UACES Facebook Summersweet Clethra (Clethra alnifolia)

Landscape Shrubs
Summersweet Clethra
(Clethra alnifolia)

 

It is often hard to find shrubs that flower in the summer—and even better yet, shrubs that flower well in the shade—but summersweet clethra (Clethra alnifolia) fits the bill for both. Clethra is native along the coast from Maine to Florida and around the peninsula to Texas. Clethra is a challenge for garden centers to sell because it is in full flower when Arkansas is in the midst of the summer heat (when most gardeners are taking refuge in front of their air conditioners). Like that summer heat, this plant is HOT when it is in flower.

Starting in mid-July and lasting for four to six weeks, clethra puts out quite a flower display. The very small white flowers are born on upright racemes that make the plant look like it is lit with Fourth of July sparklers. The racemes are about ¾ of an inch wide and 4-6” long. As you can imagine from the common name, summersweet, the flowers have a very sweet fragrance. Clethra is a favorite of bees.

Flowers actually come in two flavors: white and pink. ‘Ruby Spice’ is one of the best pink-flowered cultivars for Arkansas. Other pink-flowered selections include ‘Fern Valley Pink,’ ‘Pink Spires,’ and ‘Rosea.’ The University of Arkansas Plant Evaluation Program is testing two new white selections called White DoveTM and ‘Sixteen Candles.’ Racemes are held more upright on these two sister seedlings. Flowers are followed by a small, dry capsule that has minimal landscape value.

Leaves, which are obovate (i.e., oval and flat) in shape with a sharply serrated (i.e. teeth-like) margin, are a rich medium green. In the fall, leaves can take on a golden yellow color similar to hickories.

Plants are typically not dense, and they have a distinctive soft texture due to the fine twigs and leaves. Since the plant suckers, it is difficult to give a final width, but an average plant will be 5’–7’ tall with a spread of 6’–8.’ There are a number of cultivars with a more compact habit and my favorite is ‘Hummingbird.’ This white-flowered selection is distinctly more compact than the rest of the species.

The best exposure for clethra in Arkansas is a partially shaded location. While they will grow in full sun, you must supply adequate and constant moisture during our rugged summers. The plant seems to benefit from a slightly acid soil that is rich in organic matter.

Clethra does not appear to be susceptible to any serious disease or insect problem. Plants can be pruned in early spring, since flowers are born on new growth.

There are several other fabulous clethras that you might want to keep on your garden radar screen. All of these species are more upright in habit and can be trained as a multi-stemmed large shrub or small tree. These species include C. barbinervis (Japanese clethra) and C. tomentosa (wooly summersweet). These species are also solid contributors to the summer landscape with their showy flower displays in July and August. An older Japanese clethra will have exfoliating bark that rivals many crapemyrtles. Our Statewide Plant Evaluation Program is evaluating ‘Cottondale’ wooly summersweet, and so far it is doing exceptionally well and growing very fast. The racemes appear much longer (10-12”) than many other clethras.

  • Common Name: summersweet clethra
  • Varieties to look for: ‘Ruby Spice’
  • Flower Color: white or light pink
  • Blooming period: mid-July
  • Type: deciduous shrub
  • Size: medium; 6’ tall x 8’ wide
  • Exposure: partial sun
  • Soil: amend with organic matter
  • Watering: moist best
  • When to prune: early spring
  • Suggested use: naturalizing, mixed shrub border

 

'Ruby Spice' cultivar flowers

Picture closeup of Summersweet Clethra (Clethra alnifolia 'Red Spice') reddish flowers.

Form with flowers

Picture of Summersweet Clethra (Clethra alnifolia) shrub form with white flowers.

Flower structure

Picture closeup of Summersweet Clethra (Clethra alnifolia) white flower structure.

Form

Picture of Summersweet Clethra (Clethra alnifolia) shrub form.

Leaf and early flower structures

Picture closeup of Summersweet Clethra (Clethra alnifolia) leaf structure and early flower structure.