Sweet peas are one of the UK's most-grown and most-loved cutting flowers, raised for their intense fragrance and extraordinary colour range. They climb quickly, flower generously when picked regularly, and thrive in the cool British spring. But they are also prone to a handful of problems that all tend to show up the same way: leaves that curl, pucker or distort, and flowers that start to suffer. Getting to the right diagnosis quickly saves the season.
Why sweet pea leaves curl: the two main causes
The vast majority of curling on sweet pea leaves comes down to one of two problems: an aphid infestation or powdery mildew. Both are extremely common in UK gardens, both peak in warm, dry conditions, and both are very treatable when caught early. A close look at the leaves themselves will usually tell you which one you are dealing with before you even need to touch the plant.
Cause 1: Aphids
Sweet peas attract two species of aphid in particular. The pea aphid (Acyrthosiphon pisum) is larger than a typical greenfly, pale green or pinkish-green, and tends to gather in loose colonies on shoot tips, tendrils and the undersides of young leaves. The black bean aphid (Aphis fabae) forms dense, almost sooty-looking masses, often clustering so thickly around growing tips that the stem is barely visible beneath them. Both species pierce plant tissue to feed on sap, and the physical damage from that feeding is what causes leaves to curl downwards and inwards at the edges. As colonies grow, the curling becomes more pronounced and the growing tip may become stunted or deformed entirely.
Alongside the curling, look for a sticky, shiny film on leaves and stems (honeydew, a sugary waste product the aphids excrete), and for black sooty mould that develops on top of the honeydew. Both are reliable secondary indicators that aphids are or have recently been present. There is also a more serious secondary problem: some aphid species, including pea aphid, can act as vectors for plant viruses. Once a virus is transmitted, the leaf distortion becomes permanent and the plant cannot recover from that particular damage, even after the aphids are removed.
How to fix an aphid infestation
For light to moderate infestations, a firm jet of water directed at the colonies is often enough to knock them off. Do this early in the morning so the foliage can dry out before evening. Repeat every two or three days for a fortnight to catch newly hatched nymphs. For heavier infestations, an insecticidal soap spray (available as a ready-to-use product from most garden centres) applied directly to the colonies is highly effective and breaks down quickly without harming pollinators once dry. Spray in the evening when bees are less active to be safe. Encourage natural predators by planting companion flowers nearby: ladybirds, lacewings and hoverflies will all help to keep aphid numbers in check over the season. Avoid over-feeding sweet peas with nitrogen-rich fertiliser, as soft, sappy new growth is the most attractive to aphids.
Cause 2: Powdery mildew
Powdery mildew on sweet peas is caused by the fungus Erysiphe pisi, the same pathogen responsible for mildew on garden peas. It is a surface fungus and appears as irregular white or grey powdery patches on the upper surface of leaves, though it can also affect stems and flower buds. Affected leaves typically curl or pucker at the edges, take on a slightly bleached look, and may begin to yellow where the fungal growth is most dense. In severe cases the entire plant takes on a dusty, unhealthy appearance and flower production drops off sharply.
Powdery mildew on sweet peas in the UK is most common from June onwards, during warm spells with dry soil. Unlike many fungal diseases, powdery mildew actually prefers dry conditions at the root zone combined with warm, humid air around the foliage, which is exactly what happens on a south-facing wall during a British summer. Plants that are already stressed by inconsistent watering or root competition are far more susceptible than well-established, well-watered ones.
How to fix powdery mildew
Remove and bin (do not compost) any severely affected leaves and stems to reduce the spore load. Water deeply and consistently at the base of the plant rather than overhead, since wet foliage at night encourages many other fungal problems even as it has little effect on powdery mildew itself. A dilute solution of bicarbonate of soda (one teaspoon per litre of water, with a few drops of washing-up liquid to help it stick) sprayed onto affected foliage can slow the spread. Proprietary fungicide sprays labelled for powdery mildew are available if the infection is widespread, though they control rather than cure. The single most effective long-term measure is to keep roots moist through dry periods, since a plant that is not drought-stressed is far less vulnerable to infection in the first place.
