Kidney vetch (Anthyllis vulneraria) is one of the most ecologically important wildflowers in the UK. This sprawling or cushion-forming perennial legume grows naturally on chalk downland, limestone grassland, sea cliffs, and rocky slopes, forming low mats of grey-green pinnate leaves topped from May to August with clusters of woolly-calyxed flowers in cream, yellow, orange, or red. The silky hairs on the calyx give the flowerheads their distinctive soft, woolly appearance and the colours vary remarkably, especially in coastal populations.
If you are growing kidney vetch in a wildlife garden, a meadow restoration project, or on a calcareous bank and have noticed the small leaflets curling, the cause is almost always one of a small number of problems. Because Anthyllis vulneraria is the sole larval foodplant of the small blue butterfly (Cupido minimus), the UK's smallest butterfly, identifying and treating any problem correctly matters well beyond the health of one plant.
Cause 1: Aphid infestation
Pea aphids (Acyrthosiphon pisum) and closely related species are the most common cause of curling leaves on kidney vetch. Colonies build up on soft shoot tips during spring and early summer. The feeding action of dense clusters of aphids causes the small pinnate leaflets to curl inward and downward, protecting the colony from wind and predators. You may also notice sticky honeydew deposits on the stems and leaves below, and sometimes sooty mould growing on the honeydew.
In a wildlife grassland or meadow setting, do not reach for an insecticide. Kidney vetch growing in or near a small blue butterfly habitat must not be treated with any pesticide. Even a single application of a contact insecticide to a plant in a colony site can kill small blue eggs laid on the flowers, caterpillars feeding on the developing seeds, and adults visiting for nectar. Many insecticide active ingredients also persist in the soil and on plant surfaces long after application. Parasitic wasps, ladybird adults and larvae, lacewing larvae, and hoverfly larvae will naturally regulate aphid populations over two to three weeks. Patience is the right response in a conservation context.
In a purely ornamental garden where small blue butterflies are genuinely absent, the only safe mechanical treatment is a strong jet of water directed at the shoot tips to dislodge the colony. Repeat every few days until natural predators take over.
Cause 2: Powdery mildew
Powdery mildew caused by Erysiphe trifolii or a related Fabaceae-specific species produces a white or pale grey powdery coating on the upper surfaces of the leaflets and sometimes on the stems. Affected leaflets curl upward at the edges and may yellow. The condition is most common in warm, dry weather combined with poor air movement around the plant. Although the silky leaf surfaces and woolly calyxes of kidney vetch provide some physical protection, mildew still occurs, particularly on plants growing in sheltered spots or crowded plantings.
Improve air movement by thinning crowded vegetation around the plant. In a garden setting, plant or sow kidney vetch in an open, well-drained position from the outset. Avoid overhead watering late in the day. Removing badly affected shoots is usually sufficient for an otherwise healthy plant. Fungicide sprays are unnecessary and potentially harmful in a wildlife context.
Other causes of leaf curl
Drought stress. Kidney vetch is a plant of thin, dry, calcareous soils and is highly drought-tolerant once its deep taproot is established, typically from its second season onward. However, seedlings and transplants in their first season may curl their leaves during prolonged hot, dry spells as a water-conservation response. Water sparingly at the base during the first summer if conditions are extreme, then reduce or stop watering entirely as the plant establishes.
Rust. Uromyces sp. rust on Anthyllis is relatively rare but does occur. Look for orange or rust-brown powdery pustules on the undersides of the leaflets, sometimes accompanied by pale or yellow patches on the upper surface and mild leaf curl. Affected leaves can be removed. No fungicide treatment is necessary or advisable in a wildlife setting.
Slug damage on young seedlings. Slugs will graze on kidney vetch seedlings, particularly in damp conditions. Ragged edges and a silvery slime trail are the giveaway. Thin patches or complete seedling loss in wet spells are more common symptoms than leaf curl specifically, but heavily grazed seedlings may distort as they try to regrow. Use physical barriers such as grit or crushed shells around a seed bed rather than slug pellets.
Iron chlorosis on very alkaline heavy soils. Although kidney vetch prefers alkaline conditions, very high-pH waterlogged or heavy soils can lock up iron sufficiently to cause interveinal yellowing and leaf distortion. This is uncommon on the free-draining soils where kidney vetch naturally thrives; if it occurs, improving drainage is a more effective solution than attempting to lower the soil pH.
