Clethra alnifolia, known as sweet pepper bush or summersweet, is one of the most rewarding late-summer shrubs you can grow in a UK garden. Its long spikes of tiny white or pale pink flowers open from July through September, filling warm evenings with a powerfully sweet fragrance that earns the plant its name. The flowers are magnets for bumblebees and honeybees at exactly the point in summer when many other shrubs have finished blooming. Come autumn, the foliage turns a reliable golden yellow before dropping. Popular cultivars include 'Ruby Spice' with its deeper pink flowers and the compact 'Hummingbird', which stays under a metre and suits smaller spaces.
When clethra starts curling its leaves, the plant is almost always telling you one of two things: the soil is too alkaline, or it is not getting enough water. Both problems are fixable, though the long-term solution to alkalinity sometimes requires moving the plant altogether.
Cause 1: Alkaline soil (the most common problem by far)
Clethra belongs to the family Clethraceae and, like rhododendrons, camellias, and blueberries, it is a confirmed acid-lover. It performs best at a soil pH of around 5.5 to 6.5. In neutral or alkaline conditions, the chemistry of the soil locks up iron and manganese, making these nutrients unavailable to the plant even when they are physically present in the ground.
The first sign is interveinal chlorosis: the leaf tissue between the veins turns pale yellow while the veins themselves stay green. As the deficiency worsens, the leaf edges scorch and the whole leaf begins to curl. Symptoms typically worsen as the growing season progresses and the plant draws more heavily on nutrients it cannot reach. New growth is usually hit hardest.
Gardeners in much of lowland England, particularly on chalk or heavy clay that has been limed, will encounter this problem regularly. Hard tap water in these areas adds a steady trickle of calcium and carbonates every time you water, which gradually raises soil pH even in beds that started out reasonably acid.
For an immediate response, water the soil around the plant with a product containing sequestered iron (also sold as chelated iron). This form of iron remains available even at higher pH levels and will green up the foliage within a couple of weeks. Follow up with an ericaceous fertiliser. Switching to rainwater for any future watering will stop the problem from worsening.
For lasting health in alkaline ground, the most reliable solution is to grow clethra in a large container filled with ericaceous compost. Use only collected rainwater to irrigate, as mains water in hard-water regions can push the compost pH upward over a single season. Repot or top-dress with fresh ericaceous compost every two to three years.
Cause 2: Drought stress
Clethra's native habitat is the moist woodland edges and boggy ground of the eastern United States. It evolved with wet feet and does not cope well with prolonged dry conditions. In a UK summer, which increasingly delivers dry spells through June and July precisely when the plant is building its flower buds, moisture stress is a real and common problem.
A drought-stressed clethra curls its leaves inward to reduce the surface area exposed to sun and wind, a simple but effective response to water loss. The leaf margins begin to yellow, and in severe cases the plant will drop leaves early. The shrub rarely dies from a single dry summer, but it will flower poorly and may take two seasons to fully recover its vigour.
The single most effective thing you can do for clethra in a UK garden is to mulch it generously each spring with a 7 to 10 cm layer of composted bark, wood chip, or leaf mould. This keeps the root zone cool and moist through dry spells. Clethra planted beside a pond, in a rain-shadow-free north-facing border, or at the edge of a bog garden will almost never show drought stress at all. If your plant is in a sunnier or drier position, plan to water it during any stretch of dry weather in June, July, and August.
Other causes worth checking
Aphids colonise the soft new growth of clethra in spring and early summer, and their feeding causes the young leaves to curl downward and pucker. Look at the underside of the growing tips for clusters of small pale or green insects. A forceful jet of water knocks most colonies off, and a follow-up with insecticidal soap will deal with stragglers. Aphid damage is unsightly but rarely serious; the plant recovers quickly once the infestation is cleared.
In containers, vine weevil grubs are a significant risk. The grubs feed on roots through winter and early spring, and by the time the plant shows leaf curl and wilting in late spring, the root system may already be badly damaged. If a containerised clethra suddenly collapses despite adequate watering, tip the pot out and inspect the compost for white C-shaped grubs. Treat with a nematode drench (Steinernema kraussei, applied in autumn while soil is above 5C) as a preventive measure if vine weevil is common in your garden.
