Clethra alnifolia
If you’re looking for a flowering shrub that thrives in moist soil and delivers late-season interest, hardy summersweet (Clethra alnifolia) is a top contender.
Also known as sweet or coastal pepperbush and white alder, this low-maintenance native tolerates a variety of soils, performs well in partial shade, and erupts in a profusion of sweetly fragrant blooms during the dog days of summer.
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Its upright, bottlebrush-shaped flowers are a magnet for pollinators, and once established, the plant requires little in the way of upkeep.
Read on to find out more about growing this versatile North American shrub. Here’s what I’ll cover:
What You’ll Learn
A member of the Clethraceae family, summersweet is something of a botanical outlier.
Ranging from southeastern Canada all the way south to Texas, C. alnifolia is the northernmost member of its family and the only Clethra species endemic to North America.
For much of the year, this deciduous shrub blends into the background, its alternately arranged, serrated green leaves forming a dense screen.
But come July or August, C. alnifolia transforms as it bursts into bloom, filling the air with a heady, honeyed perfume that you’ll likely smell before you see.
The upright floral spikes, typically white to pale pink with prominent yellow stamens, are intensely fragrant, often described as cloying or even overpowering.
But this intoxicating scent, combined with the buzz of bees and flutter of butterflies, makes summersweet a sensory showstopper few shrubs can match.
The seed heads are gray skeins of many, small capsules filled with tiny, dark, dry seeds.
Quick Look
Common name(s): Hardy summersweet, coastal or sweet pepperbush, white alder
Plant type: Deciduous shrub
Hardiness (USDA Zone): 3-9
Native to: Eastern North America
Bloom time / season: Mid to late summer (July-August)
Exposure: Full sun to partial shade
Soil type: Moist, loamy, well-draining
Soil pH: 5.0-6.5, slightly acidic
Time to maturity: 3-5 years
Mature size: 3-8 feet tall, 4-6 feet wide
Best uses: Rain gardens, borders, hedges, naturalized areas, pond edges
Taxonomy
Order: Ericales
Family: Clethraceae
Genus: Clethra
Species: Alnifolia
Hardy in USDA Zones 3 to 9, summersweet or pepperbush grows wild in woodland glades, thickets, pond edges, and riverside habitats.
In the landscape, it is well-suited to wet, sunny locations like pond margins, streambanks, and rain gardens. In these conditions, the plant can reach its full potential, often growing up to eight feet tall and wide.
Don’t have a water feature on your property? No problem.
Summersweet also does well in consistently moist garden beds and is a wonderful choice for planting near patios or porches where you can enjoy its fragrance and the flurry of pollinator activity up close.
Keep reading to learn how to get this native beauty established in your landscape.
How to Grow
The secret to cultivating hardy summersweet is really no secret at all. Find a spot that suits its needs, and this native shrub will largely take care of itself.
Naturally found in moist, well-draining soils throughout its native range, C. alnifolia is happiest when consistently hydrated, but not waterlogged.
While it thrives near ponds or streams, it doesn’t tolerate standing water for long periods. Think pond shore, not marsh; streamside, not bog.
If you’re planting summersweet in a garden bed, you’ll need to water regularly, especially during dry spells. Be careful not to let the soil dry out completely.
A one-inch layer of mulch around the base of the shrub helps retain moisture and keeps roots cool in summer.
Climate
Hardy in USDA Zones 3 to 9, summersweet is impressively cold-tolerant. In fact, it often produces more robust blooms following a cold winter.
It also handles humid conditions well, which makes it an excellent candidate for southeastern gardens.
Light
Summersweet is very accommodating when it comes to light levels. While it will bloom most profusely in full sun, it only needs about six hours of direct light per day.
In warmer parts of its hardness range, Zone 7 and above, summersweet will benefit from some afternoon shade. The dappled light along a partially shaded pond or stream is ideal.
Soil
This shrub isn’t fussy about soil texture and will grow in sandy, loamy, or even clay soils, as long as it remains moist but drains well.
