Clethra

clethra

( KLETH - ruh )

At-A-Glance Information

This genus of deciduous shrubs and evergreen small trees (commonly called Sweet Pepperbush or Summersweet) needs partial shade and regular water for best performance. They are native to a variety of habitats, including woodlands, swamps, and rocky areas in eastern and southeastern Asia. Their flowers will attract hummingbirds and butterflies to the garden. Flowers can be creamy white to pink and bloom in mid- to late summertime. Use Clethra in rain gardens, bioswales or along pond or pool edges. Take care when planting Summersweet in the landscape, as plant parts of some species are toxic to humans and pets if ingested.

Clethra Species and Varietals

The genus Clethra includes about 75 species, and is a member of the plant family Clethraceae. The best growing zones for Clethra trees and shrubs vary depending on the species. For more information on how to grow these plants, see the descriptions below. To find your zone, click here: Zone Map.

Clethra acuminata (Mountain Pepper Bush, Cinnamon Clethra, Cinnamonbark Clethra)
This species is native to the Appalachian Mountains in Pennsylvania, West Virginia, Virginia, North Carolina, South Carolina, Georgia, Alabama, and Tennessee. These understory shrubs can reach up to 20 feet tall and have foliage that is concentrated towards the ends of branches. Flowers are white and rich in nectar, blooming in June through August. Bark on older plants peels and splits in thin sheets, revealing cinnamon-colored wood underneath. This species grows best in USDA hardiness zones 5-8.

Clethra alnifolia (Coastal Sweetpepperbush, Summer Sweet, Common White Alder)
This species is native to eastern North America and is a wetland or bog-dwelling deciduous shrub that reaches 9-10 feet tall at maturity. The species grows well in USDA hardiness zones 3-9. Fragrant flowers are white to pink, and are favorites of native bumblebees. The specific epithet 'alnifolia' refers to the fact that the foliage looks like that of Alder trees. These plants are typically used as ornamental garden shrubs and can be valuable as erosion-control plants in sloped areas. Several cultivars have been developed, including 'September Beauty', 'Ruby Spice', 'Sixteen Candles' and 'Hummingbird'.

Clethra arborea (Lily-Of-The-Valley Tree)
This evergreen tree grows best in USDA hardiness zones 8-10 or Sunset growing zones 15-17 and 21-24 and is native to Madeira. It has a stiffly upright growth to about 20 feet tall and is about half as wide. Leaves are dark green and flowers are white, held in clusters at branch tips. Foliage will burn in frost, but the entire plant comes back strong once warmer weather begins again. Spider mites can become a problem in summertime if not controlled. This species is considered invasive in the Azores and Portugal.

Clethra barbinervis (Japanese Sweet Shrub, Japanese Clethra, White Alder, Tree Clethra)
This deciduous shrub or small tree grows well in USDA hardiness zones 5-8 or Sunset growing zones 5-9 and 14-24, slowly reaching 15-18 feet tall and about half to 2/3 as wide. Bark is grey-brown and peeling, and white flowers bloom in drooping clusters. Fall color is bright yellow. Leaves are deeply toothed. These plants are native to eastern Asia, including China, Korea, and Japan.

Clethra scabra (Summersweet, Sweet Pepperbush, Coastal Sweetpepperbush, Rough Clethra, White Alder, Soap Bush)
This species is native to the eastern Andes Mountains and surrounding mountainous forests in Brazil, Bolivia, Paraguay, and northwestern Argentina. It grows well in USDA hardiness zones 3-9. These trees reach about 82 feet tall when mature, and produce terminal racemes of fragrant white flowers in March.

Caring for clethra

Clethra trees and shrubs are best when grown in full sun to partial shade. In warmer climates, these plants will need some late afternoon protection from the sun to avoid sunburn. These trees do best in moist, organic, slightly acidic soil that is well-draining and has a pH of between 5.0 and 6.5. Clethra can grow to reach between 4 and 8 feet tall and wide, and should be planted about 4-6 feet apart to allow for good air circulation and to accommodate their mature size. Plant with the root ball level to the soil or slightly above; avoid planting too deeply to prevent root rot issues from taking hold. Mulch around the base of the plant after installation to suppress weed growth and to maintain soil moisture and temperature. For the first year or so in the ground, keep the soil consistently moist. These plants do not tolerate drought well, and will struggle during warmer periods of the year if irrigation is not sufficient. Established shrubs can be watered regularly especially during dry periods. Apply water to the base of the plant to keep foliage dry and to reduce the risk of fungal diseases.

