largely determined by inherited factors leading to variations in petal formation and the extent to which stamens have been transformed through breeding to inner petals or petal-like structures.

A peony flower's elegant beauty belies the strength of this garden plant. Herbaceous peonies can take a leisurely approach to establishing themselves and blooming freely, but once they settle in, over a period of about three years, a well-sited plant is robust and should be easy to care for. These plants need to experience a period of chill in winter to become fully dormant and complete a healthy annual cycle. For that reason, they are harder to grow on the warmer margins of our USDA zone 7. Gardeners in the New York City area must take particular care not to plant them too deeply or overprotect the roots in winter or plants will lose vigor and flower less.

Matelich Peony Border at NYBG; photo by Marlon Co
Matelich Peony Border at NYBG; photo by Marlon Co

Growing Herbaceous Peonies

Choose the right location

Peonies that are sited well can survive and bloom generously for decades with minimal care. When planting your herbaceous peony, choose a location in full sun, with at least six hours of direct sun (not just sunny, the sun must be visible in the sky from the site) each day, to ensure vigorous blooms. Have your soil tested for pH by your county's cooperative extension office to see if amendments are needed to raise the pH to the 6.5 to 7 target zone. Select an area with very good drainage; standing water will suffocate the roots and is the leading cause of plant failure.

Plant in the fall

Plant bare root herbaceous peonies in fall, not spring, as the falling temperatures of autumn trigger the initiation of root growth. Spring planting for bare root plants has a low likelihood of success but container-grown plants can be introduced to the garden in spring with care taken that the roots are not damaged. Container-grown plants have a greater risk of producing inferior plants in the garden.

Allow generous space

In a few years, your peony will become a large, green, perennial bush. Most diseases affecting peonies, both fungal and viral, are encouraged by poor air flow around the plant and leaves that dry slowly, so allow a space of about 3 - 4 feet for good air circulation between your peony and other permanent plantings and structures. A lack of competing plants will also support strong branch and root development and the nutrition needed to form large flowers. The American Peony Society recommends that you avoid planting herbaceous peonies near lilacs which compete too directly with the cultural needs of peonies. If there are large trees or shrubs nearby, give your peonies even more space.

Prepare the soil

Keep the peony roots shaded and slightly moist until planting. Herbaceous peonies prefer a humus-rich, well-drained soil with a pH of 6.5 - 7.0. Organic compost and calcitic lime are two amendments that are useful to most soils, to increase fertility, improve texture, raise pH and add calcium. (You will not need the calcitic lime if you have tested your soil and it falls in the ideal pH range.) A very heavy clay soil will also require the addition of sand for drainage. The site you prepare should have good soil at least 18 inches deep and 2 feet in diameter.

Don't plant too deeply

Herbaceous peonies have fleshy tuberous roots with buds (sometimes called eyes) that are planted "buds up", with those buds no more than 2 inches below ground level. The plant needs to become fully dormant during winter to remain vigorous and in the New York City area, USDA zone 7, deep planting impedes flowering. Check that the plant does not sink as you water it in.

Wait until early winter, after the ground freezes and the plant reaches dormancy, to mulch well with a few inches of leaves or other material for winter protection. In most climates, this heavy mulching is only necessary in the first season, as roots develop, to prevent heaving (frost pushing the plant up out of the soil).

Paeonia 'Cameo Lullaby' in NYBG's Matelich Peony Collection; photo by Marlon Co
The herbaceous hybrid Paeonia 'Cameo Lullaby' in NYBG's Matelich Peony Collection; photo by Marlon Co
 

Caring for your peony

Use hoops to support your plant

While some herbaceous peonies have sturdier stems than others, it is typical to support your plants with a ring early in the season, when shoots emerge from the soil, before they become too leafy. Without the ring, the plant may not be able to hold its heavy blooms above the ground, and its branches may break under the load. Peony flowers that slump onto the ground will not last as long. Garden centers offer all sorts of wire-ring and linking-stake supports for peonies and other perennials. Leave some stems and foliage outside the support for a more relaxed, natural-looking plant and to hide the wire.

The beautiful peony stands used at NYBG attract a lot of attention and questions. These stands were made for us by Bob Keating’s Metal Works. They are available through the Shop at NYBG or you can contact the Metal Works directly.  Note that the peony supports arrive unpainted, with a black coating that is carbon from the forging process. Gradually, the carbon wears off and the supports begin to rust, eventually becoming a uniform, deep brown color that looks quite at home in the garden. At NYBG the supports have been painted with a coating of anti-rust paint, but it requires regular upkeep.

