Clarkia and godetia are two of the most rewarding hardy annuals a UK cut-flower grower can direct-sow. Clarkia elegans produces spikes of ruffled double flowers in pink, red, purple, and white; Clarkia amoena (sold as godetia or Godetia grandiflora) offers the same colour range in broader, more open blooms. Both belong to the same genus and share the same growing requirements and the same problems, including a tendency for their soft, lush spring growth to suddenly start curling.
Leaf curl on clarkia and godetia is almost always caused by one of two things: aphids or downy mildew. The other causes listed below are less common but worth ruling out, particularly if you are growing under glass or in a season that has been unusually wet. Working through each cause systematically will get you to the right answer within a few minutes of inspection.
Cause 1: Aphids
The peach-potato aphid (Myzus persicae) and the foxglove aphid (Aulacorthum solani) are the two species most likely to colonise clarkia. Both are soft-bodied, pear-shaped insects roughly 1.5 to 2 mm long. Myzus persicae ranges from pale green to pinkish depending on the time of year; Aulacorthum solani is apple-green with a dark green patch at the base of the cornicles (the small tubes near its tail end).
Aphids feed on plant sap by inserting needle-like mouthparts into young tissue at shoot tips and along the midribs of unfurling leaves. The feeding causes cells to collapse unevenly, drawing the leaf edges downward and inward into a tight curl that shields the colony from both predators and sprays. Once curled, leaves rarely flatten back out even after the aphids are removed.
Alongside the curl you will usually see honeydew, the sticky, clear liquid aphids excrete as they process sap. Honeydew coats the upper surfaces of leaves below the colony and quickly becomes colonised by black sooty mould fungi, which look alarming but do not directly damage tissue. The mould is a reliable sign that aphids are present somewhere above.
Clarkia is particularly vulnerable in spring because its growth is fast, soft, and nitrogen-rich at exactly the time overwintering aphid queens are laying their first asexual generations. Autumn-sown plants are somewhat larger and tougher by the time this flush arrives, but no clarkia is immune.
What to do. Check plants every few days from late March onward, paying close attention to shoot tips and the underside of the youngest leaves. A strong jet of cold water from a hose dislodges colonies effectively and causes significant mortality if repeated every two to three days. For heavier infestations, spray with an insecticidal soap solution (follow label rates), making sure to saturate the undersides of curled leaves and the growing tip. Fatty acid and pyrethrin-based sprays are also effective and have minimal impact on beneficial insects when applied in the evening. Ladybirds, lacewings, and hoverfly larvae will all help regulate populations if you can avoid broad-spectrum insecticides. Pinching out the worst-affected shoot tips removes the densest part of the colony at a stroke and is often enough for a minor outbreak.
Cause 2: Downy Mildew
Downy mildew on clarkia is caused by Peronospora arthuri, a water mould (oomycete) specific to Clarkia species. It is not the same pathogen as the powdery mildews that attack roses or phlox, and it behaves very differently.
The first sign is usually a pale yellow or lime-green patch on the upper surface of a leaf, roughly angular in outline because the pathogen is bounded by the leaf veins. Turning the leaf over reveals the diagnostic feature: a dense grey-white or pale purple furry coating of spore-bearing structures (sporangiophores) directly beneath the yellow patch. In cool, humid weather these can appear almost overnight.
As the infection progresses, affected leaves curl upward at the margins, yellow further, and drop. The disease tends to work from the base of the plant upward, and seedlings and young transplants are the most vulnerable because their tissue is tender and their root systems are not yet deep enough to buffer moisture stress. Cool, wet springs of the kind common in the UK from March through May create near-ideal conditions for Peronospora outbreaks, particularly in sheltered, still spots where dew sits on foliage through the morning.
What to do. Remove affected leaves as soon as you spot the grey fuzz underneath, sealing them in a bin bag rather than composting them. Thin crowded plants to open up airflow between stems; clarkia is very often sown too thickly and the resulting congestion keeps humidity high at leaf level. Water at the base of plants, not overhead, and try to water in the morning so any splash dries quickly. Copper-based fungicides can slow the spread and are approved for use on ornamentals by home gardeners in the UK, but they will not cure already infected tissue. Avoid saving seed from infected plants, as the pathogen can persist in seed coats.
Other Causes to Rule Out
Drought stress. Unlike most leaf-curl causes, drought produces an upward roll along the long axis of the leaf, curling the margins toward the midrib to reduce the surface area exposed to sun and wind. The plant will also look slightly dull or wilted in the afternoon heat. Clarkia does not need heavy watering, but it cannot tolerate drying out completely at the roots during rapid stem elongation. A deep soak once or twice a week during dry spells is usually enough.
