Lathyrus Leaves Curling

Lathyrus is a genus of around 160 species in the pea family Fabaceae, ranging from scrambling annuals to long-lived perennial climbers and compact non-climbing border plants. The most famous is Lathyrus odoratus, the sweet pea: an annual climber reaching 2 metres or more, grown for its intensely fragrant flowers in every shade from white through pink, red, and purple. Sweet peas are among the most popular cutting flowers in UK cottage gardens, and the Spencer varieties in particular are a summer staple from June to September. Beyond the sweet pea, L. latifolius (everlasting pea) is a tough, fragrance-free perennial climber for covering fences and banks, L. vernus (spring pea) is a compact, clump-forming perennial flowering in March and April, and L. sylvestris is a native UK species found on chalk scrub and woodland edges. All lathyrus produce the characteristic leaves plus stipules and, in the climbing species, curling tendrils that grip their support. When those leaves start curling, the cause is usually biological, but it helps to know which species you are growing before reaching for a solution.

Cause 1: Aphid infestation

Aphids are by far the most common cause of curling and puckering leaves on lathyrus in UK gardens, and on sweet pea in particular they can be severe enough to stop a plant flowering entirely. Two species are mainly responsible. Pea aphid (Acyrthosiphon pisum) is a large, pale green aphid that colonises the shoot tips and upper stems of sweet peas and other lathyrus species. Black bean aphid (Aphis fabae) is the small, dense, jet-black species that forms tightly packed colonies on sweet pea stems and growing tips from June onwards, and in some years is spectacularly damaging. Both species feed by inserting their mouthparts into the soft new tissue at the growing tips and extracting sap. The feeding prevents the cells from developing normally, and the resulting leaves emerge curled, puckered, blistered, and distorted. In heavy infestations, the shoot tips look as though they have been crumpled. Tendrils are deformed and cannot grip supports properly. The plant looks generally sickly, stem growth slows or stops, and flower production falls sharply.

Black bean aphid on sweet peas is often worst in June and July, which is precisely when the plants should be at peak flowering. Colonies spread rapidly in warm conditions and attract ants, which actively farm the aphids and protect them from predatory insects like ladybirds and hoverfly larvae. If you see ants running up and down the supports of your sweet peas, investigate the shoot tips immediately: the ants are a reliable indicator that an aphid colony is present. A sticky barrier applied to the base of canes and supports stops ants climbing and removes this protection, allowing natural predators to access the colonies.

For small colonies, simply remove the affected growing tips by hand and dispose of them away from the garden. This removes the colony before it has spread. For larger infestations, spray with pyrethrum or insecticidal soap, applying the spray directly to the undersides of all affected leaves and shoot tips where the aphids are densest. Repeat after five to seven days to catch any survivors or newly hatched nymphs. Ladybirds, hoverflies, lacewings, and parasitic wasps are all effective natural controls; avoid broad-spectrum pesticides that kill these beneficial insects alongside the aphids, as this allows the aphid population to rebound unchecked once the chemical effect fades. The key practical step is to inspect sweet pea growing tips at least twice a week from May onwards and act immediately when you see the first colony, before it has time to multiply and spread to neighbouring plants.

Cause 2: Powdery mildew

Powdery mildew is the second most significant cause of leaf problems on Lathyrus odoratus in the UK, and it is particularly associated with the heat and drought stress of late summer. The fungal pathogen Erysiphe trifolii produces a white, powdery coating on the upper surfaces of the leaves and stems. Affected leaves curl at the margins and may develop a slightly bleached, pale appearance under the coating. In severe cases the whole upper portion of the plant takes on a grey-white look and growth slows considerably.

Mildew on sweet peas is almost always a late-season problem, typically appearing from August onwards on plants that have been growing in warm, dry conditions without sufficient soil moisture at the roots. The disease is a sign of stress rather than an aggressive pathogen, and prevention is far more effective than treatment. The three most important prevention measures are: maintain consistent soil moisture throughout the growing season by watering at the base during dry spells (never over the foliage); space plants generously at least 30 cm apart in a row to allow free air movement between stems; and do not grow sweet peas in the same position year after year (see the section on soil sickness below). If mildew appears, remove the worst affected leaves and spray with potassium bicarbonate or a seaweed-based extract. Plants well into their season and already carrying a good flower crop will not recover fully from late mildew, but the condition will not kill them before the season ends naturally.

Other causes

Herbicide drift is the most dramatic single-incident cause of leaf distortion on lathyrus and is more common than many gardeners realise. Hormone weedkillers, including lawn treatments containing 2,4-D, mecoprop, or dicamba, and products based on glyphosate applied nearby, can cause severe cupping, twisting, and downward curling of leaves even in very small concentrations. The distortion typically appears suddenly, affecting multiple plants in a pattern that follows the direction of wind drift, and the new growth emerges crinkled and cup-shaped. The older leaves below the point of exposure usually remain normal. There is no remedy. Remove and compost affected material; some plants produce visibly normal new growth from the unaffected base in subsequent weeks, but others remain distorted for the rest of the season. The practical lesson is to avoid applying any weedkiller near lathyrus on breezy days and to check what neighbouring properties may be applying over the fence.

