Though a few species peonies may be found through specialists’ catalogs and seed exchanges, most garden peonies are hybrids. The basic types are herbaceous and tree peonies, both descended from Chinese species. A new third category, the intersectional hybrids, combines the best traits of herbaceous and tree types. All peonies are extremely long-lived plants of significant size; they provide choice cut flowers and are a mainstay of big perennial borders.
Plant peonies in fall, either as bare-root plants or from nursery containers. Ideally, the planting site for peonies should be deeply dug at least several days before planting. Work in plenty of compost, especially in heavy soil, and incorporate a high-phosphorus fertilizer; then allow the soil to settle before planting.
Peonies of all types can be grown in large (18—24-in.) containers. Replant every third autumn in the same or a slightly larger pot, replacing most of the soil when you do.
Tree (actually shrub) peonies produce flowers on permanent woody branches and are chiefly descendants of P. suffruticosa, a 6-ft. shrub. Tree peonies usually start out as grafts on herbaceous peony roots, but in the long run they do best on their own roots. When shopping for tree peonies, look for plants that have already started to form their own substantial root systems.
Set tree peonies with the graft line 6–8 in. below soil surface (the aim is to get the shrubby top graft to root on its own). In cold-winter areas, plant at least 6 weeks before the ground usually freezes; mulch only after the ground has frozen. Plants are unlikely to bloom the first spring after planting, but they should bloom every year after that. with handsomely divided blue-green to bronzy green leaves. Single to double flowers, typically very large (to 10–12 in. across), appear in spring.
These peonies seldom show their full potential until they have spent several years in the garden, but the spectacular results are worth the wait. Catalogs offer named varieties of Japanese origin in white and shades of pink, red, and purple. These are generally semidouble and display their flowers well. Some of the older European hybrids in pink and yellow are fully double,with flowers so heavy they hang their heads. More recent and more expensive hybrids come in yellow, copper, and a coral that approaches orange; all result from crosses of P. suffruticosa with P. delavayi and P. d. lutea. These bear semidouble blooms that face outward and upward.
Tree peonies require much less winter chill than herbaceous peonies. The large flowers are fragile and should be sheltered from strong winds. Snip off blooms when they fade.
After leaf drop in fall, prune to remove dead wood and to control the height of mature plants; do this by removing about half the current season’s growth on the tallest branches that have bloomed, always making your cuts just above an outward-facing side branch.
In coldest climates, protect from winter sun and wind with a burlap curtain.
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