Sweet peas are one of the most rewarding annual climbing flowers you can grow in a UK garden. The fragrance of a well-grown Spencer variety in full flower is genuinely extraordinary, and the colour range, from pure white through every shade of pink, red, purple, and bicolour, is hard to match among annuals. They climb to 1.5 to 2.5 metres on canes, netting, or pea sticks, flower prolifically from June through August when picked regularly, and reward careful growers with armfuls of cut flowers all summer. They are also, compared with their relatives in the vegetable garden, surprisingly susceptible to a handful of problems that show up first as curling or distorted leaves. Getting to the cause quickly matters, because some of those problems are treatable and some, particularly virus, are not.
Cause 1: Aphids (the most common cause)
Pea aphid (Acyrthosiphon pisum) and black bean aphid (Aphis fabae) are the insects most commonly found on sweet peas in the UK and the most frequent reason leaves curl. Both species target the soft, actively growing parts of the plant: the shoot tips, the young unfurling leaves just below the growing point, and the undersides of the most recently expanded leaves. Their feeding draws sap out of the plant tissue and causes those young leaves to curl, pucker, and twist as they expand into distorted shapes rather than flat, normal growth.
The two aphid species look quite different from each other. Pea aphids are large for aphids, soft-bodied, and range from pale green to pinkish-green depending on the individual. They tend to form loose, spread-out colonies on the stem and leaf undersides rather than dense clusters, which can make them harder to spot at first glance. Black bean aphids are smaller, almost black, and pack into extremely dense, conspicuous colonies that coat shoot tips and young stems so thickly that the plant tissue underneath is barely visible. Both species excrete sticky honeydew as they feed, which coats the leaves and stems below the colony and provides the growing medium for black sooty mould. A sooty, sticky coating on leaves below a shoot tip, even when you cannot immediately see the insects themselves, is a reliable sign of aphid activity.
On sweet peas in particular, controlling aphids early is more urgent than on many other plants. The reason is that both pea aphid and black bean aphid are vectors for several viruses that are devastating on Lathyrus odoratus. A colony that has been on the plant for a week in warm weather can transmit virus to multiple growing points before the population is large enough to be obvious. Acting at the first sight of any aphid colony, rather than waiting until numbers are high, is the single most effective thing you can do to protect your sweet peas through the season.
Treating aphids on sweet peas
Knock colonies off with a firm jet of water directed at the shoot tips and undersides of young leaves. This physically removes the majority of the insects and is immediately effective. Repeat every two to three days for ten days to catch any individuals that survived or re-colonised. For heavy infestations, insecticidal soap spray applied directly to the colonies is highly effective and breaks down quickly, leaving no persistent residue that would harm pollinating insects once dry. Apply in the evening when bee activity is lowest. Avoid broad-spectrum insecticides entirely; sweet pea flowers are pollinated by bumblebees and other insects, and a contact insecticide applied while the plant is in flower kills beneficials alongside the pests. Planting nasturtiums nearby works as a sacrifice plant: aphids often prefer nasturtiums to sweet peas and a colony that builds on nasturtiums is easy to monitor and control without risking the main crop.
Cause 2: Virus (the most serious cause)
Pea enation mosaic virus (PEMV) and other legume viruses transmitted by aphids are the most serious cause of leaf curling on sweet peas and the one problem that cannot be fixed once it is established. Virus symptoms on Lathyrus odoratus are more severe than simple aphid leaf curl and look distinctly different on close inspection. Affected leaves show a mosaic pattern of pale yellow and dark green that is irregular and blotchy across the whole leaf surface, not just at the margins. The leaves curl inward as well as becoming mottled, and growth slows dramatically. Severely infected plants become stunted, produce distorted, puckered leaves on every new shoot, and may stop flowering almost entirely. The whole plant has a sickly, dull appearance that is quite distinct from the vigorous, deep-green look of a healthy sweet pea in full growth.
Virus is transmitted by aphids feeding on an infected plant and then moving to a healthy one. It can spread rapidly through a row of sweet peas during a period of high aphid activity. Once a plant is infected the virus is systemic throughout its tissues and cannot be removed by any treatment. The only action available is to remove and bin the affected plant immediately, before aphids feeding on it transmit the virus to neighbouring healthy plants. Do not compost virus-infected material. Control aphid populations on the remaining plants by water jet and soap spray to reduce the rate of further spread, and inspect the other plants regularly for the first symptoms of mosaic mottling over the following two weeks.
The difference between virus symptoms and simple aphid leaf curl is worth knowing clearly. Aphid curl affects the young leaves at the shoot tip where the insects are feeding, while the older leaves further down the stem remain a normal flat green. Virus produces mottling and distortion throughout the plant, including on leaves that formed after the aphids were removed. If the mottled, distorted leaves keep appearing on new growth even after you have cleared all the aphids, the plant has virus and should come out.
Other causes of leaf curling on sweet peas
Powdery mildew
Powdery mildew caused by Erysiphe pisi is very common on sweet peas during warm, dry spells in July and August. It appears as a white or pale grey powdery coating on the upper surfaces of the leaves and on the stems, and causes the leaf margins to curl upward and the overall foliage to look dull and bleached. Unlike aphid damage, there are no insects on the plant; the white coating is the visible fungal growth. Mildew spreads fast in conditions of warm days, dry soil, and slightly humid evening air, and a sweet pea growing in a crowded position without good airflow around the stems is particularly vulnerable. It rarely kills an otherwise healthy plant but it reduces vigour, spoils the appearance, and can cut the flowering season short if it takes hold before August.
