Restharrow is one of the most characterful plants of the British chalk downland. Run your fingers along a stem of Ononis spinosa (spiny restharrow) or O. repens (common restharrow) and you will notice the distinctive sticky, slightly tacky feel of the glandular leaves and the faintly resinous smell that clings to your hand afterwards. It is a perennial legume in the Fabaceae family, sprawling and creeping across dry calcareous soils from June to September, producing pink pea flowers beloved by bumblebees and the green hairstreak butterfly.
When restharrow leaves curl, it is almost always a sign of stress or pest pressure. The plant's small, compound leaves and sticky texture make the symptoms slightly harder to read than on a large-leaved garden plant, so it pays to look closely. This guide covers the two most common causes and a handful of secondary ones.
Aphid infestation
The pea aphid (Acyrthosiphon pisum) and several related species are the first thing to check when restharrow leaves curl and cluster at the shoot tips. Colonies settle on the soft new growth, piercing the plant tissue to feed on phloem sap. As numbers build, the leaves curl inward and downward, effectively enclosing the colony and making it harder for predators to reach them.
Look closely at the tight, curled tips of young shoots. You will often see small, pale green or pinkish soft-bodied insects packed together, sometimes accompanied by white shed skins. On restharrow, the sticky leaf surface adds a further wrinkle: dead or dislodged aphid bodies can stick to the leaf and give the impression that an active infestation is still underway even after the colony has collapsed. Rub a leaf gently between your fingers to check whether the insects are alive.
Honeydew deposits from feeding aphids can also attract sooty mould, leaving a dark, slightly dusty coating on affected leaves. This is secondary and clears once the aphids are gone.
In a wildflower or meadow setting, the correct response to aphids on restharrow is patience. Ladybirds, hoverfly larvae, and lacewing larvae are reliable predators that typically reduce colonies within two to three weeks. Applying any insecticide in this context would harm the broader food web, including the pollinators the plant attracts. If you have a young restharrow in a garden bed and the infestation is severe, a firm jet of water directed at the shoot tips on a dry morning is the safest mechanical intervention.
Powdery mildew
Powdery mildew on restharrow is caused by Erysiphe trifolii or a closely related Fabaceae-specialist species. It appears as a white or pale grey powdery coating on the surfaces of the small, sticky leaves. Affected leaves may curl, yellow at the margins, and feel dry to the touch despite the characteristic tackiness of a healthy restharrow leaf.
Powdery mildew thrives in warm, dry conditions with poor airflow, which makes restharrow something of a candidate: it grows on free-draining chalk and limestone soils where summer drought is common, and it often grows in low, sprawling mats where air circulation around individual shoots can be restricted. Paradoxically, the spores spread most easily when humidity is moderate and temperatures are warm (around 18 to 25 degrees Celsius), rather than in wet conditions.
Established restharrow on well-drained chalk soils tends to recover from mild mildew without intervention. If the plant is in a garden setting where you want to reduce the spread, improving airflow by thinning neighbouring vegetation is the most practical step. Avoid wetting the foliage when watering nearby plants. Severe cases on garden specimens can be treated with a diluted solution of potassium bicarbonate or a sulphur-based fungicide, though neither is warranted in a wildflower meadow context.
Drought stress
Restharrow is famously tough on chalk and limestone soils, and an established plant can withstand extended summer drought without much visible distress. Its deep, wiry root system reaches moisture and nutrients that shallower-rooted plants cannot access. However, newly planted or recently transplanted specimens have not yet established that root system, and they can show leaf curling and yellowing in dry spells during their first season.
Drought stress produces a general inward curl of the leaves combined with a slight softening or wilting of the whole shoot. Unlike aphid damage, there will be no insects present and no sticky residue beyond the plant's natural stickiness. Unlike powdery mildew, there will be no white coating. The leaves will feel dry and the soil around the plant will be very dry to depth.
Water newly planted restharrow during its first summer if conditions are very dry, directing water to the root zone rather than the foliage. Once established, the plant should need no supplementary watering at all. This is one of its great virtues on thin chalk soils where irrigation is impractical.