Botrytis (grey mould). Botrytis cinerea attacks clarkia in wet weather or very humid conditions under glass, producing fluffy grey mould on stems, buds, and leaves. Affected tissue collapses rather than curls. Remove infected material, improve ventilation, and avoid wetting foliage.
Root rot. Clarkia originates from the dry slopes of western North America and will not tolerate sitting in waterlogged soil. In heavy clay or compacted ground, roots suffocate and rot, and the plant loses the ability to take up water. The result is leaf curl and yellowing despite apparently adequate moisture in the soil. Working grit into the planting area before sowing, or growing on a slight slope, prevents this entirely.
Virus. Cucumber mosaic virus (CMV) and impatiens necrotic spot virus (INSV) can both infect clarkia, typically arriving via aphid feeding. Infected plants display mottled yellow-green leaf patterns, distorted or twisted growth, and stunted stems rather than clean curling. There is no cure. Remove and bin infected plants and focus on controlling the aphid vectors to protect nearby healthy plants.
Spider mite. A problem mainly for clarkia grown in pots or under glass, spider mite (Tetranychus urticae) causes fine stippling on the upper leaf surface, pale mottling, and eventually a bronzed, papery texture. Fine webbing appears on shoot tips in heavy infestations. Outdoors in the UK the pest is rarely significant on clarkia.
Prevention for the Next Sowing
Thin plants ruthlessly at the seedling stage. Clarkia seed is small and easy to sow too densely; seedlings that touch each other create a humid microclimate that invites both downy mildew and botrytis. Aim for plants standing 20 to 30 cm apart at maturity, thinning in stages rather than all at once.
Consider an autumn sowing in September or October. Plants that germinate in autumn develop a strong root system during mild winter weather and are far more resilient to the spring aphid flush and to mildew pressure than young spring seedlings. Autumn-sown clarkia also flowers three to four weeks earlier, extending the cut-flower season significantly.
Water at the base of plants, preferably in the morning. Keeping foliage dry dramatically reduces downy mildew risk and removes the moisture that aphid honeydew needs to develop sooty mould. Choose well-drained ground or amend with grit before sowing, and deadhead spent flowers regularly to keep plants producing new growth and to reduce the decaying material that encourages botrytis.
Frequently Asked Questions
Why are my clarkia leaves curling downward?
Downward curling in clarkia is almost always caused by aphids, usually the peach-potato aphid (Myzus persicae) or the foxglove aphid (Aulacorthum solani). Check the underside of young shoot tips and newly opened leaves for soft-bodied insects, sticky honeydew residue, or black sooty mould. If you find aphids, a strong jet of water or an insecticidal soap spray will bring them under control quickly.
What causes grey fuzz on the underside of clarkia leaves?
Grey or white furry growth on the underside of leaves, paired with yellow patches on top, is the characteristic sign of downy mildew caused by Peronospora arthuri. It thrives in the cool, damp conditions of a UK spring. Remove affected leaves promptly, improve air circulation by thinning plants, and avoid overhead watering. There is no curative fungicide available to home gardeners, so prevention through spacing and airflow is the main tool.
Can overwatering cause clarkia leaves to curl?
Yes. Clarkia and godetia are native to dry, open hillsides and perform poorly in waterlogged soil. If roots sit in poorly drained ground, they begin to rot, reducing the plant's ability to take up water and nutrients. The leaves may curl, yellow, and collapse. Improve drainage before sowing by working in grit or sharp sand, and always sow into a free-draining seedbed. Raised beds and pot growing suit clarkia well.
My godetia leaves are curling and mottled. Could it be a virus?
Mottled, distorted leaves that curl without any obvious pest or fungal sign point toward a viral infection, most likely cucumber mosaic virus (CMV) or impatiens necrotic spot virus (INSV). Both are spread by aphids, so controlling aphid populations is the single most important preventive step. There is no cure for infected plants. Remove and bin any plant showing mottled distortion to prevent aphids from spreading the virus to healthy plants nearby.
When should I sow clarkia in the UK to get the healthiest plants?
Clarkia and godetia can be sown in autumn (September to October) or in spring from March through May. Autumn sowings produce robust, deep-rooted plants that establish during mild winter weather and come into flower earlier the following summer. Because they are larger and sturdier by the time the spring aphid flush arrives, they tend to shrug off pest pressure more easily than spring-sown seedlings. Both timings work well in the UK, but autumn sowing rewards you with stronger plants and earlier flowers.