When to prune clethra?

Summersweet clethra

Summersweet clethra (Clethra alnifolia) photo: John Hagstrom Tree & Plant Care

Oval to rounded form reaching 6 to 8 feet high and 4 to 6 feet wide
Cultivars vary in size and color.
Best in moist to wet soils, tolerant of soil pH, part shade to full sun.
Slow to leaf out in spring.
Spreads by suckering roots forming wide colonies.
Flowers on new wood, prune to control size in spring.
Tolerant of aerial salt spray.

Disease, pests and problems

No serious problems

Native geographic location and habitat

Native to the eastern coast of the United States.

Attracts birds & butterflies

Songbirds, including robin, goldfinch, and warblers are attracted to the seed capsules.
Nectar source for swallowtail butterflies and hummingbirds.

Bark color and texture

Bark is brown; multi-stemmed.

Leaf or needle arrangement, size, shape, and texture

Simple, alternate leaves; 2 to 4 inches long, sharply serrated, medium to dark green above, paler beneath, turns a golden-yellow in fall.

Flower arrangement, shape, and size

Fragrant, five-petaled flowers in upright spikes on end of branches. Color varies by cultivar from white to pink.

Fruit, cone, nut, and seed descriptions

Interesting, delicate, dried seed capsules persist through winter.

Cultivars and their differences

Hummingbird (Clethra alnifolia ‘Hummingbird’): 2 to 3 feet high and wide; rounded form; white flowers; flower clusters are larger and open earlier than the species.

Ruby Spice (Clethra alnifolia ‘Ruby Spice’): 4 to 6 feet high and wide; oval to rounded form; rosy pink flowers.

Sixteen Candles (Clethra alnifolia ‘Sixteen Candles’): 2 to 3 feet high and wide; rounded form; white flowers.

Vanilla Spice™ (Clethra alnifolia ‘Caleb’): 4 to 6 feet high and 3 to 5 feet wide; upright to rounded form; white flowers are larger than those of the species.

Summersweet Clethra

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

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.

Description

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.

Landscape Use

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

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.

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. 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).

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, 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.

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

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

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

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

Problems

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.

Pseudocercospora leaf spot is a late season disease of summersweet clethra (Clethra alnifolia).
Joey Williamson, ©2018 HGIC, Clemson Extension

Summersweet (Clethra alnifolia) for Difficult Growing Conditions

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Don’t let heavy shade or damp and clay soil stop you from gardening. Consider adding a Summersweet, Clethra alnifolia, to your landscape.

This North American native can be found growing in swampy woodlands, marshes, along stream banks and seashores. This suckering shrub has dense branching and grows 3 to 6 and occasionally 8 feet tall and 4 to 6 feet wide. It is hardy in zones 3 to 9.

The fragrant white flowers attract butterflies and bees, while brightening the landscape for 4 to 6 weeks in July and August. But the show doesn’t end there. The green leaves turn an attractive yellow in the fall. Plus, the tidy appearance makes it a nice addition to the winter landscape.

It grows best in full sun to part shade and moist to wet soil.

Use summersweet in rain gardens, shrub borders, narrow spaces and perennial gardens where its four-season beauty can be enjoyed.

A bit more information: This versatile shrub, also known as Sweet Pepperbush, is generally trouble-free. The cultivar Hummingbird is more compact, slow spreading and grows about 3 to 4 feet tall. Sugartina is even smaller at 30 inches with clear white flowers. Pink Spire has pink buds that open into pinkish white flowers.

How Much Can I Prune Or Cut Back Ruby Spice Clethra

Answer #1 · Maple Tree’s Answer · Hi Jean-The Ruby Spice Clethra develops blooms on the current years new growth. It should be pruned late winter or very early spring. This is done before the flower buds develop. If you wait until mid to late spring, you will remove the buds and your plant will have very few or no blooms that year. This annual pruning in late winter will promote vigorous new growth each year and hopefully an abundance of blooms for you in July and August. You can prune your plants down to about 1 foot below where you would like them to be when blooming.
Heavier pruning to keep your plant within a desired size can be done immediately after flowering. At this time you can cut out up to 1/3 of the old and longest stems. Any sucker shoots can also be cut out even with the surface of the ground if desired to keep the plant’s width in bounds. Any dead, bare, or branches that are rubbing against others should also be cut off level with the surface of the ground at. If you have a broken or branch growing to tall these can be pruned anytime of the year.
Im thinking by pruning your plants in the fall, early buds may have been damaged by late frost and only lower, newer growth is producing your flowering this year.
I noted below the link that will take you to the ‘Ruby Spice Clethra’ plant file in Gardenality. This will give you other information you may like to see regarding this plant. Just click on the link to go directly to the plant file.

Hope this helps.
John)

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