Kidney vetch and the small blue butterfly
The small blue butterfly (Cupido minimus) is the UK's smallest resident butterfly, with a wingspan of only 16 to 27 mm. The caterpillars feed exclusively on kidney vetch flowers and developing seeds. There is no alternative larval foodplant. This means that wherever small blue colonies exist, they are entirely dependent on the local kidney vetch population. Both have declined significantly across the UK as the short, open, well-drained calcareous grassland they depend on has been lost to agricultural improvement, scrub encroachment, and development.
Kidney vetch is also an important nectar source for bumblebees, the green hairstreak butterfly (Callophrys rubi), and a range of other pollinators. If you discover kidney vetch growing on your land or are establishing it for conservation purposes, protecting it from pest problems using only wildlife-safe methods is not optional: it is the point.
Prevention
The best way to keep kidney vetch healthy and free from serious pest and disease problems is to establish it correctly from the start. Sow directly into calcareous, neutral to alkaline, or free-draining thin soil in situ. Kidney vetch has a deep taproot that does not tolerate transplanting: seed sown where the plant is to grow will always establish better than a transplant, however carefully handled. Sow in late summer or autumn to allow cold stratification over winter.
Choose an open position with good air movement to reduce the risk of powdery mildew. Do not fertilise: kidney vetch is a nitrogen-fixing legume and does not need feeding; rich soils encourage the coarser grasses that will shade it out. Water sparingly in the first season and not at all once established. Keep the surrounding sward short to maintain the open, low-competition habitat kidney vetch needs. Allow natural predator populations to control aphids rather than intervening with any pesticide.
Frequently asked questions
Why are my kidney vetch leaves curling?
The most common causes are aphid colonies on soft shoot tips (particularly pea aphids), powdery mildew in warm dry conditions, drought stress in newly sown plants, rust fungus causing orange pustules on the undersides of leaves, and slug damage on young seedlings. Check the undersides of curled leaves first: dense aphid clusters are the most frequent culprit.
Is kidney vetch important for the small blue butterfly?
Yes. Anthyllis vulneraria is the sole larval foodplant of the small blue butterfly (Cupido minimus), the UK's smallest butterfly. The caterpillars feed exclusively on the flowers and developing seeds, so without kidney vetch the small blue cannot complete its life cycle. Small blue colonies are entirely dependent on kidney vetch populations, and the decline of both is directly linked to the loss of short, open, well-drained calcareous grassland. If you grow kidney vetch in or near a known small blue habitat, treat any pest problem with extreme care and never use insecticides.
Can I use an insecticide on aphids on kidney vetch?
Not if there is any chance of small blue butterflies being present. Even a contact insecticide applied to a single plant can be lethal to small blue eggs, caterpillars, and adult butterflies visiting nearby flowers, and many insecticides persist in the soil. In a wildlife grassland setting, rely entirely on natural predators: parasitic wasps, ladybird adults and larvae, and hoverfly larvae will usually bring aphid colonies under control without any intervention. In a purely ornamental garden setting, a strong water jet aimed at the shoot tips is the only treatment that is safe for all wildlife.
How do I grow kidney vetch for wildlife?
Sow directly into thin, well-drained, calcareous or neutral to alkaline soil in late summer or autumn. Kidney vetch transplants very poorly due to its deep taproot, so sow in situ and do not attempt to move established plants. It thrives on chalk downland, limestone grassland, coastal cliffs, and road verges. Do not fertilise: rich soils favour coarser grasses that out-compete it. Keep the sward short around it (rabbit grazing, sheep grazing, or careful strimming in late summer) to prevent it being shaded out. Once established it is very drought-tolerant.
What does powdery mildew look like on kidney vetch?
Powdery mildew on kidney vetch appears as a white or pale grey powdery coating on the upper surfaces of the small leaflets and sometimes on the stems. Affected leaflets often curl upward at the edges and may go yellow. It is caused by Erysiphe trifolii or closely related Fabaceae-specific mildew species and is most common in warm, dry weather with poor air movement. Improve airflow around plants and avoid overhead watering late in the day. There is no need to spray fungicides on a conservation plant; removing badly affected shoots and improving air circulation is sufficient.