Tetragonolobus purpureus, the asparagus pea or winged pea, is one of the most unusual vegetables you can grow in a UK kitchen garden. It is a low-growing, trailing annual in the legume family, Fabaceae, reaching 30 to 45 cm across with softly silvery-grey foliage and, from June through to September, a succession of striking dark crimson-red pea flowers that stop people in their tracks. The flowers give way to the pods that make this plant worth growing: four-winged, ribbed structures that are eaten whole when they reach 2 to 3 cm, at which point they have a mild, distinctly asparagus-like flavour. Let them go longer than that and they become fibrous and inedible. Native to southern Europe and the Mediterranean, asparagus pea has been grown in Britain since at least the eighteenth century but has never found its way into mainstream UK vegetable growing, despite deserving a much wider audience.
The plant works in two roles simultaneously. At the front of a border or in a large container, the combination of the silvery trailing stems, the crimson flowers, and the architectural winged pods is genuinely ornamental. In a kitchen garden or potager, it produces edible pods through the summer months from a plant that takes up very little space. It is well suited to growing at the edge of a raised bed, where the trailing stems can spill over the sides. Sow after the last frosts, typically in May in most parts of the UK, or start indoors in April and transplant out once the weather settles. The seeds germinate readily and the plants grow quickly, with the first flowers usually appearing six to eight weeks after sowing.
When asparagus pea leaves start to curl, the cause is usually one of two things: pea aphid colonies on the soft young growth, or powdery mildew building up on the foliage in warm, dry conditions from midsummer. Other causes including root rot, slug damage, bean seed fly, drought stress, and mosaic virus account for a smaller number of cases but are worth working through if the two main culprits do not match what you are seeing.
Cause 1: Aphids
The pea aphid, Acyrthosiphon pisum, is the primary pest of most legume crops in the UK and the main reason asparagus pea leaves curl during the growing season. Pea aphids are large for aphids, pale grey-green, and they congregate specifically on the softest, most actively growing parts of the plant: the shoot tips, the flower buds, and the undersides of the youngest leaves. Their feeding causes the affected leaves to cup and curl downward, enclosing the colony and making it harder for predators to reach. In young seedlings that have been in the ground for only a few weeks, a heavy aphid infestation can cause significant distortion and check the plant's growth during the critical early weeks before it flowers.
The asparagus pea is particularly vulnerable in its early weeks after planting out. The plants are small, the growth is soft, and the natural predator populations that build up through the garden during May and June have not yet reached the levels needed to keep aphid colonies in check. A colony that arrives on a seedling in mid-May can multiply rapidly through warm weather, producing several generations and hundreds of individuals within a fortnight. By contrast, plants that establish well and begin to flower in June are less badly affected: the colony is diluted across a larger plant, natural predators are more abundant by then, and the plant's growth rate is better matched to the damage being done.
Signs to look for include the characteristic downward cupping of the shoot tips, a sticky coating of honeydew on the leaves and pods below the colony, ants moving up and down the stems to tend the aphids, and the small pale green bodies of the aphids themselves crowded onto the undersides of the youngest leaves.
What to do
Begin with physical control: a strong jet of water aimed at the shoot tips and the undersides of the curled leaves dislodges many aphids immediately without any chemical residue. Repeat this every few days while the colony is active. Yellow sticky traps placed on canes near the plants intercept flying adults before they land and slow the rate of reinfestation. An aluminium foil mulch laid around the base of the plants reflects light upward from below, which disorients aphids and significantly reduces the rate at which they land and colonise the plant. This mulch also retains soil moisture and suppresses weeds, so it is worth using as a matter of course.
Do not reach for a systemic insecticide. Asparagus pea flowers are visited by bees and other pollinators, and systemic treatments taken up by the plant contaminate the flowers as well as the pests. Natural predators, including ladybirds, lacewings, and hoverfly larvae, establish in outdoor gardens by June and July and are effective at reducing aphid populations once they arrive. Where the infestation is severe enough to risk losing the whole plant, a targeted spray of insecticidal soap applied directly to the colony, avoiding the flowers, is the least harmful chemical intervention.
Cause 2: Powdery mildew
Powdery mildew on asparagus pea is caused by Erysiphe pisi, the same fungal species that affects garden peas, broad beans, and other legumes across UK gardens. It appears as a white or pale grey powdery coating on the upper surfaces of the leaves and along the stems, and causes the leaf margins and tips to curl upward as the affected tissue dries and distorts. The disease spreads most rapidly during the warm, dry spells of July and August, when the warm daytime temperatures accelerate spore germination and the dry air reduces the washing-off of spores by rain. It is most common on plants that are growing closely together, where the restricted airflow between stems allows humidity to build up and spore concentrations to increase.
Erysiphe pisi is a specialist of legumes and does not carry over to unrelated plants in the garden. The spores overwinter on plant debris in the soil and on nearby weeds that are also legumes, producing a fresh infection the following season. Asparagus pea plants that are already under stress from drought or root restriction are more susceptible to mildew than vigorous, well-watered plants, because stressed plants produce less waxy cuticle and are therefore easier for the fungal spores to penetrate.
There are no mildew-resistant varieties of Tetragonolobus purpureus currently available to UK gardeners, unlike garden peas where resistance breeding has produced a range of reliable options. Management therefore depends entirely on cultural practices: spacing, watering technique, and pod picking frequency.
What to do
Space asparagus pea plants at least 30 cm apart to allow airflow between them. Thin seedlings if they have germinated more thickly than intended. Water at the base of the plant rather than overhead: wetting the foliage creates the humid leaf surface conditions that favour spore germination. Harvest pods every two to three days once they appear, which opens up the interior of the plant and improves air movement through the canopy. Remove heavily mildewed leaves and dispose of them in the bin rather than composting them, to reduce the spore load in the immediate area. At the first sign of white coating on the leaves, apply a potassium bicarbonate spray to the affected foliage, which disrupts the fungal cell membrane. Sowing early and growing the plants through to the end of their season before serious mildew builds up in late summer is the simplest and most effective long-term approach.
Other causes worth checking
Root rot from overwatering or waterlogged soil is a serious problem for asparagus pea. The plant needs well-drained soil and does not tolerate standing water around the roots. Plants in waterlogged conditions develop yellowing leaves, a general wilting of the whole plant even when the soil is wet, and a dark, soft rot at the base of the stem. If you pull the plant gently and the stem pulls away from the roots cleanly at the base, root rot has already taken hold. The best prevention is planting in free-draining soil or a raised bed with a gritty compost, and avoiding watering in cool wet weather when the soil is already moist.
Slug and snail damage is a significant threat to asparagus pea seedlings immediately after planting out. The soft, fleshy stems of recently transplanted or newly germinated plants are ideal slug food. Slugs eat ragged holes in the leaves and can demolish the growing tip of a seedling entirely, after which the remaining foliage curls and distorts as the plant attempts to push out lateral shoots. Protect seedlings with a generous collar of horticultural grit or sharp sand around the base of each plant. Copper tape around a raised bed or container provides effective deterrence. Check plants in the morning during the first few weeks after planting, when slug damage from the night before is easy to spot and you can remove any slugs sheltering nearby.
Bean seed fly, Delia platura, is a less frequently discussed but genuinely damaging pest of legume seeds and young roots. The larvae hatch from eggs laid near seeds or seedlings and tunnel into the germinating seed or the roots of young plants, killing them before they establish or causing weak, distorted growth. The emerging seedling looks pale, stunted, and may curl at the base. Bean seed fly is more of a problem in soils that have recently had large amounts of organic matter incorporated, which attracts egg-laying females. If you suspect bean seed fly, sow seeds later in the season when fly populations are lower, or start seeds indoors and transplant out as established plants with a root ball large enough to be less vulnerable.
Drought stress in dry weather causes asparagus pea leaves to curl inward along their length and the plant to lose its usual fresh, silvery appearance. The foliage becomes dull, and the flowers may drop before setting pods. Asparagus pea is not drought-tolerant in the way some Mediterranean plants are: it needs consistent moisture to produce a good crop of pods. Water at the base during dry spells, particularly when the plant is flowering and setting pods, and use the aluminium foil mulch to retain soil moisture.
