Why Are My Mimulus Leaves Curling?

Drought stress

Mimulus is naturally a moisture-loving plant, and many species grow wild alongside streams, in ditches, and in boggy ground. Unlike drought-tolerant bedding plants such as pelargonium or portulaca, mimulus needs consistently moist soil to perform well. When the compost dries out in containers, the soft leaves lose turgor quickly and curl inward along their length, accompanied by wilting of the stem tips. Flower buds may drop before opening and the plant can look near-dead within a few hours of severe drought in hot weather.

What to do

• Water mimulus before the compost dries out completely. Check containers daily in warm weather.
• Mimulus tolerates much wetter conditions than most bedding plants: it can be grown in containers standing in shallow trays of water, or planted at the margins of a garden pond.
• Use a moisture-retentive compost and incorporate water-retaining gel for container planting in exposed or sunny positions.
• Wilted mimulus recovers quickly once watered if the drought has been brief. Cut back any stems where the leaves have already crisped.

Aphid infestation

Aphids are the most common pest on mimulus. They colonise the soft shoot tips and the undersides of young leaves, feeding on the sap and causing the leaves to pucker, curl, and distort. Green aphids are the most commonly encountered species, though blackfly also occurs on some mimulus varieties. The sticky honeydew they produce can promote sooty mould on the leaves and attracts ants that tend and protect the colonies from natural predators.

What to do

• Pinch out and dispose of shoot tips bearing the heaviest aphid colonies.
• Knock remaining aphids off with a jet of water or rub them away gently with a gloved hand or damp cloth.
• Spray with insecticidal soap every four to five days for two to three weeks for persistent infestations.
• Mimulus flowers attract beneficial insects including hoverflies whose larvae predate aphids: avoid broad-spectrum insecticides that would kill these natural predators.

Heat stress

Most mimulus varieties are cool-season plants that flower best in spring and early autumn. In the hottest part of the UK summer, mimulus frequently goes into a semi-dormant state: flowering stops, the stems become lax, and the leaves may curl or look generally uninspiring. This is entirely normal behaviour and not a sign of permanent decline. The plant conserves energy during unfavourable conditions and typically regrows vigorously when autumn temperatures arrive.

What to do

• Move container plants to a cooler, slightly shadier position during the hottest weeks of summer, away from full afternoon sun.
• Cut plants back by about half in midsummer to remove tired, heat-stressed growth and encourage fresh compact growth from the base.
• Keep consistently watered during the semi-dormant period. Heat and drought stress together can kill mimulus permanently, but either alone is usually survivable.
• Once temperatures drop in late August and September, mimulus typically reflowers strongly before the first frosts.

Overwatering

Although mimulus enjoys moisture, it does not thrive in stagnant, waterlogged compost. There is a difference between moist soil with good oxygen levels and saturated, airless compost. When containers have poor drainage or when water sits in saucers for extended periods, the roots can still rot even on a moisture-loving plant. Yellowing, curling leaves in very wet compost without signs of pest damage suggest root problems rather than drought.

What to do

• Ensure containers have adequate drainage holes. Mimulus grown in standing water in a tray benefits from this arrangement, but stagnant, airless compost is different from fresh, oxygenated water at the base of the pot.
• If root rot is suspected, remove the plant, trim away any dark and mushy roots, and replant in fresh, well-aerated compost.

Powdery mildew

Powdery mildew can affect mimulus in warm, dry conditions with poor air circulation, which seems counterintuitive for a moisture-loving plant but reflects the fact that mildew spores need dry leaf surfaces and warm temperatures to germinate. Infected leaves develop a white powdery coating and the margins may curl and dry out. It is most common on plants in crowded containers or very sheltered spots in late summer.

What to do

• Improve air circulation and avoid planting too densely.
• Remove and dispose of infected leaves promptly.
• Spray with a potassium bicarbonate or sulphur-based fungicide at the first signs of infection, repeating every seven to ten days.

Frequently asked questions

Why are my mimulus leaves curling?

Mimulus leaves curl most often from drought stress or aphid infestation. Mimulus needs consistently moist soil and wilts quickly when the compost dries out. Aphids colonise shoot tips and cause the young leaves to pucker and curl as they feed on the sap.

How often should I water mimulus?

Mimulus prefers consistently moist soil and should be watered before the compost dries out. In containers, check daily in warm weather. Many mimulus species are naturally adapted to stream banks and boggy ground and tolerate much wetter conditions than most bedding plants.

Why has my mimulus stopped flowering and the leaves are curling?

Mimulus often stops flowering and looks tired in the hottest part of summer. Cut the plant back by about half, water well, and feed with a balanced liquid fertiliser. It typically produces a second flush of flowers in late summer and autumn when temperatures moderate.

Is mimulus a perennial in the UK?

Some mimulus species are hardy perennials in the UK, including Mimulus guttatus and its hybrids, which can naturalise in boggy or streamside conditions. The brightly coloured bedding mimulus sold in spring are typically half-hardy annuals or short-lived perennials treated as annuals.