Marigolds (Tagetes spp.) are a staple of UK summer beds and borders, valued for their long flowering season, their cheerful orange and yellow blooms, and their well-documented role as companion plants alongside tomatoes and brassicas. French marigold (T. patula), African marigold (T. erecta), and signet marigold (T. tenuifolia) are all widely grown, started under glass from March to April and planted out after the last frost. When the foliage starts to curl, pucker, or discolour, two pests account for the great majority of cases: aphids and red spider mite. Knowing which one you are dealing with determines how to respond.
What causes marigold leaves to curl?
Marigold leaves curl as a physical response to damage or stress. Insect pests that feed on sap cause cells on one side of a leaf to lose turgor while the other side continues to develop normally; the leaf bends toward the damaged surface. The same mechanical response occurs when cells are killed by fungal infection or die back from root problems. In each case the leaf is telling you something about what is happening at the point of damage. Aphid colonies on the growing tips produce characteristic downward curling of young leaves, while spider mite feeding across the whole leaf underside creates a finer, more generalised puckering with associated discolouration. Stress from overwatering, drought, or disease produces different but overlapping symptoms. Running through the visual signs methodically makes diagnosis straightforward in most cases.
1. Aphids (Aphis fabae and Myzus persicae)
Aphids are the most common reason marigold leaves curl in UK gardens. Two species are responsible for most infestations: black bean aphid (Aphis fabae) and peach-potato aphid (Myzus persicae). Black bean aphid is the more visible of the two, forming dense, conspicuous colonies of shiny black insects on marigold stems and shoot tips from late spring onward. Peach-potato aphid is pale green to yellowish and slightly smaller; its colonies are less dense and easier to miss against light-coloured stems, but its feeding is equally damaging. Both species feed by piercing plant tissue with their stylets and withdrawing phloem sap, removing plant sugars and simultaneously injecting saliva that disrupts normal cell signalling.
The result on marigolds is a characteristic downward and inward curling of the young leaves closest to the colony. As the shoot tip is where new growth emerges, aphid feeding there distorts the leaves as they unfurl, leaving them permanently curled even after the aphids have been removed. Heavily infested plants produce dense clusters of small, malformed leaves at the shoot tips and flower less prolifically. The aphids excrete sticky honeydew onto the tissue below the colony, and this honeydew supports the growth of black sooty mould, which further reduces the plant's ability to photosynthesise. In UK gardens, black bean aphid populations on marigolds often peak in June and July, just as the plants come into full flower, because the aphid's primary host plant (the field bean, Vicia faba) is senescing and releasing winged migrants that colonise alternative hosts. Stressed marigolds, including those planted in partial shade, in poor soil, or immediately after transplanting, are significantly more vulnerable to aphid colonisation than plants that have established well and are growing vigorously.
How to fix it
Check the shoot tips and stem undersides of marigolds weekly from late May. Aphid colonies are visible to the naked eye: look for clusters of black or pale green insects at the growing tips and on young stems. For small colonies detected early, rubbing them off with your fingers or knocking them from the plant with a jet of water from a hose is sufficient. This physical removal is effective and avoids any chemical use. For larger colonies, spray all affected surfaces thoroughly with insecticidal soap or a diluted neem oil solution, making sure to wet the undersides of all curled leaves where aphids shelter. Repeat every five to seven days until the colony is cleared. Encourage natural predators: ladybirds, lacewing larvae, and hoverfly larvae all feed voraciously on aphids, and avoiding broad-spectrum pesticides allows these beneficial insects to establish in the garden and provide season-long control. Deadheading regularly, which keeps marigolds vigorous and producing new growth, makes plants less attractive to aphids than stressed or neglected ones.
2. Red spider mite (Tetranychus urticae)
Red spider mite is the second major cause of marigold leaf curling in the UK. Despite its name, Tetranychus urticae is not always red; in summer the mites are typically yellowish-green with dark markings, turning brick red in the cooler months. They are tiny, around 0.5 mm in length, and visible to the naked eye only as moving specks on the leaf underside when you look closely. The first visible sign of an infestation is almost always not the mites themselves but the characteristic pale stippling they leave on the upper leaf surface as they feed. Each mite feeds by piercing individual leaf cells on the underside and draining the contents; the emptied cell on the upper surface appears as a tiny pale or yellowish fleck. As the mite population grows across the leaf, these flecks merge into a generalised bronzed, silvery, or pale mottled appearance. The leaves begin to curl and brown at the margins, and fine silky webbing becomes visible on the undersides and between nearby stems as the population reaches high levels.
