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. The flowers produced by these plants will attract hummingbirds and butterflies to your garden. Flowers are fragrant and smell sweet, coming in creamy white and pink and blooming in mid to late summertime. Use Clethra in rain gardens or bioswales, or along pond or pool edges. Take care when planting Summersweet in the landscape, as some species are toxic to humans and pets.

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)
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 has 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.

​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. 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 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 conditions, but the entire plant comes back strong once warmer weather begins again. Spider mites can become a problem in summertime if not controlled. 

Clethra barbinervis (Japanese Sweet Shrub)
This deciduous shrub grows well in 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 longispicata
These trees reach 30 feet tall and have fissured brown bark. Fragrant flowers are white and bloom in spring and summer. This species is native to Borneo, the Philippines and Sulawesi.

Clethra scabra
This species is native to the eastern Andes Mountains and surrounding mountainous forests in Brazil, Bolivia, Paraguay, and northwestern Argentina. These trees reach about 82 feet tall when mature, and produce terminal racemes of fragrant white flowers in March. 

Caring for and pruning clethra Trees

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 foliage sunburn. These trees do best in moist, organic, slightly acidic soil that is well-draining. They prefer soil to have a pH of between 5.0 and 6.5. These plants 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 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.

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
  1. 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. If you don't have any potting medium on hand, you can make some. Just mix together equal parts (by volume) moss peat and horticultural-grade sand or grit sand.
  2. 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.
  3. 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.
  4. 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
  1. 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. If you don't have any potting medium on hand, you can make some. Just mix together equal parts (by volume) moss peat and horticultural-grade sand or grit sand. You will also, after a few weeks, need individual 3-inch pots for each seedling that grows. 
  2. 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. 
  3. 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. Once you begin to see seedlings sprout and they are able to be handled, transplant them to their own individual 3-inch pots, filled with the same potting medium as described in Step 1. Loosen the seedlings from the potting medium by digging down with the tines of your fork about an inch and a half, gently working the seedling up until it is out. Then, replant the seedling in its individual pot.
  4. 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

Clethra is susceptible to developing problems with the following:
  • Aphids
  • Spider Mites
  • Leaf Spot
  • Powdery Mildew
For more information on these issues, including how to solve them, consult the following links:

Gallery

These shrubs are great at providing places for local pollinators to feed.
Clethra shrubs are deer-resistant, and add texture to mixed beds.
Here, Clethra alnifolia is paired with ferns and Rudbeckia.

Videos

Clethra shrubs are great garden additions for local pollinators.
Learn all about Clethra alnifolia.
Clethra alnifolia 'Ruby Spice'.
How to identify a Sweet Pepperbush.

Resources

Clethra Summersweet - Pruning, Winter Care and Fertilizing
Hardy Summersweet: How to Grow Clethra Alnifolia
How to Grow and Care for Summersweet
How to Plant, Prune, Fertilize, Water and Care for Clethra Summersweet Plant
Clethra, Our Favorite Native Shrub
Clethra: A Growing Guide for Summersweet Shrubs
Wikipedia: Clethra
The New Sunset Western Garden Book: The Ultimate Gardening Guide (2012) - Page 254, Clethra
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