As I write on this mid-March day, a pile of thick, crusty snow lies on the ground while the sky continues to shower the Arboretum with more of the icy mess. It has been a long winter. But the longer days give me hope that spring is just around the corner and soon we will see the blooms of old botanical friends. One of these is Spiraea prunifolia var. simpliciflora, a delicate spirea collected as seed by John George Jack in 1905 during his trip to Korea (for more about this extraordinary plantsman, see the article starting on page 2 of this issue). Jack’s original plant (accession 18283-A) still grows below a canopy of hickory trees along Valley Road. An earlier accession of the same species (accession 3138) came from the Royal Botanic Gardens, Kew, in 1887 and still grows in the Bradley Rosaceous Collection. This spirea is native to eastern Asia and grows wild throughout China, Korea, and Japan. Ernest H. Wilson had also collected the species in Korea, but that accession perished long ago.
S. prunifolia var. simpliciflora is a fine-textured shrub that functions well as a single specimen, group planting, or in the mixed border. It reaches 5 to 6.5 feet (1.5 to 2 meters) tall and at least as wide. When allowed to reach its full size, the long stems grow gracefully from the center and arch up and away to create a vase shape. Cold hardy through USDA Zone 5 (average annual minimum temperature -10 to -20°F [-23 to -29°C]), plants grow well in full sun to partial shade and tolerate most soils.
The dark green leaves are small, reaching 1 to 2 inches (2.5 to 5 centimeters) long and about ½ to ¾ inch (1.3 to 2 centimeters) wide. They somewhat resemble those of cherries (Prunus), which can be divined from the specific epithet prunifolia. The foliage develops gold to orange tones in the autumn, brightest when plants are grown in full sun.
But it is the flowers that are the most interesting part of this horticultural and botanical story. The type variety of this species is Spiraea prunifolia var. prunifolia, commonly known as bridalwreath spirea. (A type variety is the botanical representative for the species and is indicated by an autonym, which duplicates the specific and varietal epithets—prunifolia var. prunifolia in this case.) Interestingly, the flowers of Spiraea prunifolia var. prunifolia are double, resembling tiny white pompons, and are produced in great quantities along the stems. This botanical variety does not represent a wild-occurring plant, but rather a horticultural oddity that is sterile, something we might now select and identify as a cultivar (cultivated variety). When Siebold and Zuccarini discovered and named the species (S. prunifolia) in the mid-nineteenth century they were looking at the double-flowered form, long cultivated as an ornamental in Japan. Following the protocols of plant naming, this variant became the original type variety, which meant that when Japanese botanist Takenoshin Nakai later described the plants that grow wild—those with simple, five-petaled, and fully fertile flowers—he had to give it a different varietal name: S. prunifolia var. simpliciflora (in this case, the varietal name translates literally to “simple flower”).
Though unusual, this is not the first time that a horticultural variety was designated as the botanical type. Other examples in which the species types are represented by double-flowered oddities include Viburnum plicatum and Rhododendron yedoense. In order to describe and name the single-flowered forms once they were discovered in the wild, new botanical varieties were created—V. plicatum var. tomentosum and R. yedoense var. poukhanense in these cases.
Spiraea prunifolia typically starts flowering in late April. The type variety with sterile double flowers (var. prunifolia) can bloom for up to three weeks, whereas the blooms on the single-flowered variety (var. simpliciflora) last a shorter time. This is because the petals on the fertile flowers of var. simpliciflora wither and drop after pollination. This spring, the blanket of white flowers on both varieties will provide a welcome change from winter’s blanket of snow.
Michael S. Dosmann is Curator of Living Collections at the Arnold Arboretum.