Scale insects can appear on established woody stems, appearing as brown or white crusts. A heavy infestation weakens the plant and can contribute to poor leaf quality and curl. Scrub them off with a soft brush or treat with a winter plant oil when the plant is dormant.
Frost damage to the growing tips in an exposed position causes blackening and curl on the youngest leaves in spring. Clethra alnifolia is reliably hardy throughout the UK but the fresh spring growth is tender. A fleece cover during a predicted late frost will prevent it. Note that Clethra arborea, the lily-of-the-valley tree, is a much more tender species suited only to sheltered mild-climate gardens; it is far more vulnerable to cold damage than C. alnifolia.
Powdery mildew can affect clethra in warm dry conditions, producing a white coating on the leaves that causes distortion and curling. It is unusual on plants that receive adequate water. Improved airflow and consistent watering usually prevents it without the need for fungicide treatment.
Prevention
Plant clethra in acid soil at pH 5.5 to 6.5, in a position that retains moisture well. A north-facing border, a pond margin, or a bog garden suits it perfectly. Mulch the root zone heavily every spring and water through any dry spell from June onwards. Feed each spring with an ericaceous fertiliser to maintain soil acidity and ensure a good supply of the trace elements clethra needs. In hard-water areas, collect rainwater for any containerised plants and check the compost pH annually with an inexpensive test kit.
Propagation from suckers is easy and gives you extra plants for free. Detach a rooted sucker from the base of the parent plant in autumn or early spring and pot it into ericaceous compost. Grown in the right conditions, clethra is genuinely low-maintenance and will reward you with exceptional late-summer fragrance and golden autumn colour for decades.
Frequently asked questions
Why are my clethra leaves turning yellow and curling?
Yellowing with curling almost always points to alkaline soil. Clethra is an ericaceous plant that needs acid conditions (pH 5.5 to 6.5). In alkaline or limed soil it cannot absorb iron and manganese, causing interveinal chlorosis followed by leaf curl and marginal scorch. Treat with sequestered iron immediately and feed with an ericaceous fertiliser. For a lasting fix, move the plant to acidic soil or transfer it to a container filled with ericaceous compost.
Can dry weather cause clethra to curl its leaves?
Yes. Clethra alnifolia naturally grows at bog edges and in moist woodland, so it is far less drought-tolerant than many garden shrubs. In dry spells the leaves curl inward, develop yellow margins, and may drop early. The plant rarely dies from drought alone but it will flower poorly and look tatty. A generous layer of bark mulch around the roots and regular watering through dry summers is the single most effective thing you can do to keep clethra healthy.
Is clethra alnifolia acid-loving?
Yes, strongly so. Sweet pepper bush belongs to the same family as rhododendrons and prefers a soil pH of around 5.5 to 6.5. It will decline steadily in chalky or clay soils that have been limed. If you are on alkaline ground, grow it in a large container of ericaceous compost and water exclusively with rainwater to avoid adding the calcium and carbonates found in mains water in hard-water areas.
What pests cause curling leaves on clethra?
Aphids are the most common culprit, clustering on soft new shoots and causing the young leaves to curl downward. A blast of water or an application of insecticidal soap clears most infestations quickly. In containers, vine weevil grubs can damage the roots badly enough to cause wilting and leaf curl across the whole plant. Scale insects can appear on older wood. Powdery mildew is possible in warm dry conditions though it is uncommon on well-watered plants.
How do I keep clethra healthy in a UK garden?
Plant in acid soil at pH 5.5 to 6.5 in a position that stays consistently moist, such as a pond margin or bog garden. Mulch heavily with composted bark each spring to retain moisture and slightly acidify the soil over time. Water during dry spells, particularly in summer when the plant is flowering and building next year's buds. Feed each spring with an ericaceous fertiliser. If you are in a hard-water area and growing clethra in a container, always water with collected rainwater rather than tap water to prevent gradual alkalinity build-up. Propagate new plants from rooted suckers that appear at the base.