Water consistently at the base rather than overhead, improve airflow by thinning crowded stems, pick flowers and remove spent blooms frequently to keep the plant actively growing, and remove heavily affected leaves to reduce the spore load. A dilute solution of bicarbonate of soda can check a mild outbreak. Mildew in a wet, cold summer is less common; the disease peaks in the combination of dry soil and warm humid evenings.
Drought stress
Sweet peas are thirsty plants and wilt quickly in hot weather, particularly in containers or in free-draining soil in a raised bed. Drought stress causes the leaves to curl inward along their length and lose colour, and the plant slows its growth and may stop flowering. Container-grown sweet peas may need watering twice daily in a July heatwave. Mulching the root zone with compost or straw conserves moisture and keeps the root zone cooler, extending the time between waterings and reducing drought stress significantly. Plants that are watered consistently from the base, rather than overhead, are less prone to both drought stress and fungal problems.
Lack of support
Sweet peas produce tendrils that need something to grip as the plant grows. If the canes run out or the netting becomes too tangled, the tendrils curl around themselves and distort the nearby leaves as the stems bunch up and compete for space. This produces leaf and stem deformation that can look like pest damage at first glance, but there are no insects and the affected stems are simply twisted and compressed. Providing more support, untangling crowded stems, and tying in wayward growth sorts this out immediately.
Botrytis (grey mould)
In wet, cool, humid conditions, botrytis (grey mould, Botrytis cinerea) can affect sweet pea flowers, stems, and leaves, causing grey fuzzy growth and collapse of affected tissue. It is most common in a cold, damp June or in a polytunnel or greenhouse with poor ventilation. Removing affected material promptly and improving airflow is the primary fix.
Prevention: keeping sweet peas healthy through the season
- Sow early. October sowings under glass or March sowings in a cold frame give the plant time to establish deep roots and strong stems before summer heat arrives. Early-established plants tolerate stress far better than late-sown ones.
- Pinch out the growing tips when the seedlings are about 10 to 15 centimetres tall, removing the top pair of leaves. This encourages branching and produces a bushier plant with more flowering stems.
- Provide good support from the outset. Canes, pea netting, or pea sticks should be in place before the plants go out so the tendrils can start gripping immediately.
- Control aphids from the first day you spot them. Check the shoot tips every time you pick flowers. A water jet at the first sign keeps populations below the threshold where virus transmission becomes likely.
- Remove any plant showing mosaic mottling immediately. Do not compost it. The speed with which you remove an infected plant determines how many of the others you save.
- Water daily in hot weather, especially for container-grown plants. Consistent moisture is one of the biggest factors in keeping sweet peas productive and healthy in summer.
- Deadhead religiously. Pick every flower as it opens and remove every fading bloom. Allowing a single seed pod to develop signals the plant to shift from flowering to seed production and the display deteriorates fast.
Frequently asked questions
How do I stop sweet pea leaves from curling?
The fix depends on the cause. If aphids are present on the shoot tips and leaf undersides, knock them off with a firm jet of water and follow up with insecticidal soap spray repeated every few days. If the leaves show mosaic mottling and yellowing alongside the curl, the plant has a virus and must be removed and binned immediately; there is no cure. For drought curl without pests, water daily in hot weather and apply a mulch to the root zone to conserve moisture. For powdery mildew showing as a white coating, improve airflow, water at the base, and remove affected leaves. Consistent watering, early sowing, regular deadheading, good cane support, and controlling aphids from the moment the plant goes out are the best prevention.
What is the difference between virus symptoms and aphid symptoms on sweet peas?
Aphid damage causes leaf edges to roll downward and inward, and the curl is concentrated where the insects are feeding, usually on shoot tips and young leaves. The leaf colour between curled areas is otherwise normal green. Virus symptoms look different: the leaves show a mosaic or mottled pattern of yellow and dark green that is uneven and blotchy across the whole leaf surface, combined with puckering, downward curl, and severely stunted growth. An aphid-infested plant can look almost normal once the insects are removed; a virus-infected plant does not recover and keeps deteriorating. If the curl is accompanied by yellow mottling and you can find no aphids, treat it as virus and remove the plant.
How do I extend the flowering season on sweet peas?
Deadheading is the single most important thing you can do. Sweet peas set seed the moment a flower fades; once they begin forming seed pods the plant prioritises seed production over flowering and the show deteriorates rapidly. Pick every flower and remove every fading bloom at least every two to three days, ideally more often. Water consistently and never let the soil dry out. Liquid feed weekly with a balanced or high-potash fertiliser once the plants are established. Sowing early, in October under glass or March in a cold frame, gives a longer growing window before summer heat shuts the plant down. Spencer varieties grown as single-cordon plants on canes extend the season because they produce longer stems and more flowers per node than bush-grown plants.
Why are my sweet pea flowers small or disappearing?
Missed deadheading is the most common cause; once a single seed pod is allowed to develop fully, the plant begins to shut down flowering. Hot weather is the other major factor: sweet peas are cool-season plants and stop flowering freely when temperatures consistently exceed around 25C in July and August. Keeping plants well-watered and providing some afternoon shade in very hot spells helps extend the season. If the plant is healthy and deadheaded but still not flowering well, check whether it has run out of climbing space and stems are tangling without being able to extend upward.
Can I compost sweet pea plants that had aphids or powdery mildew?
Plants with aphids and powdery mildew can go into a hot compost heap once all the foliage has died back at the end of the season. However, any plant showing virus symptoms including mosaic mottling, severe stunting, or distorted leaves must go in the bin and not the compost. Garden compost heaps rarely reach the temperatures needed to destroy virus particles reliably, and composting virus-infected material risks spreading it to next year's crop if the compost is not fully broken down.