Water judiciously

Peonies are not particularly sensitive to dry weather but can be damaged by too much water in soil that doesn't drain well. Water only when the soil feels dry 6 inches below the surface. Water at the base of plant, avoiding sprinkling water on the leaves. Leaves that stay wet allow fungal spores to germinate and disease to spread. In the spring, when the plant is emerging from its dormancy and creating leafy growth, keep an eye on its watering needs as it will use moisture more heavily. Irrigation is not necessary for peony plants in most locations.

Support the soil for good nutrition

If the pH of your soil is not in the correct range (6.5 to 7), peony plants will not be able to access the nutrients and micro-nutrients available to them, so soil management is more important than fertilizer. Fertilize herbaceous peonies with restraint and always with low nitrogen fertilizer. A single feeding at the beginning of the growing season and another after blooming is adequate. There many appropriate fertilizers, including organic fish and seaweed fertilizers and bulb fertilizers, but check that the one you choose has a low first number in its N-P-K value.

Paeonia 'Bu Te' at NYBG; photo by  Marlon Co
Paeonia lactiflora  'Bu-Te', an American Peony Society Gold Medal winner; photo by Marlon Co

Cut flowers for display

Cut flowers that have not fully opened for the longest display life. Every leaf that you remove from the plant represents a loss of energy to next year's plant so don't cut off leaves unnecessarily. Let the cut peonies stand in a cool place, out of direct sunlight, in tepid water for several hours (preferably overnight). Add some cut-flower food; this will extend their bloom time by several days.

Peony flowers can also be cut and then refrigerated for later use. Assessing peony flowers to cut for temporary storage is referred to as the marshmallow test. For denser peony flowers like Fully Double varieties, press the base and top of the bud and cut when the softness resembles a marshmallow. Less dense peonies that open more easily, like Single, Japanese and Bomb form varieties, should be cut slightly harder than a marshmallow.

Flower buds can be stored in a home refrigerator with great results for more than a week. Wrap the unopened bud and stem in a plastic bag first to hold in humidity. Re-cut the stems when you are ready to display the peonies and allow them a day in a vase of water out of the direct sunlight to begin to open.

Propagate by dividing at the end of the growing season

Plants will emerge from underground in the spring, growing into a larger clump each year. After 5 or more years, you can dig and divide your peony in the fall, to create more plants. Peonies don't require division but if you want to create more plants or space has gotten too tight, division of the roots can be very successful. To transplant a mature herbaceous peony, dividing it first is often the most successful technique to encourage fresh vigor. Work slowly and with care for the best results. Use clean gardening and cutting tools.

In early fall, as leaves collapse and begin to brown, cut foliage back to about six inches. Dig a circle as wide as the plant in full leaf (up to 12 to 18 inches out from the center for a large plant) and work your digging tool all the way under the root system. Gently rock the root system until it can be pulled from the ground. Clean the roots with water so that the pink stem buds are visible. Keep the root ball damp and out of the sun if you will not be dividing it immediately. Waiting several hours to divide the root ball can make it softer to work with.

Sterilize your cutting tool and shorten roots to about 6 inches long, cutting away entirely any diseased or damaged roots. Then divide the root clump so that each new segment has several buds and a generous portion of 6 inch roots to support it. Treat the root segments as immature plants and follow the instructions above for preparing a site and planting no more than 2 inches below the soil surface. As would be the case with a newly purchased, bare root plant, the division will take a few years to mature and flower freely.

Deadhead flowers and practice good plant hygiene

After blooming in late spring, it is best to deadhead any spent flowers before seed production diverts the strength of the plant. In the fall, peony leaves and stems will collapse and turn brown. Cut down the stems and leaves to a few inches above the ground, taking care not to damage the crown of the plant. It is very important to remove debris from the garden to prevent overwintering of disease. Compost this material only if there is no indication of disease anywhere on the plant.

Mulch cautiously

Remove any winter mulch in late winter/ early spring, before peony shoots begin to emerge from the ground. Like removing old foliage and stems, removal of over-wintering mulch reduces the incidence of disease transference to new peony shoots. You can add summer mulch for water retention and soil temperature stabilization if the site requires it, but only after the stems form. Clean this away with old foliage in the fall as part of your maintenance routine.

