News and Articles

Growing Woody Peonies

Nate Bremer… Solaris Farms Owner and Grower

Woody peonies have not always received good press, leading to the misconception they are difficult to grow.  This belief may have more to do with Chinese suffruticosa, the least well adapted group to North American or European climates, being the most commonly available and planted group.  While each group has its own optimum growing requirements, most can be grown successfully throughout much of the United States.

Conditions in Balance.

As gardeners we often concentrate on single measurables (temperature, precipitation, air movement etc…) or growing conditions, but fail to consider that these factors must work in concert with one another for optimum outcomes.  The idea of viewing relationships between temperature, precipitation, soil moisture, soil structure, soil fertility, air movement, light, ground cover and humidity adds complexity to decision making, but can yield answers to better growing.  If you grow woody peonies, observe them closely and make adjustments that bring these factors into harmony for best growth.

Light.

All woody peony plants will grow best in full sun the entire day during the growing season.  Plants grown under these conditions will attain greater size, produce more flowers and have more vigor than those grown in less optimum lighting.  In reference to ‘Conditions in Balance’, remember plants in full sun require more water due to greater transpiration from heat and their own growth.  Soil temperatures will also become higher and mulching may be required in warmer growing areas to conserve water and reduce soil temperatures.

Plants may be grown successfully with a half day of sunlight or in high shade, but less vigorous growth and fewer flowers can be expected under such conditions.  Stem length, leaf size and beauty may be enhanced due to lesser exposure to the sun, but plants are more easily damaged by wind.  If grown near trees ‘Conditions in Balance’ again becomes important as trees will reduce soil temperatures by shielding solar radiation but may also produce competing roots systems robbing the soil of water and nutrients.  Flowers produced on woody peonies in shaded locations often last longer and do not get sun burned, but are produced in less profusion.  Thus sacrificing amount of bloom for beauty of bloom is a consideration you will need to make.

We have done well with woody peonies planted near tree-lines that protect them from direct sun during the heat of the day, while exposing them to early and late day slanting light.  Evergreen trees with open branching are preferred over high water consuming deciduous trees or densely branched/needled evergreens.

Temperature. 

1. Winter temperatures: Like their herbaceous peony relatives, all woody peonies require a period of cold dormancy to be grown successfully.  Winter temperatures and duration of cold vary widely across the United States.   rockii cultivars have the greatest cold tolerance, followed by Japanese suffruticosa and then the lutea hybrids with the Chinese suffruticosa the least cold tolerant.  All woody peonies prefer stable winter temperatures with a graduated cool down in autumn and warm up in spring.  Temperatures that warm in mid-winter and are followed by very cold temperatures are problematic because they cause stem tissues to move sap and buds to swell, before freezing again.  This causes hydrated cells to burst due to expansion during freezing, resulting in dead or damaged stems and buds.  Stem hardiness in relationship to temperature is often confused with overall hardiness.  While lutea hybrids lack low temperature stem hardiness, the crowns and roots differ little from the other two woody groups.  Lutea hybrids often lose stems at temperatures that fall below 0°F, but crowns and roots are seldom killed.  In fact, lutea hybrids are masters at producing ground shoots after winter stem kill and the new stems often produce heavy bloom the following spring.  In contrast, Japanese suffruticosa and rockii hybrids mostly bloom on old wood buds and when winter damaged will produce few, if any, blooms.

Many gardeners rely on the USDA Hardiness Zone system to provide information concerning temperature tolerances of plants selected for their gardens. While this can be helpful, it only supplies a fraction of pertinent information for those wanting to grow woody peonies.  Consider the following:

• It was developed for plants that have persistent woody stems exposed to air temperatures, not for their root systems.
• It cannot account for winter soil temperatures which are impacted by snow cover, moisture or lack of it, reflection of solar radiation, and other variables.
• The system does not account for accumulated duration of temperatures in winter or how quickly autumn cool down and spring warm-up occurs.

Thus a single zone rating is not nearly the same for everyone belonging to a zone and only provides one piece of information – the average annual minimum winter air temperature.

