Hedysarum coronarium, known as French honeysuckle, sulla, or sulla clover, is a short-lived perennial or biennial in the legume family (Fabaceae) that produces some of the most intensely coloured flowers of any hardy plant for the UK garden. The upright racemes of vivid crimson-red pea flowers are carried on stems reaching 60 to 90 centimetres through May, June, and into July, and the colour is genuinely striking: a deep, saturated red that stands out even in a mixed border full of colour. The pinnate leaves are composed of small paired leaflets that give the plant a soft, feathery texture through the growing season. Traditionally cultivated in Mediterranean countries as a fodder crop, hedysarum has a long history of use in the agricultural landscape as well as the ornamental garden, and the flowers are edible, with a mild sweet flavour suited to salads.
As a short-lived plant, hedysarum typically germinates in one year and flowers in the next, setting seed freely and then dying back. This natural cycle is fundamental to understanding how to grow it successfully: the colony is maintained by self-seeding, not by individual plants persisting indefinitely. In a good open position on freely-draining chalk, limestone, or sandy soil in full sun, hedysarum establishes itself as a reliable self-seeding presence in the garden, perpetuating from one generation to the next without much intervention. The closely related Hedysarum multijugum, Mongolian sweetvetch, is a more fully shrubby species with pink-purple flowers that suits larger borders and does not share the biennial habit. Hedysarum hedysaroides, sweetvetch, is a native alpine species found in montane habitats in Europe.
The vivid flowers make hedysarum an outstanding choice for wildlife gardens and cottage-style planting, particularly for bumblebees and long-tongued bee species that visit the flowers abundantly. This pollinator value is one reason to think carefully about how you treat pest and disease problems: broad-spectrum insecticides used near the flowers can harm the very bees that make the plant so worthwhile. When the pinnate leaves curl, it is worth identifying the cause to respond proportionately. Here is what to look for.
Aphids
Aphids are the most common cause of leaf curling on hedysarum and the most likely explanation when the youngest leaves at the shoot tips and flower stems are curling downward and inward during spring and early summer. The pea aphid (Acyrthosiphon pisum) and related legume-specialist species are strongly attracted to the soft, fast-growing new growth of hedysarum, establishing colonies on the undersides of the youngest leaflets and on the tender growing tips of both the vegetative stems and the emerging flower racemes. The feeding causes the affected leaflets to curl downward, and in a heavy infestation the growing tips become congested and distorted, with the youngest leaves tightly bunched around the stem and the colony plainly visible on close inspection as dense clusters of soft-bodied, pale green or pink insects.
One complication specific to hedysarum is the timing. The vivid crimson-red flowers are the plant's visual centrepiece, and the eye is naturally drawn to them during May and June when the plant is at its peak. An aphid colony building at the shoot tips or on the stems just below a flowering raceme can be well-established before it is noticed, because the flowers draw attention upward and outward while the infestation develops in the less conspicuous growing tips below. It is worth checking the shoot tips and the undersides of the youngest leaflets regularly from April onwards, before the flowering display begins, to catch colonies early when they are easiest to deal with.
Honeydew deposits on the leaf surfaces below the feeding sites are another indicator: a sticky, slightly shiny coating on leaves that were not treated with anything is a reliable sign that aphids are active above. On hedysarum, which is relatively compact at 60 to 90 centimetres, honeydew can coat the upper leaves of the plant as well as the stems and lower foliage.
The most important thing about treating aphids on hedysarum is what not to use. Broad-spectrum insecticides applied to a flowering plant will kill or deter the bumblebees and long-tongued solitary bees that visit the flowers, eliminating much of the plant's value as a wildlife garden species. On a plant you are growing specifically for its appeal to pollinators, systemic or contact-kill insecticides are counterproductive. Begin with a strong jet of water directed at infested shoot tips to physically dislodge the aphids. Insecticidal soap or a dilute washing-up liquid solution applied to the affected shoot tips kills aphids on contact without the persistent residue of systemic products. Apply in the evening when bees are less active, and target the shoot tips rather than the open flowers. Pea aphid colonies in spring are typically self-limiting once natural predators establish: parasitic wasps, ladybird larvae, hoverfly larvae, and lacewing larvae are all active through late spring and early summer and provide effective biological control once the prey population reaches a sufficient level to attract them. On an established hedysarum colony with multiple plants at different stages, the combination of natural predation and physical removal is usually sufficient to prevent lasting damage.
Powdery mildew
Powdery mildew affects hedysarum most commonly in warm, dry conditions from midsummer onwards, and it is particularly associated with plants in their second or third year that are approaching the end of their natural lifespan. The pathogen responsible on leguminous plants including hedysarum is Erysiphe trifolii, one of the powdery mildew fungi that specialise in the Fabaceae. It produces the characteristic white powdery coating on the upper surfaces of the leaflets, and affected leaves curl at the margins as the fungal growth distorts the leaf tissue. In a moderate infestation the white coating is visible from a short distance, giving the foliage a dull, floury appearance that contrasts noticeably with the healthy sheen of unaffected leaves. Severely affected leaflets may yellow and drop early.