In our area, only a light mulch should be added in the fall after a mature peony has become dormant, if at all. 

Paeonia 'Charismatic'; photo by Marlon Co
The vibrant herbaceous hybrid peony Paeonia 'Charismatic' is a semi-double form; photo by Marlon Co

Common problems

An influx of ants on peony flower buds may be distressing, but developing buds produce a sappy substance that is highly desirable to ants. Scientists have demonstrated that ants and peonies have a biologically mutual relationship but that the peony does not rely on ants to bloom. The ants enjoy the sugary fluids that are released from the extrafloral nectaries around a peony bud and even emit a pheromone trail to lead other ants to the feast. Peony buds, in return, have the protection of the ants from other insects while the flower forms. The ants will disappear once a peony blooms and, while unattractive, are not harming the plant.

Ants are an aesthetic problem for those who wish to cut flowers for indoor use. You can shake any hitchhikers off your flowers, or swish them in a bucket of water, or arrange them in a vase and leave them outdoors in shade overnight and the ants will leave to go back to their nest.

A white powdery substance on peony leaves may be powdery mildew. Powdery mildew is a common fungal disease that can affect many plants, especially in late summer, but it is considered more of a cosmetic problem than a risk to the safety of your plant unless it causes premature defoliation. Because it usually occurs at a late point in the growing cycle, you do not need to do anything to your peony. The flowers finished earlier in the season, and everything will be dying down soon enough. Be sure to remove all infected plant material and dispose of it as described in good plant hygiene above. Consider the location of your peony and whether it has sun, great air circulation, soil that drains well and avoids water splashing on the leaves, all factors needed to reduce the incidence of fungal disease. Without making improvements, your peonies may continue to have problems.

Gray mold visible on stems and buds with stems wilting and collapsing following damp, spring weather is probably botrytis peony blight, a fungal disease caused by Botrytis paeoniae. Leaves may be heavily spotted. The spore-heavy, diseased material spreads the infection by wind, insect and water to other parts of the plant. The plants should be cut down and cleaned away in fall as described in good plant hygiene above. Destroy rather than composting the debris and add a 2-inch layer of mulch to trap any stray fungus. Reconsider your planting location and whether it has sun, great air circulation, soil that drains well and avoids water splashing on the leaves. All of these site limitations favor the spread of fungal disease. If you continue to grow a peony in this location, make improvements for better results and contact your county's cooperative extension office for information on fungicides effective in your area to apply in early spring.

Peonies don't develop buds or peony buds form then fail to grow (bud blast). Herbaceous peony flowers are large and dense; they take an enormous amount of energy to form fully. Strong leaf development with no buds or buds stunted and brown is apt to occur for one of these reasons that deprive the plant of the strength needed to complete the flowering process.

  • The plants are young and not mature enough to bloom yet or have been recently divided and need to recover.
  • The leaves of the peony were cut back before they were fully brown last season or too many leaves were removed when flowers were cut for display last season. They need to photosynthesize as long as possible to gather strength to re-bloom in the following growing season.
  • Plants around the peony have grown up and are creating increased shade; full sun is needed.
  • Mulch or compost has been added and built up burying the roots too deeply, or heavy winter mulch was put down too soon in the season. Our increasingly warm winter weather can add to the problem. Without a good winter chill to allow complete dormancy, peonies get weak and may not complete the blooming process.
  • Crowding and competition with other plants reduces the resources the peony plant has to thrive and  bloom.
  • The buds may have been damaged after forming by strong frost in the spring just as buds are developing or by botrytis mold infection. (If botrytis is the issue, you will probably also see damage to new leaves and shoots).
  • An over-fertilized plant (or one hit with stray, high-nitrogen, lawn feed application) may have been signaled to produce lush foliage rather than flowers.

Deeply colored flowers fade more quickly than light colored ones. The problem is exposure to strong sun in the late afternoon. Planting these cultivars in a part of the sunny garden bed that with light, late-afternoon shade has some benefit.

Paeonia Charles Burgess, a Japanese form peony with identifiable but thickened stamens, termed staminodes; photo by Marlon Co
A Japanese form peony, Paeonia lactiflora 'Charles Burgess', in bloom at NYBG. Japanese flower forms have identifiable but thickened stamens, termed staminodes; photo by Marlon Co

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