Horticulture Highlight: Clethra alnifolia

November 28, 2011

“Warm summer sun,
Shine kindly here
Warm southern wind,
Blow softly here”

-Mark Twain

This year countless flowering plants have been luxuriant, if early, in their floral displays thanks to plentiful sunshine and the abundant early spring rainfall.  During August many herbaceous perennials and annuals enhance our landscape but there are fewer trees and shrubs that flower this late in the summer. One exceptional flowering shrub is Clethra alnifolia, summersweet which is also known as sweet pepperbush.

The genus Clethra is comprised of over thirty species of deciduous and evergreen shrubs and small trees found in temperate and neo-tropic zones of Asia, Malaysia, Central America, South America, and North America. This is the only genus within the family Clethraceae.

Clethra alnifolia is a deciduous shrub attaining a height of between 3-8 feet, with a native range from Nova Scotia to Florida. Mount Auburn’s visionary founder, Dr. Jacob Bigelow (1787-1879) Lot 116, Beech Avenue, writing in his seminal flora of New England, Florula Bostoniensis, (1824, second edition) timelessly described this species as “A tall, elegant, white flowering shrub. Leaves about three inches long, and from one to two broad, inversely ovate [fattest above the middle], serrate [toothed margin], downy underneath in one variety, glabrous [not hairy] in another. Flowers in long racemes or loose spikes with downy stalks.”

Bigelow’s taxonomic description is herein coupled with a coincidental William Shakespeare (1564-1616) observation, “The summer’s flower is to the summer sweet.” Our summersweet’s 2-6 inch long racemes are densely composed of numerous five-petaled, white flowers, each 1/3 inch across. Best of all these bottlebrush-shaped flowers are delightfully fragrant, sweetening the air well in advance of your approach. The shrub is not covered with blossoms all at once but produces flowers over a four to six week period that are highly attractive to bees, butterflies, and hummingbirds.  When fertilized these flowers produce tiny, dust-like seeds contained within multiple peppercorn-sized, dry capsules that allude to its sweet pepperbush common name. These dry capsules persist on the plant and provide a great winter identification characteristic.

Summersweet naturally is found in moist and swampy habitats, although not exclusively, in shady to full sun situations. Here at Mount Auburn there are approximately four dozen of these plants including a selection of compact cultivated varieties, located at many sites. Halcyon Lake, Auburn Lake, Willow Pond, the former site of Alice’s fountain, and Consecration Dell are a few of these habitat correct locations. At Consecration Dell, a glacial kettle hole with a rare urban vernal pool, summersweet is one of the many native woodland species the horticulture department has planted to help restore a sustainable ecologically appropriate plant community.

On your next visit to Mount Auburn seek out and enjoy the summersweet blossoms and while you’re here give a well-deserved thank you to the dedicated horticulture workers you encounter.

 *This Horticulture Highlight was originally published in the August 2010 issue of the Friends of Mount Auburn electronic newsletter.

About the Author: Jim Gorman

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