What does this mean for the gardener wanting to grow woodies?  All groups of woody peonies may be grown in areas that experience -35^oF temperatures, as long as good snow cover insulates the root systems.  The following is a temperature guide for woodies:

• Rockii cultivars, rockii, P. decomposita and P. rotundiloba: These generally tolerate temperatures to -28°F without significant damage.  Of course inconsistent winter temperatures may impact these tolerances.  Some rockii cultivars may not be as temperature hardy, likely due to the influence of P. suffruticosa in their genetic makeup.  Not all rockii cultivars are created equal!
• Japanese suffruticosa cultivars, qiui and P. jishanensis: Generally tolerate temperatures to -15°F without significant damage to stems. As with the rockiis, some cultivars have greater cold tolerance than others.
• Lutea hybrids: F1 hybrids such as most A.P. Saunders cultivars and are the least stem hardy and will tolerate temperatures to around -5°F without significant stem loss.  Many advanced generation hybrids will tolerate temperatures to -12°F to -15°F without stem loss.  The lutea hybrids vary widely in their stem hardiness, but are the only group of woodies capable of blooming heavily on new stem growth produced from below the ground (ground shoots).

Conditions in balance:  Winter temperatures provide a huge challenge for northern gardeners since keeping the multiple factors involved in balance is daunting.  Wind combined with low temperatures may cause desiccation of stems and buds.  Droughty soils allows for greater cold penetration, allowing temperature to impact root systems.  Overly wet soils are slow to warm in the spring and low soil temperatures may cause icing to occur. Inconsistent snow cover will impact all parts of the plant.   Providing for a balanced temperature is a bit out of the gardeners control and is something we often don’t think about. Mulching and wind breaking are recommended to assist in temperature balance.  If you live in an extreme winter climate, woody peonies can be enclosed in insulating boxes, cones or even wrapped to mitigate the impact of temperatures.  Protecting stems from solar radiation in the winter is likely as important as protecting them from cold, as winter sunlight often warms stems above freezing during the daylight hours, only to freeze them again at night.  Again keeping conditions nearly constant or balanced is advantageous to stems and plants.

2. Summer temperatures. Woodies are all quite tolerant of high temperatures in the summer months.  Air temperatures in the 90°’s to low 100°’s are tolerated by all groups, as long as water is available in the soil.  Low humidity combined with high air temperatures also may stress plants, causing them to draw on root reserves if adequate water is not available in the soil.  Here again conditions in balance become important.  Rockii and its hybrids are most tolerant of high summer air temperatures, but none of the groups appear to be highly impacted by extreme heat if ample water is available to established root systems.

3. Soil temperatures. Woody peonies prefer soil temperatures that do not rapidly change.  Keeping them evenly moist in most situations will prevent overheating.  Plants may be mulched in high temperature areas to conserve water, but also to prevent soils from becoming hot.  Woody peony roots are active in warmer soils than herbaceous peonies, but may also be damaged by high heat, which often occurs in conjunction with drought.

Water.

Like all peonies, woody peonies grow best with suitable amounts of water in the air and soil.  Woody peonies should never be planted in an area that becomes wet during any season.  Midwest United States’ spring seasons are notoriously wet and cold, which provides some challenges, even in the suitable planting sites. Other areas of the United States are also likely to experience similar conditions.   However, these conditions do not cause insurmountable obstacles in growing these wonderful plants.  We do not use any fungicides, but rather rely on observation and physical disease prevention to combat excesses in water.  In almost all cases, disease from excess water can be dealt with by creating or selecting a good planting location and removal of diseased plant parts so that they do not become problematic for the entire plant.

• High humidity (air moisture) can provide botrytis and other fungal diseases optimum conditions to attack stems and leaves. Woody peonies (especially P. suffruticosa) are particularly susceptible to fungal diseases to their stems during spring conditions that are both wet and cool.  Often, these fungal diseases attack the area in which new growth arises from the previous year’s stem growth.  Apparently the bracts at the base of the young shoots collect water containing the disease organism’s spores or reproductive vessels and provide a good location for them to grow.  Once drier and warmer conditions prevail, stem diseases subside and often cease.  New shoots showing wilt are indicative of infection and should be cut off well below the area in which they join the older woody stem.  This will arrest the disease before it moves downward toward the woody peony’s crown.  Removal of diseased plant parts before the entire plant is impacted is an important preventative measure.  Once disease enters the crown just below the soil line, persistent problems may be encountered for years to come.