These plants do not need fertilizing; a yearly application of compost is typically sufficient to provide nutrients for the growing season. They are also easy to prune; simply remove dead, damaged, crossing, or diseased branches and foliage in late winter or early springtime before new growth pushes out for the year. A light shaping can also be done at this time. These plants naturally have a dense, rounded habit. Spent flower heads can be removed as they fade to encourage a tidy appearance and encourage further blooms to develop. In deciduous varieties, remove some of the older wood from the base annually to rejuvenate the plant. Clethra shrubs and small trees can live for between 20 and 30 years in the landscape before needing to be replaced.

Propagation

Clethra can be propagated either by using cuttings or from seed. To ensure that your propagated plant will have the exact same qualities and characteristics as the parent plant, use the cutting method. If you would like to experiment and see what plant you can grow from seed, just keep in mind that it will not necessarily be the same as the parent plant. To propagate your Clethra, follow the instructions below:

Propagating Clethra by Cutting

Gather your materials. You'll need a clean, sanitized, sharp pair of garden shears, rooting hormone, a pencil, and a 3-inch pot filled with well-draining compost or other organic material. A good substrate for propagation of cuttings is a light blend of 1/3 pasteurized soil or compost, 1/3 sand, vermiculite or perlite, and 1/3 coconut coir or peat moss.
In early summertime, choose the plant that you'd like to propagate. Ensure that this plant has had a good watering the night before, as this will improve the chances your cutting will take root. Cut one or more 3-4 inch portion(s) of the end of a non-blooming branch and strip the leaves off of the lower half. Make a hole in your planting medium by pushing your pencil into the soil. Dip the end of the stem in rooting hormone, and plant it in your pot. Fill in soil around the cutting to support it, but do not pack it down too much.
Moisten with water, and ensure that the pot has good drainage. Your pot should be kept in warm, sunny conditions indoors (and not in direct sunlight). Water whenever your soil is dry under the top layer. After about four to six weeks have passed, your cutting should be rooted. When you notice new growth, you'll know you have roots.
Keep your new plant in an area with bright light (though not direct sunlight) and temperatures between 55 and 70 degrees Fahrenheit. Ensure that the soil stays moist, but not wet, until the next spring, when you can transplant it to its new home. Ensure that the last frost of the year has occurred before doing this, as it can seriously damage your new shrub.

Propagating Clethra by Seed

Keep in mind that the plant you get from seed will not necessarily mimic all of the characteristics of the parent plant. To propagate by seed, you will need a paper bag, a pot or tray filled with well-draining compost or other organic material, a spray bottle and a fork. The best potting medium for seed germination is a light blend of 1/3 pasteurized soil or compost, 1/3 sand, vermiculite or perlite, and 1/3 coconut coir or peat moss.
Harvest your seeds when they are ripe, which should be in fall. Seed capsules will be brown and dried. Pods can contain anywhere between 6 and 17 seeds each. They will open naturally when they are ripe.
Water your prepared seed tray or pot filled with soil, and ensure that good drainage is occurring. Place your seeds in your prepared pot or seed tray about a quarter inch to a half-inch under the soil surface. Cover them with soil and water with your spray bottle. Place your tray of seeds in a warm area that receives bright, indirect light. A greenhouse is the most ideal climate, but a planting tray inside a window will do as well. Germination usually takes 7-14 days from sowing.
Keep the seedling's potting medium moist, but not wet, by spraying the soil with your spray bottle. Keep your new plant in an area with bright light (though not direct sunlight) and temperatures between 55 and 70 degrees Fahrenheit. Ensure that the soil stays moist, but not wet, until the next spring (after the danger of frost has passed), when you can transplant it to its new home.

Clethra seed pods are round and small.

Clethra seeds are very tiny.

Problems and Pests

Some species of Clethra can be considered invasive in certain regions; check the descriptions above for more information. Check with local authorities before planting. Clethra is susceptible to developing problems with the following: