Ornithopus sativus, serradella, is an annual or short-lived perennial legume in the Fabaceae family, native to southern Europe and long cultivated in Portugal, Germany, and the Netherlands as a fodder crop and green manure for light acid sandy soils. It is the legume that works where others fail: on very poor acid sandy soil with a pH as low as 4.5, where clover yellows and dies, where vetches fail to nodulate, and where most green manures struggle to establish, serradella grows reliably, fixes atmospheric nitrogen through root nodules, and produces a substantial leafy mass that improves soil structure and fertility when incorporated. Its fine pinnate leaves give it a soft, feathery appearance, and the small pink or white pea flowers are attractive to bees. The distinctive seed pods, which are jointed and curved like a small bird's foot, give the genus its name: ornithopus is derived from the Greek for bird's foot.
Ornithopus perpusillus, common bird's-foot, is a very different plant in scale. This UK native wildflower is tiny and delicate, typically reaching just 5 to 20 cm in height, with hair-fine pinnate leaves and miniature pink flowers with fine red veining. It grows in open, disturbed, and short sandy and gravelly grassland, heathland margins, rabbit-grazed ground, and sandy tracks on acid or neutral soils. It is a plant of a very particular and declining habitat. Across much of England it has disappeared as its open sandy ground habitat has closed over with rank grass and scrub, or been lost to development. It remains locally frequent in the Breckland of Norfolk and Suffolk, the Surrey heaths, Dorset, and coastal sandy areas in southern England. Both species produce the same characteristic jointed seed pods that split into individual segments, each containing one seed, and both fix nitrogen.
When serradella or bird's-foot leaves start to curl, the two most common causes are aphid colonies on the soft new growth and waterlogging of the root zone. Several other causes are worth checking once the main two are ruled out.
Cause 1: Aphids
The pea aphid, Acyrthosiphon pisum, is a large, pale green aphid that feeds on many legume species and frequently colonises ornithopus. It is joined by several related species that are recorded on members of the Fabaceae family. Colonies form rapidly on the soft new shoot tips and the undersides of the youngest, most tender leaflets, particularly through late spring and early summer when the flush of new growth provides the most nutritious feeding sites. The small pinnate leaflets of serradella respond to the feeding by curling tightly downward, cupping around the colony in a way that both shelters the aphids from natural predators and makes the infestation visible as a cluster of distorted, rolled foliage at the shoot tips.
A sticky film of honeydew, the sugary excretion of aphid feeding, coats the leaf surfaces below the colony and the soil immediately around the plant. Ants tending the aphids for this honeydew are a useful secondary indicator that aphids are present even before the leaf curl becomes obvious. In larger colonies, the honeydew can support the growth of a grey-black sooty mould, which further reduces the photosynthetic function of the affected leaves.
In most cases, aphid colonies on ornithopus are self-limiting. Ladybirds, hoverfly larvae, and parasitic wasps discover and exploit aphid colonies through early summer, and natural predator pressure often reduces populations without any intervention. The plant grows through the leaf distortion at the shoot tips without lasting harm, producing new growth above the damaged leaves as conditions improve. In a wildlife garden where you are growing bird's-foot or serradella specifically to support insect life, leaving the colony to the natural predators is always the right decision.
For heavier infestations on young plants or crop sowings where establishment is the priority, direct the spray from a garden hose at the curled shoot tips and the undersides of the affected leaves. The physical displacement of aphids by a strong water jet reduces populations rapidly without leaving chemical residues. Insecticidal soap spray applied to the colony directly is effective and breaks down quickly. In wildlife gardening contexts, avoid any insecticide; the bees that visit ornithopus flowers and the other insects visiting the plant are too valuable to risk.
Cause 2: Waterlogging
Ornithopus sativus evolved on the light, acid, freely draining sandy soils of southern and central Europe. It has been selected and cultivated specifically for its ability to grow on the poorest, sandiest, most acid soils where almost all other agricultural legumes fail. The drainage and aeration characteristics of these soils are the opposite of heavy clay or waterlogged ground. When serradella is planted or sown in heavy clay, in a low-lying position where water pools, or in soil that remains saturated for more than a few days after rain, the roots are deprived of oxygen and begin to rot. Waterlogging is one of the fastest ways to kill ornithopus.
The symptoms of waterlogging develop over one to three weeks. The first visible sign is a general yellowing of the lower leaves as the root system becomes unable to take up nutrients. The leaves then begin to curl and wilt, drooping at the tips of the shoots despite the soil remaining wet. This is the paradox of waterlogging symptoms: the plant wilts not because it lacks water but because its damaged roots can no longer transport water to the shoots, even in saturated soil. If you pull a waterlogged plant gently you will find the roots are dark, soft, and may have a sour smell of anaerobic decomposition rather than the pale, firm roots of a healthy plant.
In heavy clay soils that retain moisture, the appropriate response is to move ornithopus to a better-drained position entirely. Where drainage is adequate but a particular bed or corner collects water after rain, improving drainage by incorporating sharp horticultural grit to a depth of 20 to 30 cm before resowing makes the site suitable. Serradella is particularly valuable on poor acid sandy soils precisely because these soils drain freely and are too acid and infertile for competitive legumes; using it on clay or heavy loam undercuts its main strength.
Other causes worth checking
Powdery mildew caused by Erysiphe trifolii and related species in the legume family can affect ornithopus in warm, dry summer conditions. The fungal infection produces a white floury coating on the surface of the pinnate leaflets and causes the leaf margins to curl and distort. It is most common in plants grown in crowded conditions without adequate air circulation, or in dry spells when the plants are under mild drought stress. On short-season crops of serradella grown as a green manure, mildew appearing late in the season is not usually worth treating because the plants are approaching incorporation anyway. In longer-term sowings or in ornamental wildlife gardening, improving airflow by thinning dense stands and removing the most affected growth reduces the spread.
Slug damage on seedlings is a real problem in wet springs. Slugs rasp holes and notches across the small leaflets of young ornithopus plants and may eat the growing tips entirely, leaving the remaining foliage to curl and distort as the plant attempts to recover with side growth. Protect seedlings with a layer of sharp grit or coarse sand around them; slugs dislike crossing abrasive surfaces. This is especially useful for serradella sown as a green manure where getting a complete stand established quickly is important.
Drought stress on young plants before they are fully established causes inward curling of the leaflets along the midrib and a loss of the soft, fresh green colour. Serradella is described as drought-tolerant and it is, but this tolerance develops as the root system extends into the soil over several weeks. Seedlings and young plants in the first few weeks after germination have a limited root zone and need moisture to establish well. Water at the base of the plant during dry spells in the first four to six weeks; once established, ornithopus manages most UK summer conditions without supplementary watering.
Nitrogen fixation failure is a less obvious cause of poor growth and leaf problems in ornithopus. Like all legumes, serradella fixes atmospheric nitrogen through a symbiotic relationship with soil bacteria called rhizobia, which colonise the plant's roots and form visible pink-coloured nodules. On very acid soils below pH 5.0, the relevant rhizobium strains may not be naturally present in the soil in sufficient numbers to colonise the roots effectively. Without active nodules, the plant cannot fix nitrogen, grows poorly on the lean sandy soils where it is typically sown, and develops yellowed, stunted foliage that may curl. The solution is to inoculate the seeds before sowing with a commercial rhizobium inoculant sold specifically for legume green manures or serradella seed. Mix the inoculant with the seed before sowing according to the manufacturer's instructions, keep the inoculated seed out of direct sunlight, and sow promptly. On soils where serradella has been grown before, rhizobium populations are usually adequate without inoculation.
Prevention
Sow serradella in freely draining acid to neutral sandy soil with a pH of 4.5 to 6.5. This is its natural habitat range, and the conditions in this range are where it establishes most reliably and provides the most benefit. Avoid clay, heavy loam, or any position where water stands after rain. On very acid soils below pH 5.0, inoculate the seeds with a rhizobium inoculant before sowing to ensure effective nitrogen fixation from the start. Monitor young plants for aphid colonies on the shoot tips through late spring and early summer, and protect seedlings from slug damage with a grit mulch around the base. Keep the soil around young plants consistently moist for the first four to six weeks after germination to support establishment, then reduce watering as the plant develops its root system. Incorporate serradella as a green manure before the plants flower, when the green biomass is at its maximum, by digging the top growth and roots into the top 15 to 20 cm of soil.