• High amounts of water in the soil may bring on root rots, basal stem rots, botrytis and other fungal diseases. Avoid allowing any mulch applied for winter protection to remain in contact with the base of the plant. While we do not mulch our growing fields, our display beds are mulched for appearance and any mulch is pulled 6 inches away from the plant bases.  This allows soil to dry at the base of the plant and removes the wet conditions diseases most prefer.  In areas of high rainfall, the creation of a mound which sheds water away from the plant is helpful and has proven to be a successful tactic.

• Excess water in the winter has become highly problematic as climate change causes snow to melt and then turn to ice or standing water to accumulate over frozen soil. If the soil remains frozen around the crown and water does not infiltrate the soil, little or no damage will occur.  However, if the surface soil thaws and accepts water, a zone of super saturation around the crown often causes damage.  Mulch plants in areas that experience conditions of winter wetness combined with thawing and refreezing.  Mulching prevents soils from thawing quickly, thus preventing water penetration into the soil during winter.  Mounding as described above will also help to remove water from the crown area.

• Areas of the United States that experience dry seasons may require supplemental water. In Wisconsin, we have never needed to water peonies, even in drought years, probably due to our soil’s water holding capacity (heavy clay).  If water is needed during summer months, soaking the ground thoroughly is recommended at each watering.   Since woody peony roots extend for quite some distance and depth from the plant, a period of an hour or more of constant water may be necessary to impact the root zone of the peony.  Never water the foliage, as this may only encourage disease on stems and leaves.

• Soil moisture in August through the fall season is extremely important for the formation of strong buds that will be used for flowering and growth the following year. Plants that grow in drought conditions will make poor growth the year after these conditions.  Also, extremely dry soils in winter allow for greater cold air penetration and may cause damage to root systems.  Evenly moist in a well-drained situation is best year round.

• Lutea hybrids are the most tolerant group to atmospheric and soil water. Thus if conditions in the climate tend toward high humidity and greater rainfall, this group will likely do best in combating associated diseases.

Soil.

Soil constituents and conditions vary widely throughout the United States and from year to year due to other climate variables.  Soil science is complex and generalizations can be difficult to make, considering the multiple variables that are involved.  Again the ‘Balance of Conditions’ comes into play.  Much literature points to growing woody peonies in highly organic friable soils for best results.  This information is the opposite of our experience, as our woody peonies are primarily grown on mineral soils that mainly consist of clay which overlays limestone.  We have visited numerous woody peony plantings that utilize native soils that are have high organic material, are friable and in almost all cases the plants do not grow as vigorously as those sited on clay based soils. A number of soil conditions are certain:

1)  The soil must drain and should never be water logged, but also needs to be water retentive.

2)  Sandy soils are not recommended as they have little nutrient value and woody peonies do not perform well on them.

3)  Clay based soils with underlying limestone is preferred.

4)  Woody peonies grow quite well in soils with a pH of 6.3 to 7.0.

5)  The soil should have good water retention, since woody peonies appear to make use of water throughout the summer season (more so than do herbaceous peonies).

6)  Avoid amending soils with peat or peat based mixes, as this appears to cause disease issues with P. suffruticosa cultivars.  This is likely due to the high water holding capacity of peat.

7)  If fertilizer is needed, use one that includes high calcium as this appears to help stem strength and assists in warding off disease.  Volcanic ash amendments such as Azomite have been found to be highly affective.  A soil test is recommended before any type of fertilizing is performed.  If in doubt-do not fertilize.

8)  Avoid the use of manure as a soil amendment, as the use of this material is directly related to disease problems in woody peonies.

Air Movement.

Woody peonies are best grown in locations that have ample air space on all sides of the plants.  Leaves and stems should be able to dry quickly after rainfall.   Good air movement provides a situation in which diseases are less likely to become problematic.  More disease issues can be expected in crowded garden situations.  Wind swept locations are not recommended because stems can be broken, however, a breezy location will produce a healthier plant.

Our woody peonies are grown in an open field and experience heavy wind from time to time, which break off weak stems.  While we dislike seeing stems lost from windy conditions, these stems are often poorly attached or unhealthy and the loss can be counted as Mother Nature doing her pruning.