Other causes to consider
Slugs and snails can damage young restharrow plants, particularly in a garden setting where the surrounding soil holds more moisture than the plant's natural chalk downland habitat. Slug damage tends to be irregular and ragged rather than a uniform curl, and you may see slime trails on the leaves or nearby soil. Protect young transplants with a layer of horticultural grit around the base, which slugs find difficult to cross.
Iron chlorosis can appear on restharrow grown in very highly alkaline soils (above pH 8 or so), particularly in heavy clay soils that have been limed heavily. The symptoms are a yellowing of the younger leaves while the leaf veins remain green, sometimes accompanied by a slight inward curl. True restharrow habitat is alkaline but free-draining, and the plant does not normally encounter the anaerobic, waterlogged high-pH conditions that trigger chlorosis. If you suspect it, a soil test will confirm the pH, and a chelated iron foliar feed can address it in a garden context.
Prevention and care for restharrow
The single most important thing you can do for restharrow is give it the right soil conditions from the start. It is a plant of thin, free-draining, alkaline soils. In a garden, this means a sunny position with sharply drained, chalky or gritty soil, low in nutrients. Rich, fertile, moisture-retentive soils encourage lush growth that is far more susceptible to aphid colonies and powdery mildew, and they suppress the plant's naturally compact, wiry habit.
Allow natural predator populations to establish around the plant rather than reaching for sprays. Avoid overwatering at all stages. For young transplants, protect the base from slugs with grit and check regularly during the first season. Ensure the planting site has enough open airflow to reduce the risk of mildew in warm weather.
Restharrow is a declining UK wildflower in many areas, lost from farmland through herbicide use and agricultural improvement. It persists on road verges, embankments, chalk grassland nature reserves, and coastal cliffs. Including it in wildflower turf or meadow seedings on chalk or limestone substrates is a genuine contribution to its conservation, and it rewards the effort with months of pink pea flowers and a plant that asks for almost nothing once established.
Frequently asked questions
Why are my restharrow leaves curling?
The most common causes are aphid colonies on the soft shoot tips, powdery mildew in dry warm conditions, and drought stress on chalk or limestone soils. Check shoot tips for clusters of small soft-bodied insects first, then look for a white powdery coating on the leaf surfaces.
Why is restharrow called restharrow?
The name comes from the plant's tough, wiry, deep-rooting stems that would stop (or "rest") farm harrows in their tracks. Before mechanised agriculture, ploughing through a patch of restharrow was a genuine nuisance, and the name stuck. It is a reminder that a plant considered a dainty wildflower today was once a persistent weed of cultivated land.
Why are restharrow leaves sticky?
Restharrow leaves are covered in glandular hairs that secrete a sticky, slightly resinous substance. The plant is famously pungent as a result, and the stickiness has been suggested as a defence against small crawling insects. Interestingly, the sticky surface does not deter aphids from colonising the softest new shoot tips, but it does mean that once aphids die or are dislodged, their bodies may stick to the leaf surface and give the impression of ongoing infestation.
Is restharrow a good wildflower for a chalk garden?
Yes. Both Ononis repens (common restharrow) and Ononis spinosa (spiny restharrow) are well suited to thin, free-draining chalk or limestone soils where most other plants struggle. They are nitrogen-fixing legumes, so they improve soil fertility over time. The pink pea flowers from June to September attract bumblebees, and the green hairstreak butterfly uses restharrow as a larval foodplant. It is an excellent choice for wildflower turf seedings on alkaline substrates.
Should I treat aphids on restharrow with insecticide?
No. In a wildflower or meadow setting, avoid insecticides entirely. Aphid populations on restharrow are almost always controlled by natural predators, including ladybirds, hoverfly larvae, and lacewing larvae, within a couple of weeks. Spraying would harm these beneficial insects and disrupt the wider food web. If the infestation is severe on a young plant in a garden bed, a firm jet of water to dislodge the colonies is the safest intervention.