Other causes worth knowing
Not every problem with sweet pea foliage comes down to aphids or mildew. The pea and bean weevil (Sitona lineatus) feeds on leaf margins and leaves a characteristic pattern of U-shaped or scalloped notches around the edges of leaflets. This notching can make leaves look curled or ragged, but it does not cause the inward rolling that aphids produce. Adult weevils are mostly active at night and are rarely seen, but the damage pattern is distinctive enough to diagnose without catching them in the act. Plants usually recover without intervention once growth outpaces the damage.
Fusarium wilt, caused by Fusarium oxysporum, is a soil-borne fungal disease that causes sudden wilting and rapid yellowing, often appearing to kill a plant almost overnight. It can be confused with drought stress but does not respond to watering. There is no treatment; affected plants must be removed and the soil in that bed should not be used for sweet peas or other legumes for at least four years to avoid reinfection.
Bud drop, where flowers abort before opening, is almost always a response to heat or drought stress rather than disease. It is especially common on plants grown against a south-facing wall during a hot June or July. Consistent deep watering and a thick mulch over the root zone will reduce it significantly. Finally, leaf scorch showing as brown or crispy leaf edges sometimes follows an overenthusiastic feed with a high-nitrogen fertiliser. Sweet peas do not need heavy feeding; a balanced fertiliser used sparingly once flowering begins is sufficient.
Prevention: keeping sweet peas healthy all season
- Water deeply at the roots two or three times a week in dry weather rather than watering little and often at the surface. Deep watering encourages roots to go down where the soil stays moister for longer.
- Apply a 5 to 8 cm layer of mulch (garden compost, bark or well-rotted manure) over the root zone after planting to retain moisture and keep soil temperatures even.
- Train plants carefully up their supports from the start and tie in regularly. Good airflow up a well-spaced row of canes or a trellis dramatically reduces the conditions that favour both aphids and mildew.
- Pick flowers as soon as they open, before any seed pods begin to form. Sweet peas stop flowering once seed is set; regular cutting keeps them productive for months.
- Avoid high-nitrogen feeds, which produce the soft sappy growth aphids find most attractive. If you do feed, use a balanced fertiliser or one with a higher potassium content (such as a tomato feed) to encourage flowering rather than leafy growth.
- Consider autumn sowing (October into deep root trainers in a cool greenhouse or cold frame) for plants with stronger, deeper root systems that are better placed to cope with summer drought stress. Autumn-sown plants also flower earlier in the UK season.
- Choose mildew-resistant varieties for exposed or dry sites. Varieties such as Alan Titchmarsh, Jilly, and others bred for garden performance have been selected partly for their tolerance of the conditions that trigger mildew in older Spencer types.
Frequently asked questions
Why are my sweet pea leaves curling downwards?
Downward curling on sweet peas is almost always caused by aphid infestations, most often pea aphids (large, pale green) or black bean aphids (dense, dark clusters). Check the underside of leaves and around shoot tips. A blast of water or an insecticidal soap spray will remove them quickly.
Can I still cut sweet peas with powdery mildew?
Sweet peas are grown for cutting and fragrance, not for eating. Flowers from mildew-affected plants are still safe to cut and use in arrangements, though the blooms may be slightly smaller. Removing infected foliage and improving airflow will slow the spread and keep flowers coming for longer.
My sweet peas wilted overnight and the leaves are yellow. Is it aphids?
Sudden overnight wilt combined with yellowing is more likely to be fusarium wilt, a soil-borne fungal disease, than aphids. Aphids cause gradual curling and distortion rather than rapid collapse. Unfortunately there is no cure for fusarium once plants are infected. Remove and dispose of affected plants (do not compost them) and avoid growing sweet peas in the same soil for at least four years.
How do I stop powdery mildew on sweet peas in summer?
The key is keeping the roots consistently moist through dry spells so the plant does not stress. Water deeply at the base of the plant two or three times a week in hot weather, and apply a thick mulch to retain moisture. Thin out stems for better airflow, avoid overhead watering, and choose mildew-resistant varieties like Alan Titchmarsh or Jilly for future seasons.
When should I sow sweet peas for the best results in the UK?
The two main sowing windows are October (autumn sowing, overwintered in a cool greenhouse or cold frame for the earliest flowers and stronger root systems) and January to March (under glass or on a warm windowsill). Autumn-sown plants tend to be more vigorous and flower earlier, and stronger, deeper-rooted plants are better able to resist both aphid pressure and drought stress.