Mosaic virus in legumes, spread by aphids as they move from plant to plant, produces a distinctive mottled pattern of yellow and green across the leaf surface, often accompanied by distortion, puckering, and general stunting. Affected leaves may curl as the green and yellow tissue grow at different rates. There is no treatment for mosaic virus in asparagus pea. Remove and dispose of affected plants in the bin to reduce the reservoir of virus in the garden, control aphid populations to slow the spread, and avoid planting asparagus pea in the same position the following year.
Prevention
Sow asparagus pea in well-drained, moderately fertile soil after the last frosts. A position in full sun or very light afternoon shade gives the best combination of strong growth and good flower and pod production. Incorporate a little general fertiliser or garden compost into the planting area before sowing, but avoid very rich soil: the plant is a legume and fixes its own nitrogen, so overly fertile soil produces lush foliage at the expense of flowers and pods.
Thin seedlings to at least 25 to 30 cm apart to ensure good airflow and reduce the risk of powdery mildew. Protect seedlings from slugs with a grit collar from the moment they emerge or are transplanted. Water at the base consistently during dry weather, using the foil mulch to conserve moisture between waterings. Harvest pods every two to three days from the moment they reach 2 to 3 cm, both to eat them at their best and to keep the plant producing and the canopy open. A plant that is allowed to accumulate a few large, fibrous pods stops producing new ones and becomes more susceptible to mildew as the old pods restrict airflow. Regular picking is the single most important thing you can do to keep asparagus pea healthy and productive through the season.
Frequently asked questions
Why are my asparagus pea leaves curling?
The most common causes of asparagus pea leaves curling are pea aphid colonies on the soft shoot tips, which cause downward cupping of young leaves, and powdery mildew in warm dry conditions from midsummer onwards. Root rot from waterlogged soil causes yellowing, wilting, and stem base collapse rather than straightforward leaf curl. Slug damage on seedlings, drought stress in dry spells, and mosaic virus causing mottled yellow and green distortion are also worth checking.
When should I harvest asparagus pea pods?
Harvest asparagus pea pods when they are 2 to 3 cm long, roughly the length of your thumbnail. At this size the pods are tender, the four wings are fully formed, and the flavour has developed its mild asparagus-like quality. Leave them a week longer and the pods become fibrous and unpleasant to eat. Check the plants every two to three days once flowering starts, as pods develop quickly in warm weather. Frequent harvesting also encourages the plant to produce more pods and improves airflow through the foliage, reducing the risk of powdery mildew.
Can you eat asparagus pea pods?
Yes. The pods of Tetragonolobus purpureus are eaten whole, including the four characteristic wings. They are lightly steamed or stir-fried and have a mild flavour that resembles asparagus when harvested at 2 to 3 cm. The pods must be picked young; anything over 4 to 5 cm becomes fibrous and indigestible. The seeds inside mature pods are not commonly eaten. The dark crimson flowers are also edible and make a striking garnish.
How do I treat aphids on asparagus pea?
Blast colonies off with a strong jet of water, paying close attention to the soft shoot tips and the undersides of the youngest leaves where pea aphids congregate. Yellow sticky traps placed near the plants reduce flying adults. An aluminium foil mulch laid around the base of the plants reflects light upwards and disorients aphids before they land. Natural predators including ladybirds and hoverfly larvae usually establish in the garden by June and July and bring populations under control without intervention. Avoid systemic insecticides, which persist in the plant tissue and risk harming beneficial insects visiting the flowers.
Why is my asparagus pea getting powdery mildew?
Powdery mildew on asparagus pea is caused by Erysiphe pisi, the same species that affects garden peas, and is most common from July onwards in warm, dry conditions when plants are closely spaced. The white coating appears on the upper leaf surfaces and stems and causes the leaf margins to curl. Thin plants to improve airflow, water at the base rather than overhead, and pick pods frequently to open up the canopy. There are no widely available mildew-resistant varieties of Tetragonolobus purpureus. Sowing early and harvesting the final pods before midsummer is the most reliable way to avoid the worst of the mildew season.