Red spider mite thrives in hot, dry conditions and its lifecycle accelerates dramatically in warm weather. At 30 degrees Celsius, the mite can complete its full life cycle from egg to reproducing adult in as little as a week, allowing populations to explode very rapidly during UK heatwaves. Marigolds grown under glass in April and May before planting out are exposed to the warm, dry greenhouse conditions in which mite populations build fastest. Plants brought indoors or kept under cover in late summer face the same risk. In outdoor UK gardens, red spider mite infestations on marigolds are most severe in hot, dry summers, particularly on plants in sheltered south-facing beds or against warm walls where temperatures are elevated. Overcrowding that reduces airflow, and drought stress that weakens the plant's ability to tolerate feeding damage, both increase severity.
How to fix it
Turn a leaf underside toward the light and look for the pale stippling and any fine webbing; use a hand lens if in doubt. Act as soon as stippling is visible across multiple leaves, before the population reaches the level where webbing is obvious. Mist the undersides of all affected leaves thoroughly with water every day for a week; this dislodges mites physically and raises local humidity to a level at which mite reproduction slows significantly. Apply neem oil or a specific miticide (always to the leaf undersides, where the mites live) and repeat at seven-day intervals for at least three applications to break the egg-to-adult cycle and catch any newly hatched nymphs. Under glass, introduce the predatory mite Phytoseiulus persimilis as a biological control; it is highly specific to Tetranychus urticae, does not damage the plant, and can clear an established mite population within two to three weeks. Water marigolds consistently to prevent drought stress, and maintain good airflow around the plants by spacing them adequately and not overcrowding beds.
Other causes to consider
Botrytis (grey mould, Botrytis cinerea) is a fungal disease that affects marigolds in cool, damp UK conditions, most commonly in early autumn or during unsettled wet summers. It first appears as water-soaked patches on leaves and flower petals, quickly developing into the characteristic grey fuzzy sporulation from which it gets its common name. Affected tissue collapses rapidly, and the leaves curl and turn brown as they die. Botrytis spreads readily in humid, still air, particularly where plants are crowded and dead flowers are left to decompose on the plant. Deadheading regularly, removing dead leaves and stems promptly, spacing plants at the recommended distance to maintain airflow, and avoiding wetting the foliage when watering all significantly reduce the risk. Remove and bin affected plant material immediately; do not compost it, as botrytis spores are long-lived.
Overwatering causes root rot in marigolds, leading to yellowing lower leaves, wilting, and general collapse. Marigolds require well-drained soil and perform poorly in persistently wet conditions. In UK summers with heavy rainfall, plants in clay or compacted soil or in containers without adequate drainage holes are vulnerable. The symptoms can initially resemble drought stress, as damaged roots cannot take up water efficiently even when the soil is wet. Check the soil moisture at root depth and the condition of the root system if you suspect root rot: healthy marigold roots are white to cream; rotted roots are brown, soft, and smell of decay. If overwatering is the cause, reduce watering immediately, improve drainage, and remove any dead roots from plants in containers.
Aster yellows is a phytoplasma disease transmitted by leafhoppers. It causes yellow, distorted foliage, stunted growth, and abnormal flower development across the whole plant. It is rare in UK gardens but occasionally occurs in seasons when leafhopper populations are high. There is no treatment; remove and destroy affected plants immediately to prevent leafhoppers from carrying the phytoplasma to neighbouring plants. In the UK, marigolds showing severe generalised yellowing and distortion without any visible pest colonies are more likely to be suffering from nutrient deficiency in poor soil, or from aster yellows only if several unrelated plants in the bed are showing similar symptoms at the same time.
Drought stress causes marigold leaves to curl inward along their length and lose their rich green colour, taking on a dull, grey-green tone. Marigolds are reasonably drought-tolerant in the ground once established, but container-grown specimens and those in sandy, free-draining soil can suffer visibly in hot UK summers. The fix is straightforward: water thoroughly and consistently at the base of the plant. Avoid overhead watering, which wets the foliage and increases the risk of both botrytis and fungal spotting.
