Maine gardeners find Summersweet Clethra a native-plant success story

Those of us who extol the value of native plants are not delivering a new message. As a reminder of this fact I keep at hand two books: “Ornamental American Shrubs” by William R. Van Dersal (1942), and “American Plants for American Gardens” by Edith A. Roberts and Elsa Rehmann (1929). Both still retain that musty odor familiar to those who love old books. Both recommend Summersweet Clethra (Clethra alnifolia) to the gardens of their time.

C. alnifolia is also called sweet pepperbush (for its peppercornlike fruit), white alder, and (my favorite) “poor man’s soap” because the flowers, if crushed in water, produce lather. In Maine it is found growing in swamps, around lakes and streams, in wet woods and thickets. Despite an apparent preference for damp soils, it thrives in the well-drained soils of most gardens. It is a medium-size shrub, growing to about 10 feet in height, with brown bark and alternate, simple, toothed leaves similar in appearance to alder leaves (explaining the specific epithet). Individual plants are often narrow and straggly but a colonizing habit results in mounded clumps. Clethras should always be planted in groups to maximize this effect.

The garden popularity (and common name) of summersweet clethra is no doubt due to its fragrant summer blooms. Borne in dense upright and narrow spikes, the white flowers are spicily perfumed and can fill the summer air of a small garden for several weeks in July and August. Following the bloom are small rounded seed capsules, each rarely more than 1/8-inch in diameter and containing several seeds, all packed into the same dense spikes of their forebears. These tan-colored capsules persist through autumn and into winter, gradually darkening in color and adding textural depth to the yellow fall foliage.

More needs to be said about the fall foliage color, often neglected by those who write about this native shrub. Overall fall color varies from light yellow to gold, but to stop here does not due it justice. On close inspection, each leaf is also spotted with blotches of bronze to give an overall look of tarnished brass to the entire leaf. Overlaid with the persistent spikes of dark brown seed capsules, the autumn foliage of C. alnifolia is unique and refreshing.

Roberts and Rehmann list C. alnifolia in three chapters, “The Pond,” “The Stream-side,” and “The Seashore.” I have encountered beautiful plantings in less natural settings, however, including mixed borders in the center of the city. Unlike many native species, C. alnifolia adapts well to the modern urban garden. It is cold hardy to USDA Zone 4a, highly shade tolerant, very tolerant of wet or flooded soils, salt resistant, soil compaction resistant, and remarkably free of insect and disease problems. Only nursery-grown containerized plants should be planted, as bare-root transplants do not establish successfully.

If desired, you can look for cultivars that are compact (Compacta, Hummingbird) or pink-flowered (Pink Spires, Rosea) or rose-flowered (Ruby Spice.) Paniculata has larger flower spikes than the species and is exceptionally vigorous. I have also read about (never seen) Creel’s Calico, a rare cultivar with large splashes of creamy white on the foliage. In my opinion, these are not improvements on the species, just different.

From the gardener’s point of view, Clethra alnifolia is indeed a native plant success story, in cultivation now for nearly 100 years. The reasons for its success are apparent: easy to propagate, easy to grow, adaptable to the stressful life of modern gardens and, above all, truly ornamental in both summer and autumn. We would serve ourselves well to keep all of these characteristics in mind as we consider other native species for our gardens.

Send queries to Gardening Questions, P.O. Box 418, Ellsworth 04605, or to reesermanley@shead.org. Include name, address and telephone number.