Sweet pea soil sickness is a distinct and well-known problem that arises when L. odoratus is grown in the same bed or border for more than two consecutive years. The exact mechanism is not fully understood but involves a combination of a build-up of soil-dwelling pathogens specific to the pea family, nematode damage to the roots, and nutrient depletion from a crop with high requirements. Affected plants are noticeably stunted from the start of the season, produce pale or mottled leaves that may curl at the margins, fail to climb vigorously, and flower poorly even in otherwise ideal conditions. The plants look broadly similar to virus-affected specimens, with a general failure to thrive rather than a single identifiable symptom. The fix is straightforward: sweet peas must be grown on a minimum three to four year rotation, moving to fresh ground each season. No amount of feeding, watering, or pest control will overcome the effects of soil sickness; only moving to fresh ground resolves it.

Viruses spread by aphids, particularly pea streak virus and bean yellow mosaic virus, can cause mosaic patterning, colour distortion in the flowers, and leaf curling on sweet peas. Virus symptoms are distinguishable from aphid-induced distortion because the patterning on the leaf is mosaic or mottled rather than the pure puckering and crumpling of aphid feeding damage. There is no treatment for virus infection: remove and dispose of (not compost) affected plants and control aphid populations rigorously to limit spread to neighbouring plants.

L. latifolius (everlasting pea) is a substantially tougher plant than sweet pea and suffers far fewer problems. Established clumps growing into hedges or covering banks are rarely troubled by aphids or mildew. This perennial produces masses of magenta-pink flowers for weeks in summer with minimal attention, tolerating drought, poor soil, and exposed conditions that would kill sweet pea outright. The main thing to know if you are comparing the two plants: everlasting pea has no scent whatsoever. L. vernus, the compact spring-flowering perennial, is similarly trouble-free in the right position and asks for little beyond a reasonable soil and something to scramble into at the base of a border.

Frequently asked questions

Why are my sweet pea leaves curling and puckering at the tips?

Curled, puckered, or crinkled growth at the shoot tips of sweet peas is almost always caused by an aphid colony. Pea aphids and black bean aphids cluster at the growing tips and feed on the soft new tissue before it has hardened, distorting the cells as they develop. Check the underside of the affected tips closely: you will usually find dense colonies of green or black aphids, sometimes attended by ants. Remove the infested tips by hand if the colony is small. For larger infestations, spray with pyrethrum or insecticidal soap, covering the undersides of all affected shoot tips thoroughly.

Why do sweet peas fail to thrive when grown in the same spot year after year?

Sweet peas suffer from a well-documented soil sickness when returned to the same bed repeatedly. A build-up of soil pathogens, nematodes, and exhausted nutrients combines to produce stunted plants, pale and curling leaves, poor flowering, and a general failure to thrive that worsens each successive year. The fix is simple but non-negotiable: move sweet peas to a fresh position on a rotation of at least three to four years. The deep trench preparation that sweet peas need is best done in a new location each season rather than in the same bed year after year.

What is the difference between sweet pea (Lathyrus odoratus) and everlasting pea (Lathyrus latifolius)?

Sweet pea (L. odoratus) is an annual climber that must be grown from seed each year. It is beloved for its intense fragrance and wide range of flower colours, and the Spencer types in particular are the quintessential cottage garden cutting flower. It is not frost hardy as an adult plant and typically flowers June to September before dying. Everlasting pea (L. latifolius) is a tough, long-lived perennial climber that dies back to the root each winter and returns reliably each spring. It produces masses of magenta-pink unscented flowers over many weeks in summer and is far more tolerant of neglect, drought, and poor soil than sweet pea. Crucially, everlasting pea has no fragrance at all. The two plants look similar in habit but are grown very differently and have different problems.

Can herbicide drift cause sweet pea leaves to curl?

Yes, and this is the most common single-incident cause of severe leaf distortion in sweet peas. Even a tiny amount of hormone weedkiller drifting on the wind from a neighbouring garden can cause dramatic leaf cupping, twisting, and distortion within days. The distortion is typically very sudden in onset and affects multiple plants in a similar pattern. There is no treatment. Some plants recover partial normal growth as new shoots emerge; others remain distorted through the season. Prevention means timing any weedkiller use for still days and keeping sweet peas away from areas where lawn treatments are regularly applied.

How do I stop powdery mildew on sweet peas?

Powdery mildew on sweet peas is primarily a late-summer problem driven by hot, dry conditions and poor air circulation. The best prevention is good spacing of at least 30 cm between plants, consistent watering at the base rather than over the foliage during dry spells, and not growing sweet peas in the same spot year after year. Once mildew appears, remove the worst affected leaves and apply potassium bicarbonate or a seaweed-based spray. Plants that have already flowered well and are beginning to go over will rarely recover fully from late mildew, so focus prevention efforts on the early and mid-season period when the plants are actively flowering.