How to keep marigolds healthy
- Grow marigolds in full sun. Marigolds grown in shade or partial shade are weaker, less floriferous, and significantly more prone to aphid colonisation and botrytis than plants in a bright, open position.
- Deadhead regularly throughout the season. Removing spent flowers keeps the plant producing fresh growth and blooms, reduces the dead plant material on which botrytis spores establish, and maintains the vigour that makes marigolds resistant to pest attack.
- Water at the base and avoid wetting the foliage. Consistently moist soil at the roots with dry leaves is the ideal condition for marigolds. Overhead watering encourages botrytis and fungal leaf spots.
- Space plants adequately and maintain good airflow. French marigolds (T. patula) need around 20 to 25 cm between plants; African marigolds (T. erecta) need 30 to 40 cm. Adequate spacing prevents the still, humid microclimate in which both botrytis and spider mites thrive.
- Check shoot tips and leaf undersides weekly from late May for aphid colonies. Early detection when colonies are small makes removal easy without any chemical treatment.
- Under glass, introduce Phytoseiulus persimilis at the first sign of spider mite activity rather than waiting for the infestation to become severe. Biological control is most effective when introduced early into a low-level mite population.
- Use marigolds as companion plants but site them where they will thrive. Plant French marigolds around the perimeter of tomato beds or interplanted with brassicas to deter whitefly and suppress soil nematodes. A stressed, neglected marigold offers little companion-planting benefit; a healthy, well-grown one is a genuine garden workhorse.
Frequently asked questions
Why are my marigold leaves curling?
The two most common causes of marigold leaf curling in UK gardens are aphids and red spider mite. Black bean aphid or peach-potato aphid colonies on stems and shoot tips cause the young leaves to curl downward and inward. Red spider mite feeding on the leaf undersides produces stippled, pale foliage that curls and browns at the edges, especially in hot, dry summers. Botrytis, overwatering, and drought stress are further causes in particular conditions.
What are the black bugs on my marigolds?
Black insects clustered on marigold stems and shoot tips are almost certainly black bean aphid (Aphis fabae). This is one of the most common aphid species in UK gardens, and marigolds are a favoured host when nearby bean crops or other preferred hosts become crowded. The colonies are dense, covering stems and the undersides of young leaves. They excrete sticky honeydew that coats the plant below and supports sooty mould growth. Rub off small colonies by hand, spray with insecticidal soap for larger ones, and encourage ladybirds and lacewings by avoiding broad-spectrum pesticides.
How do I get rid of spider mite on marigolds?
Mist the undersides of affected leaves daily with water to dislodge mites and raise local humidity, as spider mites thrive in hot, dry conditions. Apply a neem oil solution or a specific miticide to all leaf undersides, repeating at seven-day intervals for at least three applications to break the egg cycle. Under glass, introduce the predatory mite Phytoseiulus persimilis as a biological control; it is highly effective against Tetranychus urticae and can clear a population within two to three weeks. Keep marigolds in full sun with good airflow, water at the base rather than overhead, and avoid stress from drought or heat.
Can marigolds get botrytis?
Yes. Botrytis cinerea (grey mould) affects marigolds in cool, damp UK autumns or whenever conditions are wet and air circulation is poor. It first appears as water-soaked patches on leaves and flower petals, rapidly developing a grey fuzzy mould. Affected tissue collapses and the foliage curls and turns brown. Deadhead regularly, space plants to allow airflow, avoid wetting the foliage when watering, and remove any affected material promptly. There is no cure for established botrytis on plant tissue, but removing infected parts and improving conditions usually stops the spread.
Do marigolds really deter pests from tomatoes?
Yes, to a meaningful extent. French marigolds (Tagetes patula) produce root exudates that suppress soil nematodes, and their scent confuses and deters whitefly from greenhouse tomatoes. Research supports planting French marigolds as a barrier or interplant in tomato beds: they do not eliminate pests entirely, but they reduce whitefly pressure and nematode populations in the soil. Marigolds are also widely used around brassicas to confuse cabbage white butterflies. For best effect, plant densely around the perimeter of the bed and deadhead regularly to keep the plants producing the compounds that make them effective.