Summersweet clethra, Clethra alnifolia, is a native plant success story, a shrub valued for its lustrous foliage, sweet-smelling flowers and golden yellow fall foliage. In the wild it grows to 10 feet in height and produces spicy, fragrant flowers in late summer.

Also called sweet pepperbush for its peppercornlike fruit, white alder for the similarity of its foliage to that of the true alders, and “poor man’s soap” because the flowers produce lather when crushed in water, summersweet has been in cultivation for more than 200 years. It is an essential plant in the pollinator garden, attracting butterflies, bees and hummingbirds, and it is ignored by deer, a draw for many Maine gardeners.

The garden popularity of summersweet clethra is no doubt because of its fragrant summer blooms. Borne in dense upright and narrow spikes, the white flowers can fill the summer air of the garden with a spicy scent for several weeks in late July and August. Following the bloom are small, rounded seed capsules, each one-eighth-inch in diameter and containing several seeds, all packed into the same dense spikes of their forebears. These tan-colored capsules persist into autumn, gradually darkening in color and adding textural depth to the yellow fall foliage.

Overall fall color varies from light yellow to gold, but to stop there does not do it justice. On close inspection, each golden leaf is spotted with blotches of bronze to give the leaf the look of tarnished brass.

Many new cultivars of summersweet have been introduced in recent years, including several varieties with pink flowers and others with growth habits more compact than the species. However, they are not all equal in terms of potential for winter dieback. The ‘Hokie Pink’ in Marjorie’s garden suffered severe dieback last winter, yet responded with new growth in spring and now are flowering. This cultivar has the potential to reach 5 to 10 feet in height, but will never do so if it is knocked back every winter – our flowering plants are barely 3 feet tall. Over recent years, several gardening friends have experienced winter dieback with various clethra cultivars.

Longwood Gardens in Kennett Square, Pa., recently completed an evaluation of 14 cultivars, 11 of which originally were selected for outstanding foliage and flower characteristics. A valuable component of this evaluation was comparison of cultivars with respect to winter dieback. Their findings are summarized in the September 2007 issue of The Avant Gardener (Horticultural Data Processors, Box 489, New York, NY 10028).

The Longwood trials indicate that there are significant cultivar differences in the potential for winter dieback. For example, ‘Hummingbird,’ a white-flowering cultivar popular for its compact growth habit, received a lower evaluation rating because of periodic dieback and persistent dead branches. Other poor growers among the white-flowering cultivars in the tests included ‘Sixteen Candles’ and ‘White Dove,’ the latter also suffering severe winter dieback. The best of the whites was ‘Tom’s Compact’ (originally sold as ‘Compacta’), which grows only 3.5 feet tall with superior foliage.

Among the pink-flowering cultivars, overall differences were not as pronounced, with all pink varieties receiving approximately equal ratings. No major differences in dieback potential among the pink cultivars were mentioned in the Avant Gardner summary of the Longwood study.

All of this leaves me seeking more information about the winter hardiness of the pink-flowering clethras. If any of my readers can shed light on the matter, it would be appreciated.

Send queries to Gardening Questions, P.O. Box 418, Ellsworth 04605, or to rmanley@ptc-me.net. Include name, address and telephone number.