Understanding why powdery mildew is more common on second- and third-year hedysarum than on first-year plants helps to put the problem in perspective. By the time a hedysarum plant is producing its main flowering display in its second year and beginning to set seed, it has largely completed its life cycle. The plant's energy is invested in reproduction rather than defence, and declining vigour in the post-flowering period makes it more susceptible to opportunistic pathogens including powdery mildew. A colony of self-seeded first-year plants growing nearby, which have not yet flowered and are still in their most vigorous phase, will typically show little or no mildew even when the older plants around them are significantly affected. This is a useful observation because it means that, in a well-managed self-seeding colony, the most mildewed plants are usually also the ones that are most expendable from a propagation standpoint, having already set seed for the next generation.
The practical response to powdery mildew on hedysarum is therefore different from the response on a long-lived perennial where you want to preserve the individual plant for many years. On a severely mildewed second- or third-year hedysarum that has already set seed, the most sensible approach is to remove the plant and allow the self-seeded replacements to take over, rather than spending effort on fungicide treatments that will extend the plant's life only marginally. Before removing a mildewed plant, check that seed has been set and that there are young seedlings or ripened seed pods present: if not, it is worth leaving the plant to complete its seeding cycle before removal, accepting the mildew as a cosmetic nuisance in the interim.
Where mildew appears on a first-year plant or on a specimen that you want to preserve through a second flowering season, improve airflow around the affected plant by thinning nearby vegetation, water during dry periods to reduce the drought stress that makes the disease worse, and apply a potassium bicarbonate fungicide spray at the first sign of white coating on the leaflets. Removing heavily affected leaves reduces the amount of inoculum available to spread the disease within the plant. A freely-draining, open position in full sun with good air movement is significantly less conducive to powdery mildew than a crowded, sheltered bed, and this is one of several reasons why hedysarum is best grown in an open, unshaded site.
Other causes of curling leaves
Waterlogging is a more serious problem for hedysarum than is often appreciated, and it is a particularly significant risk for gardeners on heavy UK clay soils. Hedysarum coronarium originates from the Mediterranean basin and is adapted to freely-draining soils, often chalk, limestone, or sandy loam, that shed water rapidly even after prolonged rain. In the clay soils common across much of lowland England, winter waterlogging creates conditions that cause root rot in hedysarum, leading to yellowing, wilting foliage, and eventual plant death. The symptoms can appear suddenly in late winter or early spring when the plant has seemed healthy through the autumn. On heavy clay, add horticultural grit thoroughly to the planting area before sowing or planting to open up the soil structure, and consider raising the bed slightly to improve drainage around the root zone. Hedysarum growing in freely-draining chalk or sandy soil rarely suffers waterlogging problems and is significantly more tolerant of wet winters than plants in clay.
Drought stress causes the pinnate leaflets to wilt and curl inward along their length as the plant reduces its transpiring surface in dry conditions. Hedysarum is more drought-tolerant than many garden plants once it is established, particularly in freely-draining chalk or sandy soils where the root system can spread widely. However, newly sown or recently transplanted plants have not yet developed the root system to access deep soil moisture, and they may wilt and curl in their first summer during prolonged dry spells. Water young plants through their first growing season during dry periods. Once established in a well-drained, open position, hedysarum manages most UK summers without supplementary watering, and plants on chalk or limestone are particularly tolerant of dry conditions.
Slug damage is a significant risk on young hedysarum plants and on self-seeded seedlings in their early stages, and it is worth taking precautions particularly through autumn and early spring when seedlings are small and most vulnerable. Slugs feed on the young foliage and tender growing tips, leaving irregular holes and ragged edges alongside curling from the direct feeding damage. A ring of horticultural grit around seedling clusters provides physical deterrence, and checking beneath surrounding low vegetation for slug colonies is worthwhile if seedlings are disappearing or showing consistent feeding damage. Once hedysarum plants have grown beyond the seedling stage and the stems begin to toughen, they become substantially less attractive to slugs.
Frost damage to the overwintering rosettes of first-year hedysarum plants is a risk in severe winters in northern and exposed UK gardens. The rosette of pinnate leaves that first-year plants form through autumn and winter is relatively hardy in most UK conditions but can be damaged or killed by prolonged hard frost in exposed positions or during exceptionally cold winters. A mulch of straw or garden compost applied over the crowns in late autumn provides insulation without smothering the rosette, and this precaution is particularly worthwhile in northern gardens or in exposed sites. Second-year plants that are already taller and more substantial before winter sets in are generally hardier than the small first-year rosettes.
