Spiraea japonica ‘Alpina’ Has Light Pink Flowers Beginning in Late Spring

BY CORINNE KENNEDY

Three dwarf Spiraea cultivars, known as Spiraea japonica ‘Alpina’, blooming in June in the Seattle Japanese Garden’s northeast corner, Area M. (photo: Aleks Monk, 6/20/12) 

I usually write about showy plants that are major features of the Seattle Japanese Garden. This month’s article, however, is about a sweetly charming but much less dominant plant that serves instead as part of the Garden’s essential “supporting cast.” A deciduous shrub that blooms from late spring to early or mid-summer, it’s a dwarf form of Spiraea japonica (Japanese spiraea). Like evergreen azaleas and other small-leaved shrubs, it’s pruned in a low, mounded, spreading shape known in Japan as tamamono. 

In early 1969, fifty-one 10-inch-tall plants of Spiraea japonica ‘Alpina’ were sent from a Virginia nursery to the Washington Park Arboretum. That May, fifty were planted in the Seattle Japanese Garden. Decades later, only three plants remain, planted in a row on the east side of the Garden’s east-side path. Together they form an attractive wavelike shape known as o-karikomi. 

Botanically, this dwarf shrub is classified in the Rose Family (Rosaceae) and its Northern Hemisphere genus Spiraea, which includes eighty species. All are small or medium sized deciduous or semi-evergreen shrubs with fine-textured, twiggy habits, leaves held alternately on the stem, and pink or white flowers. The genus name Spiraea was derived from the Greek word speira, which means wreath—a reference the showy flower clusters typical of the genus.  

Spiraea japonica, an important species: 

Spiraea japonica, one of the most important species in its genus, is a medium-sized deciduous shrub native to China, the Korean Peninsula, and Japan, where it’s common in the mountains of Hokkaido, Honshu, Shikoku, and Kyushu. Extremely variable throughout its native range, Japanese spiraea is generally described as a relatively dense, upright shrub with a rounded habit, growing four to six feet tall, with a wider spread. The leaves are oval, sharply toothed, and 1-3 inches long. Attractive to butterflies, the small flowers are rose-pink to deep red, but rarely white. They appear in flat-topped clusters (corymbs)  from late spring to mid-summer. Sparse repeat blooming may occur. This species is not considered invasive in our region, but it can spread by suckering and/or self-seeding, and has naturalized invasively in some areas of the eastern U.S. 

Woody plant authority Michael Dirr emphasizes that “this wide geographic range no doubt offers wonderful opportunities for the selection of different forms.” And in fact, Spiraea japonica is the spiraea species that has resulted in the greatest number of cultivars, developed not only in Japan but also throughout the temperate world. Popular here, they are valued for their versatility, toughness, reliability, and ease of care. They have rounded habits and attractive foliage colors ranging from green to yellow-gold and variegated color blends. Flower colors include pale pink, dark red-pink, and deep purple-pink. Blooming on new wood, older plants benefit from rejuvenation pruning in late winter or early spring. When flowers are deadheaded, some repeat blooming may occur. 

In Japan, this species and its cultivars are used for bonsai and planted in shrub borders, and their flowering branches are used in flower arranging (ikebana). The Japanese word for the species is shimotsuke, the feudal name of the province north of Tokyo where it was discovered. 

The Seattle Japanese Garden’s dwarf cultivar: 

Our Garden’s plant has been described as an excellent dwarf shrub that forms a low-growing, compact mound of foliage, maturing to about two feet tall and up to five feet wide. Commonly known as daphne spiraea or alpine spiraea, it has small, oval, sharply toothed, blue-green leaves. Its tiny light pink flowers, held in flat-topped clusters, are abundant from late spring to early or mid-summer. When grown in sufficient sun, the foliage develops attractive red and orange fall color.    

Unfortunately, our dwarf plant’s origin and botanical name remain somewhat controversial. It was first described in 1879 from a plant discovered on Mt. Hakone, on Japan’s central island of Honshu. The botanical name Spiraea japonica var. alpina, which indicates a dwarf, naturally occurring variety of a wild-growing plant, would seem to be appropriate—and is still used by some growers. However, Professor Dirr asserts that experts mostly agree that the plant currently under cultivation is instead a cultivar (that is, a “cultivated variety,” grown from cuttings, not seeds—sometimes known as a plant “of garden origin”).  In addition, unlike a naturally occurring plant, this dwarf form does not come true from seed. Its self-sown seedlings have larger leaves and more open habits than the parent plant. 

Thus, two cultivar names—Spiraea japonica ‘Alpina’ and Spiraea japonica ‘Nana’—are generally used to identify the dwarf spiraea growing in our Garden. [Despite the assertions of some authorities, Dirr believes that ‘Nyewoods’ is a distinctly different plant.]  The Washington Park Arboretum’s 1969 Accession card lists the cultivar name ‘Alpina’, and in the intervening decades the name on that card has never been revised . 

The flower clusters of Spiraea japonica ‘Alpina’ in Area M of the Seattle Japanese Garden. (photo: Aleks Monk, 6/20/12) 

Spiraea japonica ‘Alpina’ is hardy to USDA Zone 4 (minimum temperature -30 degrees Fahrenheit). It’s easily grown in the Pacific Northwest in full sun or light shade and rich, moist, well-drained soil, but it tolerates a range of soil types. It’s appropriate for small residential gardens as well as public gardens and parks, and works well as a foundation plant, at the front of mixed borders, at woodland edges, and as a low hedge. With regular pruning, it’s a lovely specimen plant for a rock garden or rockery. 

When well-grown, ‘Alpina’ has no serious insect or disease problems. However, spiraeas in general are somewhat susceptible to diseases and insects that attack plants in the Rose Family—including leaf spot, powdery mildew, root rot, aphids, leaf rollers, and scale insects.  

This dwarf shrub takes well to pruning and shaping, and it responds well to light shearing—to remove faded flower clusters and encourage repeat bloom, or to remove the brown seed heads that plants do not shed. Flowering on new wood, it can be pruned in late winter or early spring to rejuvenate unattractive plants or simply to manage their size and shape. Our Garden’s three plants have been well maintained by appropriate pruning and remain attractive after more than fifty years. 

Spiraea japonica ‘Alpina’ is a charming plant that contributes to the Seattle Japanese Garden’s harmony of shapes, textures, and colors. As the Garden transitions from spring’s bold flower colors to summer’s rich shades of green, I’m grateful for its subtle loveliness. Unfortunately, so many cultivars of Spiraea japonica have been developed that ‘Alpina’ may no longer be available locally. Interested gardeners should look for it online, searching under both cultivar names—‘Nana’ and ‘Alpina’—as well as under the name Spiraea japonica var. alpina. 

Corinne Kennedy is a Garden Guide, frequent contributor to the Seattle Japanese Garden blog, and retired garden designer.