Summersweet clethra (Clethra alnifolia) is a fantastic shrub with all-season garden interest and can be grown throughout South Carolina. This native species, also called sweet pepperbush, flowers profusely for 4 to 6 weeks during July and August when few other plants are in bloom, and the flowers fill the garden with their spicy fragrance.
Summersweet clethra is a native, deciduous upright shrub that typically grows between 4 and 8 feet tall and slowly spreads by sending up new shoots from rhizomes to form a small thicket. The species or taller selections can be grown as low hedges or as back of the garden border plants. Smaller cultivars combine well with perennials in the garden.
The inflorescence, or flower cluster (botanically called a raceme), is white on the species, varies from 3 to 6 inches long, and is extremely fragrant. The flowers are attractive to bees and butterflies, and the resulting dark brown seed capsules will attract birds during the autumn. Bloom time in the Upstate of South Carolina is mid-July through early August. Flowers form on new growth. Therefore pruning can be done in winter.
Summersweet clethra is late to leaf out in the spring, and the foliage is an attractive medium to dark green color. Leaves are alternate, typically 1 to 2½ inches long and give the plant what is considered to be a medium texture. Fall color can be a very attractive golden yellow to golden brown. Summersweet clethra typically grow from 5 to 8 feet tall and 4 to 6 feet wide.
Cinnamon clethra (Clethra acuminata) is also a native clethra and grows over the Appalachian mountain range. It has long racemes of ivory white flowers, and grows to 6 to 10 feet tall and wide.
Clethra grows best in acid, moist, well-drained soils. For planting in heavy clay soils, it is best to add a soil amendment, such as composted, ground pine bark, to enhance soil drainage. Dig planting holes at least twice as large as the root ball or container, and amend the existing soil with a soil conditioner at no more than 20% by volume. Once established, plants will tolerate short periods of drought.
This native plant will grow in full sun to total shade, but will grow best in light, dappled shade, or with morning sun and afternoon shade. These shrubs will even bloom profusely in complete shade.
Because of its tolerance for moist soils, it can be used for streambank erosion control. These shrubs sucker, which means they will slowly spread as new trunks emerge from the roots to increase the size of the planting. Clethra are considered deer resistant.
Clethra alnifolia is easy to propagate from stem cuttings in early summer or from seed planted in fall or spring.
Softwood stem cuttings are best taken in early summer for the greatest success in rooting. Take 3 to 4 inch stem cuttings in the morning from well-watered plants. Cuttings taken in the early summer may not require a rooting hormone for 90 to 100% success in 4 weeks. However, cuttings taken later in the summer will root with more difficulty and may require a rooting hormone treatment. Using rooting hormone containing IBA will enhance the number of cuttings that will root. Examples of powdered products in 2 ounce bottles with 0.1% IBA are:
- Green Light Rooting Hormone,
- Schultz Take Root,
- Miracle-Gro Fast Root Rooting Hormone
- Ferti-lome Rooting Powder
- Bonide Bontone Rooting Powder
- Garden Tech Root Boost
- Garden Safe Take Root Rooting Hormone
Liquid rooting hormones, such as Dip ‘N Grow, may also be used, as the liquid form allows it to be more readily absorbed into hardwood cuttings.
Use a well-drained medium to start the cuttings, such as a half and half mix of potting soil and perlite. The relative humidity must be kept high around the cuttings, so containers must be covered with glass or plastic, such as enclosing the container or pot in a clear plastic bag or using part of a plastic soft drink bottle as a dome over the soil. A cutting can root and grow to fill a one gallon container by the end of the season. Summersweet clethra seed can be harvested from the ripe, brown seed capsules and planted in the fall or the next spring without the need for a cold moist treatment (stratification).
There are several cultivars of summersweet clethra that are available at garden centers and through mail order catalogs. These cultivars are different from the wild form, which is white and tall, by either being more compact and floriferous (making more flowers) or by having pink buds and flowers. Some of the more commonly available cultivars are listed below.
- ‘Ruby Spice’ has 3½- to 4-inch-long flower clusters or inflorescences that are a rich rose, and the flowers hold their color well. This cultivar can reach 6 feet in height and width, and is one of the darkest pink selections.
- ‘Pink Spires’ has pink buds that open into flowers that are a soft shell pink. This pink cultivar reaches 3 to 6 feet tall and wide, and has 3½- to 4-inch-long inflorescences.
- ‘Hummingbird’ is a dwarf cultivar that grows 30 to 40 inches tall, and will be covered with 4- to 6-inch long clusters of fragrant white flowers. It has a good yellow fall color.
- ‘Sixteen Candles’ is another dwarf selection. It was found as a seedling from ‘Hummingbird’, is compact, and quite floriferous. This white cultivar tends to hold its inflorescences in a more upright habit than ‘Hummingbird’, whose flower clusters tend to splay. Floral racemes are 4 to 6 inches long with white flowers.
- White Dove™ (‘Sotite’, PP#15505) grows to only 2 to 3 feet tall and wide, has abundant white flowers.
- Sugartina® (‘Crysalina’ PP#21561) is Proven Winner® selection. It is a dwarf clethra with a tight dense shape and pure white flowers. It grows to 28 to 30 inches tall and 28 to 36 inches wide. The fall color is yellow.
- ‘Anne Bidwell’ produces white flowers that are larger than the species. Plants grow to 4 to 6 feet tall and 3 to 5 feet wide. The fall color is golden
- ‘Rosea’ is an older cultivar with pink flower buds that open pink and fade to nearly white. This cultivar grows to 3 to 6 feet tall and 3 to 5 feet wide. It has yellow to golden brown fall color.
- ‘September Beauty’ flowers with white blooms up to two weeks later than the species. The plants grow to 4 to 6 feet tall and 3 to 4 feet wide.
- Vanilla Spice® (‘Caleb’, PP#21589) is a Proven Winners® selection, and has extra-large white flowers. The plant grows 3 to 6 feet tall and 3 to 5 feet wide.
- First Snow® (‘Takeda Nishiki’) is a cultivar of Clethra barbinervis, and is hardy from USDA zones 6 to 9. This is a large cultivar that grows to 10 to 20 feet tall and 10 to 15 feet wide.
Summersweet clethra is generally disease free, but with late season rainfall, foliage may become infected with Pseudocercospora leaf spot. Because of its late season occurrence, fungicide sprays are generally not necessary. Rake and dispose of fallen leaves. Add additional mulch to cover any remaining debris.