Pepperbush prefers acidic soil, with a pH in the range of 5.0 to 6.5, but will tolerate slightly more neutral conditions.
Water
Consistent moisture is key. Summersweet will tolerate brief dry periods once established, but it performs best when watered regularly.
In garden settings with fast-draining soil, supplemental irrigation is often necessary.
Fertilizing
Summersweet generally doesn’t need fertilizer, particularly when grown in organically-rich, fertile soil.
However, if growth appears sluggish or flowering is sparse, try top dressing with a couple inches of compost in the spring and summer. This boost of food will help encourage flowering.
Alternatively you can apply a balanced, slow-release fertilizer if you prefer. Avoid formulas high in nitrogen, which can encourage leafy growth at the expense of blooms.
Cultivars to Select
Hardy summersweet is generally quite easy to find at well stocked native plant nurseries.
Many mainstream garden centers also carry a variety of cultivars, especially in midsummer when the shrubs are coming into bloom.
Here are a few cultivars to look out for:
Hummingbird
Compact and dense, ‘Hummingbird’ is a compact cultivar that grows to just four feet tall and wide making it a good choice for smaller spaces or foundation plantings.
It retains all the charm of the species, with prolific midsummer blooms and a sweet, clove-like fragrance. Hardy in USDA Zones 3 to 9.
You can find plants available at Nature Hills Nursery.
Ruby Spice
The standout feature of ‘Ruby Spice’ is its vivid pink to purplish-pink flowers.
This cultivar reaches about six feet tall and five feet wide and is ideal for hedges or back-of-border plantings. It’s hardy in USDA Zones 4 to 9.
You can find ‘Ruby Spice’ plants available via Nature Hills Nursery.
Sugartina
Neat, tidy, and petite, Sugartina® aka ‘Crystalina’ stays compact at just three feet tall and wide.
Despite its small stature, this cultivar produces a profusion of fragrant white blooms in summer and works wonderfully in containers or tight garden spaces.
A solid pick for smaller landscapes or urban gardens in Zones 4 to 9.
You can find Sugartina® plants available at Nature Hills.
Maintenance
If you’re growing hardy summersweet as a hedge or simply want a tidier look, a light pruning can help shape its naturally dense foliage.
This species doesn’t mind a good haircut and will respond with vigorous growth.
You can remove dead, damaged, or diseased branches at any time of year. For shaping or size control, prune in early spring before new leaves emerge, since summersweet blooms on new wood.
Always clean your pruning tools with rubbing alcohol or a 10 percent bleach solution before you begin.
Make your cuts at a slight angle to prevent water from pooling on the wound – this reduces the risk of fungal infections taking hold.
Feel free to cut the shrub back to your desired height or silhouette. Just avoid removing more than 25 percent of the overall height in one session.
Severe pruning can stress the plant and reduce its vigor, especially in the case of younger specimens.
Propagation
Hardy summersweet can be grown from seed, cuttings or planted as transplants.
Growing from seed or cuttings can be challenging, but once established, these plants pretty much take care of themselves.
From Seed
Starting hardy summersweet from seed is fairly straightforward, but keep in mind that it can take three to five years before your seedlings grow into mature, self-sustaining shrubs.
During that time, you’ll need to keep a close eye on weeds. Young summersweet shrubs don’t compete well.
To grow pepperbush from seed, you have two options: either buy seed online, or collect your own from a wild plant.
To harvest your own seed, wait until late fall when the shrub has dropped its leaves. The long seed heads should be dry and gray by then, with tiny, dark seeds inside.
Collect them into a paper envelope or breathable bag, and sow as soon as possible.
Fill a seed tray or flat with moistened potting soil. Scatter the seeds liberally across the surface, then cover them with a light layer of additional soil just enough to barely cover.
Gently tamp the soil down with your palm.
Move the tray to a bright, sheltered spot outdoors. While there’s some debate about whether cold stratification is necessary, it’s best to mimic nature if you’ve collected seed from a local wild plant.
Leave the tray outside through the winter so the seeds experience a natural chilling period.
