Summersweet Clethra

Prepared by Joey Williamson, HGIC Horticulture Extension Agent, Clemson University. (New 8/10.)

HGIC 1090

Printer Friendly Version (PDF)

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 (Clethra alnifolia) inflorescence with white buds and fragrant white flowers
Summersweet clethra (Clethra alnifolia ) inflorescence with white buds and fragrant white flowers.
Joey Williamson, ©2010 HGIC, Clemson University

Summersweet clethra is a 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.

Clethra alnifolia in full bloom in July
Clethra alnifolia in full bloom during July.
Joey Williamson, ©2010 HGIC, Clemson University

Description

The inflorescence, or flower cluster, 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.

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.

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.

Propagation

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.

Rooting hormones containing IBA, such as Green Light Rooting Hormone, Schultz Take Root, Miracle-Gro Fast Root Rooting Hormone or Ferti-lome Rooting Powder will enhance the number of cuttings that will root. 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).

Cultivars

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 color, and the flowers hold their color well. This cultivar can reach 6 to 8 feet in height.

Clethra alnifolia Ruby Spice inflorescence with dark rose-pink buds and flowers
Clethra alnifolia ‘Ruby Spice’ inflorescence with dark rose-pink buds and flowers.
Joey Williamson ©2010 HGIC, Clemson University

Clethra alnifolia Ruby Spice in bloom
Clethra alnifolia ‘Ruby Spice’ in bloom .
Joey Williamson ©2010 HGIC, Clemson University

  • ‘Pink Spires’ has pink buds that open into flowers that are a soft pink. This pink cultivar also reaches 6 to 8 feet tall and has 3½- to 4-inch- long inflorescences.

Clethra ainifolia Pink Spires with pink buds and very pale pink petals
Clethra alnifolia ‘Pink Spires’ with pink buds and very pale pink petals.
Joey Williamson ©2010 HGIC, Clemson University

  • ‘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.

Clethra ainifolia hummingbird a dwarf, white summersweet clethra selection
Clethra alnifolia ‘Hummingbird’: a dwarf, white summersweet clethra selection.
Joey Williamson ©2010 HGIC, Clemson University

  • ‘Sixteen Candles’ is another dwarf selection. It was found as a seedling from ‘Hummingbird’ and is also a dwarf and quite floriferous. This white cultivar tends to hold its inflorescences in a more upright habit than ‘Hummingbird’, whose flower clusters tend to splay.

Page maintained by: Home & Garden Information Center


This information is supplied with the understanding that no discrimination is intended and no endorsement of brand names or registered trademarks by the Clemson University Cooperative Extension Service is implied, nor is any discrimination intended by the exclusion of products or manufacturers not named. All recommendations are for South Carolina conditions and may not apply to other areas. Use pesticides only according to the directions on the label. All recommendations for pesticide use are for South Carolina only and were legal at the time of publication, but the status of registration and use patterns are subject to change by action of state and federal regulatory agencies. Follow all directions, precautions and restrictions that are listed.