A novel native for landscapes

There is increased interest in using native shrubs for landscaping because they are adaptable to local climates, attract pollinators and support wildlife. One native shrub that has received attention due to its showy pink floral display in summer is Spiraea tomentosa. The common name for this plant, steeplebush, is derived from the steeple-shaped flower panicles that form at the ends of upright or gently arching shoots. While colorful flowering shrubs are certainly in vogue, steeplebush has the added benefit of being relatively unpalatable to deer, making it a useful plant in areas with high deer pressure.

Steeplebush is native to eastern North America, from Newfoundland to North Carolina and west to Wisconsin and Tennessee. It can be found in small pockets at higher elevations in Tennessee, Kentucky and Delaware. In New England, plants are found in abandoned fields, wet meadows, swamps, at the margins of lakes and along roadsides. Plants are 2 to 4 feet tall and produce slender, upright stems that sucker to form thickets. In the wild, steeplebush provides food and habitat for birds like ruffed grouse and small mammals like cottontail rabbits. Steeplebush is desirable for its long-lasting, bright pink flower panicles in mid-July through August. Individual flowers are small, but they are densely clustered in narrow, pointed spikes that range from 5 to 8 inches long. The blooms open from top to bottom, and once the first blooms are pollinated, a task mostly accomplished by bees, then smaller side shoots will bloom, providing a lengthened period of color. Butterflies and beetles also visit the flowers for nectar. Fruits consist of a dried capsule and ripen to a tan color in late summer and then turn dark brown in winter.

Dried infructescences have great texture and are long lasting, and can be used for making interior arrangements. Leaves are green with a bluish cast, 1 to 3 inches long and oval shaped with toothed margins. The entire plant is densely coated with short, reddish brown hairs or pubescence, also called tomentum, which gives steeplebush a distinctive look that is quite attractive. The presence of tomentum, from which the plants’ Latin epithet is derived, is most prominent on the undersides of leaves where it develops a yellowish gray coloration.

In general, spireas are known as easy-to-grow plants, and steeplebush is no exception. Steeplebush grows best in sites with full sun exposure, but it will tolerate partial shade. Plants grow fast and can establish in a variety of soil types. Steeplebush is often sought out for sites with moist soils and for habitat restoration, meadow planting and naturalizing projects. However, steeplebush has demonstrated adaptability to drier soils in cultivation. I’ve planted this shrub in parking lot islands on the University of Connecticut campus and it has performed well in the dry, compacted and infertile soil conditions.

Steeplebush may be used in masses or in the shrub or mixed perennial border. I have seen it used effectively in repetition to accentuate the outward face of a split rail fence. Due to its suckering habit, steeplebush may be used to stabilize sunny slopes. Occasionally, landscape plants will develop mild symptoms of powdery mildew.

Spireas can suffer stem dieback in winter. While plants will re-sprout vigorously from the base, residual dead stems and seed infructescences can contribute to an unsightly appearance. A study looking at steeplebush and its cousin meadowsweet (Spiraea alba) conducted in Illinois suggested that hard pruning of plants in late winter or early spring could benefit plant appearance. We evaluated how pruned plants compared to un-pruned plants as far as plant appearance, plant form and flowering in a landscape situation using steeplebush plants installed in the UConn parking lot.

Pruning was done in late April, before any noticeable signs of leaf expansion. On plants that were pruned, all stems were cut to 6 inches, while un-pruned plants were on average 44 inches tall. By the end of the growing season, un-pruned plants did not increase in height beyond 44 inches. Un-pruned plants produced an average of 35 flowers per plant and flowers were 5.5 inches long. Peak bloom for un-pruned plants was the second week of July.

Pruned plants produced roughly equivalent height and width measurements of 28 inches and 24 inches, respectively, giving plants a more full and symmetrical appearance than un-pruned plants. So, even though pruned plants did not grow as large as un-pruned plants, they had greater aesthetic quality. Pruned plants produced 10 fewer flowers per plant, but each flower was more than one inch longer, and since they were held on a more compact plant, the effect was an outstanding floral display. One side effect of pruning is a delay in peak flowering of 7 to 10 days.

Another benefit of pruning is the removal of persistent infructescences from the previous year. Residual infructescences on un-pruned steeplebush plants detracted from overall plant appearance throughout the season. Furthermore, as infructescences build up over multiple seasons on un-pruned plants, steeplebush can become unsightly.

Steeplebush is a useful native flowering shrub, and proper pruning management of plants can help improve their appearance in landscapes.

Jessica is an assistant professor of horticulture at the University of Connecticut. Follow her on Facebook at Native Plant Gal.