I've often heard that peonies are the traditional flowers for cemetary decoration on Memorial Day. Mom's peonies did always seem to be in bloom at just the right time. My peonies barely bloomed at all this year. What have I done wrong? Perennial herbaceous peonies are incredibly long lived plants with impressive fragrant blooms, when they bloom. When peonies don't bloom, they are simply long lived plants with dark green, pseudo-shrubby, foliage. These traditional garden favorite plants are incredibly tough in their own way. Their extraordinary flowers can be hit or miss unless certain conditions are met.
Photo: P. officinalis 'Rubra Plena'; photo by Kurt Stüber at Wikimedia Commons
Peonies like sun. Sun energy is one of the ingredients in the perennial peony's recipe for bountiful blooms. Plant peonies where they'll get six good hours of sun each day. As with many flowering plants (and gardeners), some relief from a hot afternoon sun leads to a happier specimen. Morning and mid-day sun is preferred, especially for peonies grown in the marginal warmer zones (see "Planting depth" below.)
Peonies fare well in clay soils, unless those clays are boggy and poorly drained. Clay particles are naturally able to grab fertilizer and give it up to plants. Sandier soils allow fertilizer to filter away with rain and watering. Gardeners in clay soil regions might have good peony bloom with little effort at fertilizing. Most peonies are happier, and thus bloom abundantly, when well fed. Use a balanced fertilizer formulated for flowering plants. Fertilizing in spring, just as the stems are starting to grow, ensures that the fertilizer is readily available when most new leaf growth takes place. Top dressing lightly (see "Planting depth" below) with compost is always good. The Gardener's Guide to Growing Peonies recommends fall fertilization. Peonies preferred soil pH is neutral, 6.5 to 7. If your native soil is far from that, the peony cannot fully utilize the soil nutrients it needs.
Planting depth is important to many plants but can be critical to peony bloom. Horticulture professor Allan Armitage quotes his grandmother, who observed, "The colder the winter, the better the bloom" in her Montreal garden. Other plants may die outright when sunk too deeply. It seems that while other plants rot when planted incorrectly, peonies may thrive. They just won't bloom. Peonies are a plant of the colder zones, happily shivering through a zone three winter only to burst into glorious spring bloom. Their buds need a certain chill in order to bloom well. Therefore, planting depth varies with your winter cold. The deepest recommended planting is for two inches of soil covering the nubby buds on top of peony roots. Warmer zone gardeners must avoid burying those buds, because the layer of soil keeps the roots from feeling the needed cold. Plant peonies with a scant inch or less of soil in zone six or seven. Herbaceous peonies are a poor choice for gardens with tropically hot summers. In all zones, use care in topdressing and mulching, so that the correct depth remains.
Herbaceous peonies have been bred into many cultivars (named varieties.) The most popular, and beautiful, Paeonia cultivars these days are derived from Asian stock. Prior to the introduction and hybridization of the splendid P. lactiflora from Asia, European gardeners used a number of selections of their native P. officinalis. One cultivar, 'Rubra Plena', is widely cited as "The Memorial Day Peony." One of the earliest blooming peonies, its flowers are on display in many United States gardens on Memorial Day, the last Monday in May. 'Rubra Plena' is a robust cultivar which produces double flowers opening as a deep red. The medium pink 'Rosea Plena' and the white 'Alba Plena' are also called Memorial Day Peony by some sources. However, in lieu of a 'Rubra Plena', one of the many available cultivars of herbaceous peony may be the right one to come into flower at the chosen time in your garden.
Herbaceous peonies are long lived, rugged plants with flowers that amaze both eyes and nose. Few pests or diseases plague peonies. It would be a disservice to the peony to forgo blossoms for want of simple care. As a final note, bear in mind that conditions for a given plant in your garden can change over the years. As Toni Leland points out in Peerless Peonies, soil fertility and sun exposure may change over time and affect the peony's ability to flower.
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Armitage, Allan M. Armitage's Garden Perennials (Secnd Edition, Fully Revised and Updated) Portland, Timber Press, 2011. ISBN 0-88192-435-0
Page, Martin. The Gardener's Guide to Growing Peonies. Portland, Timber Press, 1997. ISBN 0-88192-408-3
Wikimedia Commons Photo by Kurt Stüber, http://commons.wikimedia.org/wiki/File:Paeonia_officinalis_officinalis0.jpg, accessed 5-31-2012