For wildlife gardening with Ornithopus perpusillus, source seed from a reputable UK wildflower supplier and sow on very open, short, disturbed acid sandy soil, ideally in late summer or early autumn. The plant needs competition-free ground to establish; it will not succeed in rank grass or dense plantings. Leaving areas of bare or sparsely vegetated acid sandy soil near the sowing is the single most important factor in giving bird's-foot the conditions it needs to persist and self-seed year on year.
Frequently asked questions
Why are my serradella leaves curling?
The two most common causes of serradella leaves curling are aphid colonies on the soft new shoot tips and waterlogging of the root zone. Pea aphids and related species cause downward curling of the small pinnate leaflets, with visible honeydew below the colony. Waterlogging causes yellowing, wilting, and leaf curl as the roots rot; ornithopus requires freely draining acid to neutral sandy soil and dies quickly in wet conditions. Other causes include powdery mildew in warm dry weather, slug damage on seedlings, drought stress before establishment, and nitrogen fixation failure on very acid soils below pH 5.0 without rhizobium inoculation.
What soil pH does serradella need?
Ornithopus sativus is specifically adapted to acid sandy soils in the pH range of 4.5 to 6.5, a range where most other legume green manures such as clover and vetches fail. It tolerates acidity that would prevent nodulation in other legumes, making it one of the few nitrogen-fixing options for gardeners with very acid sandy soils. However, on soils at the lower end of this range, below pH 5.0, the rhizobium bacteria needed for nitrogen fixation may not be naturally present in sufficient numbers, and inoculating seeds with an appropriate rhizobium inoculant before sowing gives much more reliable results. O. perpusillus, bird's-foot, is also a plant of acid to neutral sandy soils and dislikes alkaline or clay conditions.
How do I use serradella as a green manure?
Sow serradella in spring or summer on cleared, freely draining acid to neutral sandy soil. Sow seed thinly, cover lightly, and keep moist until germination. On very acid soils below pH 5.0, pre-inoculate the seed with a rhizobium inoculant sold for legume green manures to ensure effective nitrogen fixation. Allow the plants to grow for 8 to 12 weeks, then incorporate them into the soil as a green manure before they flower. Dig or till the top growth and roots into the top 15 to 20 cm of soil. The decomposing biomass releases fixed nitrogen and organic matter that improves the structure of sandy soils, making subsequent crops better supported. Serradella is particularly useful in no-dig and organic systems on acid sandy soils where alternatives like clover and field beans do not establish well.
What is the difference between Ornithopus sativus and Ornithopus perpusillus?
Ornithopus sativus, serradella, is an annual or short-lived perennial legume native to southern Europe and traditionally grown as a fodder crop and green manure on poor acid sandy soils. It grows 30 to 60 cm tall, produces small pink or white pea flowers, and is cultivated as a crop plant. Ornithopus perpusillus, bird's-foot, is a native UK wildflower, much smaller and more delicate, typically 5 to 20 cm in height, with tiny pink-veined flowers and very fine pinnate leaves. Bird's-foot is a plant of short sandy and gravelly grassland, tracks, and heathland margins, and is declining in the UK due to habitat loss. Both species produce the distinctive seed pods that look like a small bird's foot, which gives the genus its name. Bird's-foot is the better choice for wildlife gardening and heathland restoration; serradella is the better choice for green manuring and cover cropping.
Is Ornithopus perpusillus rare in the UK?
Yes. Ornithopus perpusillus, bird's-foot, is a declining species in the UK. It is a plant of open, disturbed, and short sandy or gravelly grassland, heathland edges, sandy tracks, and rabbit-grazed ground on acid soils. It has declined as these habitats have been lost to development, agricultural improvement, and the succession of open sandy ground to rank grass and scrub. It remains locally frequent in the Breckland of East Anglia, in the Surrey heaths, in Dorset, and on coastal sandy areas in the south and east of England, but has disappeared from many former sites. Including it in heathland restoration projects, sandy wildlife gardens, and gravelly path margins on acid soils is valuable for maintaining and expanding the population.