Prevention and long-term care
Grow hedysarum in full sun in freely-draining soil on chalk, limestone, or sandy loam for the best results and fewest problems. The combination of good drainage, full sun, and open air movement addresses the most common causes of difficulty: it reduces waterlogging risk, lowers powdery mildew pressure, and produces the most vigorous and floriferous plants. On clay, thorough incorporation of grit before planting is worthwhile, though chalk or sandy soils are genuinely preferable if you have the option.
Allow self-seeding for continuity. Because hedysarum is a short-lived perennial or biennial, the colony is maintained through self-seeded replacements rather than by individual plants persisting indefinitely. Leave at least some of the spent flower stems to ripen fully and shed their seed before removing them. The self-seeded plants that emerge in the following season establish with an undisturbed root system that often makes them more vigorous than container-transplanted specimens, and in a good open position hedysarum can perpetuate itself year after year with minimal intervention once the self-seeding cycle is established.
Protect young plants and overwintering seedlings from slugs, particularly through the first autumn and the following spring before the plants have grown beyond the most vulnerable seedling stage. A ring of horticultural grit or copper tape around seedling clusters, or regular checks and physical removal of slugs from the area, reduces losses significantly. This is the period when slug damage can eliminate a whole generation of seedlings and break the self-seeding continuity.
Improve airflow around plants approaching the end of their natural lifespan to reduce powdery mildew pressure. Thinning nearby vegetation and removing congested stems from old plants allows air to move freely through the foliage. On a self-seeding colony, the practical way to manage airflow is to allow the younger self-seeded plants to establish in the more open positions while accepting that the older flowering plants in their second or third year may develop some mildew in late summer without it affecting the colony's long-term continuity.
Avoid insecticides near the flowers. Hedysarum is outstandingly valuable for bumblebees and long-tongued solitary bee species, and this is one of the plant's most important qualities in a wildlife garden or cottage planting. Using contact or systemic insecticides on or near a flowering hedysarum removes much of this value. Aphid problems on the shoot tips can be managed with physical removal and insecticidal soap applied away from the open flowers, ideally in the evening when bees are not foraging.
Frequently asked questions
Why are my hedysarum leaves curling?
Aphids are the most common cause of hedysarum leaves curling, particularly the pea aphid (Acyrthosiphon pisum) and related legume-specialist species that colonise the soft new shoot tips and cause downward leaf curl. Powdery mildew causes a white coating on the leaflets and marginal curl, most often on plants in their second or third year approaching the end of their natural lifespan. Drought stress causes the pinnate leaflets to wilt and curl inward in prolonged dry conditions, while waterlogging in heavy clay soils and slug damage to young plants are less common causes.
How long does hedysarum live?
Hedysarum coronarium is a short-lived perennial or biennial, typically flowering in its second year and then setting seed and dying back. Most plants live for two to three years at most. This is entirely natural behaviour and not a sign of disease or poor cultivation. The plant maintains itself in the garden by self-seeding freely, so a well-established colony perpetuates itself year after year through successive generations of seedlings. The key is to allow at least some plants to set seed each year rather than deadheading all the spent flowers.
Does hedysarum self-seed?
Yes, and this is essential to long-term success with hedysarum in the garden. Because Hedysarum coronarium is a short-lived perennial or biennial that usually dies after flowering and seeding, the colony persists only through the seedlings that germinate from shed seed. Allow at least some of the spent flower stems to set seed fully before removing them. The seeds germinate readily in a sunny, well-drained site, and the self-seeded plants often establish more vigorously than container-grown transplants because they develop an undisturbed root system from the beginning. In a good open position on chalk or sandy soil, hedysarum can perpetuate itself indefinitely through self-seeding with no intervention.
Are hedysarum flowers edible?
Yes, the flowers of Hedysarum coronarium are edible and have a mild, sweet flavour that suits use in salads. They are used in this way in Mediterranean countries, where the plant has a long history as a traditional fodder and food crop. The vivid crimson-red pea flowers are particularly striking as a garnish or salad ingredient. As with all edible flowers, make sure the plants have not been treated with pesticides before harvest.
Is hedysarum good for bees and wildlife?
Yes, exceptionally so. Hedysarum coronarium produces some of the most intensely coloured pea flowers of any hardy legume, and the vivid crimson-red racemes are highly attractive to bumblebees and long-tongued bee species in May, June, and July. As a member of the Fabaceae, it also fixes atmospheric nitrogen through root nodules, benefiting the soil. The flowers are available at a time when spring-flowering plants are finishing and midsummer species are not yet fully open, making hedysarum a particularly valuable bridge plant in a wildlife garden. Avoid insecticides on or near the flowers to protect the bees that visit them.