Be sure to protect the tray from strong winds and extreme fluctuations in temperature. Keep the soil evenly moist but not saturated.
Do make sure, however, your seed tray is protected from any high winds, or extreme fluctuations in temperature. Make sure the soil remains moist, but not soaking.
Germination should begin in spring as temperatures warm. When seedlings emerge, thin them out to keep the strongest.
After a few months of growth, transplant the seedlings into four-inch pots filled with potting soil.
You can move them into the garden in the fall, but keeping them in containers for another year will make it easier to monitor their progress.
Cuttings
Like many deciduous shrubs, hardy summersweet can be propagated from cuttings, particularly softwood cuttings taken in spring.
This species roots most readily when young, pliable stems are harvested early in the season, before the leaves have fully unfurled. When timed right, rooting hormone typically isn’t necessary.
To begin, prepare several four-inch pots with a mix of two-thirds potting soil and one-third sand. Since pepperbush appreciates consistent moisture, water the mix thoroughly until it’s damp to the touch. Fill each pot and gently tamp down the soil.
Using clean, sharp scissors or pruners, snip four- to six-inch-long softwood cuttings from a healthy, mature shrub, making your cut just below a node or bud pair. Strip off all but the top set of leaves.
Insert the lower two inches of each cutting into the prepared soil. To maintain humidity, cover each pot loosely with a clear plastic bag. This acts like a mini greenhouse, helping the soil stay moist and the air humid.
Set the pots in a sheltered, brightly lit area outdoors, protected from wind and harsh sun. A covered patio or porch is ideal. If nighttime temperatures are still dipping below freezing, bring the pots indoors overnight.
Roots typically develop in about six weeks. You’ll know rooting is successful when the cutting begins to produce new top growth and the leaves look fresh and green.
At that point, you can remove the plastic and begin hardening off the young plants before transplanting them into the garden.
Transplanting
Planting nursery-grown transplants is by far the easiest way to establish hardy summersweet in your garden.
Choose a spot with moist, well-draining soil and at least six hours of sunlight per day.
Dig a hole as deep and wide as the container your plant is currently growing in. Carefully remove the plant from its container, set it in the hole at the same height as it was in the pot.
Water the root ball of the hardy summersweet before you fill in the hole to ensure the plant stays sufficiently hydrated. Fill in the hole, packing dirt firmly around the rootball. Then water again.
If your soil drains quickly or tends to dry out, apply a one- to two-inch layer of mulch around the base of the plant. This will help retain moisture, suppress weeds, and regulate soil temperature.
Pests and Disease
When planted in favorable conditions, hardy summersweet is as tough as they come, and virtually pest and disease free.
Pests
Pepperbush attracts a variety of beneficial insects, from butterflies and moths to beetles and wasps. For the most part, pest issues are rare.
The one occasional nuisance is spider mites (Tetranychus spp.), particularly in plants under stress from drought, overcrowding, or poor air circulation.
Early signs include tiny white or yellow stippling on the leaves, followed by bronzing, leaf drop, and fine webbing.
To identify and manage infestations, read our guide to controlling spider mites.
Disease
Like many native species, hardy summersweet is virtually disease-free when grown in appropriate conditions. However, during unusually wet summers, it may develop leaf spot.
This fungal condition causes dark, irregular patches to form on the foliage in mid to late summer. Though usually not harmful, severe cases may result in premature leaf drop.
Remove and dispose of affected leaves in the trash or by burning. Avoid composting them, as this can spread fungal spores.
Fragrant, Beautiful, and Easygoing
Hardy summersweet doesn’t just tolerate the damp conditions where other shrubs falter, it thrives in them.
Whether nestled along a stream bank or planted in a moist garden bed, this resilient native shrub offers a profusion of fragrant blooms just when most others are winding down for the season.
If you’ve got a soggy or shaded area to fill, give pepperbush a try. With its stunning midsummer floral display and easygoing nature, it’s a standout addition to any garden.
Are you growing hardy summersweet? Let us know in the comments section below!
And to learn more about other native shrubs, check out these guides next:
