Planting and Care of Woody Peonies
Nate Bremer – Solaris Farms Owner and Grower
Climate and soil conditions are major players in successful culture of woody peonies as described in our last article, but there are other variables gardeners need to consider. Root-plant configuration, planting criteria and pruning all play a role in growing these wonderful plants well.
Root-Plant Configuration: Own root vs. grafted plants.
Knowing key differences between “own root” and “grafts” is helpful in a number of regards. Recently a number of growers, gardeners and media outlets have been promoting the idea that “own root” woody peonies are better than grafted plants. The answer to this is both “yes” and “no”.
Own Root Plants
Own root woody peonies have roots that originate from the plant itself. These roots are woody peony roots and by their nature are highly compatible with the plant they support since they are naturally produced. Own root plants are produced by dividing woody peony clumps and is a slow process for most growers in cold climates. However, woody peony roots grow more slowly and need warmer conditions than the herbaceous roots used on grafted plants. Own root plants are slower to establish and are more susceptible to winter damage and failure during first couple of years in the garden. Thus a very cold winter that cools soils excessively may damage the roots. Ultimately, anyone who grows a woody peony would like their plants to grow ‘own roots’ as they will produce a better looking plant over time, but should be aware that late fall planting or very cold winters may cause plant growth problems or failure.
Grafted woody peonies are those in which a woody peony stem is attached to a herbaceous peony’s root (known as a nurse root). The practice of grafting allows the grower to produce more woody peonies than could be produced through division. Since the initial root system on a grafted plant is actually a herbaceous peony’s, it will root in cooler soils than an own root woody peony and the root system’s growth rate will initially be faster. The herbaceous ‘nurse root’ should be considered a temporary root system, until the grafted plant begins to produce its own roots. Thus this type of peony is meant to provide a transition that is beneficial for the gardener in a cold climate. However, a grafted plant is not without pitfalls. Grafted plants are sometimes sold with a nurse root that was taken from an herbaceous peony that has adventitious characteristics. The nurse root then begins to grow a plant, which is not the woody peony. Adventitious roots often produce growth that overtakes the woody peony and should never be allowed to grow if the woody peony is to live a long life. Sometimes digging down and removing the herbaceous peony’s stems will cure this problem, but in most instances the entire plant will need to be lifted and the nurse root removed completely. Of course, the woody peony must have grown its own roots before the nurse root can be removed. Finding a grower who grafts plants to non-adventitious nurse roots certainly will prevent problems like this from ever occurring (Solaris Farms does not use adventitious root stock). Grafted plants are also all too often planted incorrectly. Shallow planting of the nurse root may inhibit the woody peony from growing its own roots and also cause the plant to be pushed out of the ground as the nurse root grows. Deep planting, so that 4 to 6 inches of woody peony stem is covered with soil is advised for grafted plants. The buried stem will naturally convert to subsoil crown tissue and produce own roots once established.
In summation, both root configurations have positives and negatives, but in almost all cases, either will produce a fine specimen if treated properly. Cold climate gardeners will usually find more success planting grafted plants that have more adaptable root systems, but own root plants are quite acceptable, given they are planted earlier in the season.
Planting Criteria: Planting a woodies
Planting is best done in the fall of the year as soils begin to cool, but before the soil freezes. Air temperature or frosts have little to do with soil temperatures and planting can often occur for another month or two after the first frosts. Many inexpensive woody peonies are sold at garden centers during the spring season, but planting should be avoided at this time as many plant failures occur during warm weather planting. Since most woody peonies sold at garden centers are grafted and have a herbaceous root system, they produce few if any roots in the warming soils of spring, and summer heat will often stress plants excessively or kill them outright. No gain in size will occur from a spring planting versus a later fall planting, if the plant even survives the summer.
Select a planting site that provides proper lighting, air movement and soil. The site should be one in which the plant will be permanently located, as older woody peonies resent being moved. The planting depth of a woody peony is quite different from that of a herbaceous peony-deeper. In northern climates it is important to bury stems at least 4 to 6 inches, whether it is an ‘own root’ or ‘grafted’ plant (especially true for grafted plants). Shallow planting is likely the greatest reason for failure of a woody peony to grow. By burying a good deal of stem, the woody peony will be able to grow new roots from its subsurface stems and this also supplies a measure of protection from loss of above ground parts. Deep planting is especially important with grafted plants, since it places the union between nurse root and woody peony well below the surface where it will not be easily broken. Winters in the northern tier of the United States periodically kill the above ground stems of woody peonies, thus deep planting insures that the plant can regrow from subsurface buds.
Most growers mark a planting depth on the stems of plants they provide, but many of these plants are grown in less rigorous climates and are marked at an overly shallow depth. Use your good judgment to make adjustments to the marking where needed. Woody peony plants supplied through mail order vary widely in size and configuration. Some will have most of their stems trimmed off while others may have quite a bit of stem. Complete stems are a detriment, as the disturbed root systems struggle to support old stems. Old stems typically do not live beyond a year or two after planting anyway and new stems should be encouraged to grow that are more closely in tune with their developing root systems.
To plant a woody peony, a hole should be dug that will easily accommodate the width and depth of the root system. If the root system cannot be easily placed in the planting hole, enlarge it. Plants that are forced into a small planting hole may be frost heaved, killing the plant. Remember that the hole should be deep enough to bury 4 to 6 inches of stem above the root system. Plants may be placed in the hole so that the stem(s) are perpendicular to the surface or angled up to 45 degrees. Either method should ensure that 4 to 6 inches of stem is buried. Fill the planting hole with loose soil to approximately halfway and add water to settle the soil. Continue to fill the hole until it is a shallow depression and add more water to settle the soil again. Once all water has soaked in fill the remaining depression, firm and add a bit more soil so that the area is slightly higher than the surrounding surface. The elevated area will settle over the next several weeks and more soil may be needed so that it is never lower than the surrounding surface. In most instances further watering is not necessary after planting unless droughty conditions are prevalent.
When the ground freezes in late fall, mulch with 1 or 2 inches of bark or wood chips to prevent frost heaving. Mulching also provides a degree of insulation which is appreciated by newly planted woody peonies.
The following spring the woody peony will likely make rather poor looking growth and any above ground portion of the stems may have been winter killed. Loss of these stems should not be of concern, since the plant should grow new stems from below ground buds (if it was planted deeply enough). These new stem shoots are almost always more vigorous than old stem growth and will be the stems that are most productive in the future. Sometimes newly planted woodies will bloom in their first year, but be patient, as this does not often happen.
Keep new plantings well-watered, but never wet while root systems develop. Plants may produce a bloom or two in their second season of growth, but are more likely to bloom in year three of establishment.
Pruning: Stem care of woodies.
Woody peonies may need to be pruned for a variety of reasons, but often require little mandatory pruning. Gardeners may expect the stems of woody peonies to last many years, but truth be told, the average age of most stems is only from 1 to 4 years. Rockii cultivars will have the longest lived stems and lutea hybrids the shortest lived. Warmer climates will experience longer lived stems than those of colder areas, but stem age has little to do with overall plant longevity. Pruning and keeping the woody stems in a ‘young’ state will create a more vigorous and better looking plant. Pruning is a major factor in keeping woody peonies healthy and vigorous.
1) Old stems that have been winter damaged, killed or have simply lost vigor are best cut away in the spring. These stems often appear to have less vigorous growth, leaves are small or misshapen and may wither as the summer heat builds. Use a pruning shears or arborist’s saw to cut these stems off close to the ground. They will be replaced in coming years by fresh growth from below or near ground level. Prune in spring or early summer to address this problem.
2) Diseased stems often present themselves in spring during cool and wet periods, but may also show up as the summer progresses. Simply remove these stems with a pruning shears or arborist’s saw below the diseased stem area. It is always wise to cut away more than might be needed to arrest disease progression. These stems will also be replaced in the coming years. Prune as need arises.
3) Some woody peonies produce a profusion of new growth that is close together, creating a situation of poor air movement within the plant. This can be especially problematic with P. suffruticosa cultivars, which easily develop stem diseases in these conditions. Removal of some of the stems so that light may be seen through the plant is advised. Lutea hybrids seldom have disease issues from dense growth, but individual cultivars all have their own tolerances, thus observation will be needed to determine pruning needs within this group. Rockii cultivars typically produce growth that is well ventilated, thus little pruning is needed to keep them healthy. This type of pruning can be done in late spring after bloom.
4) Woody peonies can also be pruned to create a more pleasing shape to the overall shrub. Long branches can be trimmed back to desired lengths and unwanted branches and stems removed. Keep in mind that P. suffruticosa and P. rockii cultivars primarily bloom on two year and older stems, thus removal of stems can inhibit flower production. This type of pruning is best done in early fall.
5) Lutea hybrids often have shorter lived stems and stems that are easily killed during cold winter seasons. During the winter of 2018/2019 our lutea hybrid field experienced 100% stem loss on these plants. All plants were cut to the ground in the spring. Plants made a profusion of growth later in the spring, only to produce one of the best flowering seasons in years. Lutea hybrids treated in this manner don’t always respond with outstanding flower production, but bloom can be expected in most instances.
6) In some areas of the United States a number of boring insects use woody peony stems as homes and can cause significant damage in pruned plants. Gardeners in these areas should use a bit of paint or nail polish to cover the ends of cut stems, thus preventing entry of these pests.
7) Pruning can be used to rejuvenate old woody peonies that have lost their vigor. Old woody plants that have only a single stem (trunk) or two should never be cut completely to the ground. Removal of 2/3rds of the stem length often promotes new basal growth and then a year or two later the remaining 1/3rd of the old stem(s) can be removed to allow the new growth to take over.
8) Pruning plants heavily in the fall almost always promotes large amounts of new growth the following year on healthy plants. The practice of cutting plants down in the fall is a tactic used to promote the growth of high quality stems for grafting the following year. Large numbers of vigorous basal shoots with terminal buds are often produced, which are the best candidates for grafting.
The ‘woodies’ are relatively newcomers to American gardens and there is yet much to be learned about their culture. Each climate and garden situation will likely present its own set of variables. The woody groups and individual cultivars also add complexity, but also provide options for gardeners. We have found the woody peonies to be as easily grown as the herbaceous peonies, but the diversity of plant habit greater. All that is needed to grow this wonderful group of peonies is patience and proper selection.