Melianthus Leaves Curling

Frost damage

Frost damage is the most common cause of leaf curling, blackening, and shoot collapse on melianthus in UK gardens. Melianthus major is a South African shrub that is only marginally hardy in Britain, and the large, pinnate leaves with their dramatic glaucous-blue colouring are very sensitive to frost. When temperatures drop below about minus 3 or 4 degrees Celsius, the leaf tissue collapses and turns black or brown within hours. The affected leaves curl and hang limply from the stem, while the stem itself may remain firmer initially before the damage becomes clear. In a mild winter, only the leaf tips may show damage; in a hard winter, the entire above-ground growth can be killed back to the base. The crown and root system are considerably hardier than the stems and typically survive temperatures that kill the top growth.

Do not cut back frost-damaged melianthus stems immediately after a frost event. The dead stems provide some insulation for the crown and root system below, and cutting them away in winter increases the exposure of the vulnerable crown to further cold. Wait until late spring, once the risk of frost has passed and new shoots are visibly emerging from the base, before cutting away all dead and damaged stem material. Protect the crown through winter by heaping a thick mulch of bark, straw, or dry leaves over it in autumn before frosts arrive. In colder gardens, growing melianthus in a large container that can be moved under glass for winter is the most reliable approach for keeping the plant long-term.

Aphids

Aphids are a common pest on melianthus, particularly on the vigorous new growth produced in spring and early summer after the plant has cut back to the base and is regenerating. The soft, new stems and the developing leaflets at the shoot tips attract colonies of pale green or greyish aphids that cause the youngest leaves to curl around the insects. Honeydew deposits make surrounding stems feel sticky, and sooty mould can develop on heavily infested growth. Because melianthus produces its most attractive growth in summer after recovering from winter, protecting the new growth from aphid damage in spring is particularly worthwhile.

Treat aphid colonies on the new melianthus growth with a strong jet of water directed at the stem tips and the developing leaflets. Follow up with an insecticidal soap spray applied to all affected growth. Natural predators, particularly hoverflies attracted to nearby flowering plants and lacewing larvae, are effective at controlling aphid populations on melianthus in mixed borders. Repeat the soap treatment every five to seven days for two or three applications if the infestation is heavy. Vigorous, well-watered melianthus in its peak growing period from midsummer to early autumn produces growth quickly enough to outpace moderate aphid pressure without significant lasting damage.

Drought

Melianthus is more drought-tolerant than its luxuriant, large-leaved appearance suggests, particularly once established with a deep root system. However, the large pinnate leaves have a high water demand and will curl inward along their midrib in hot, dry conditions as the plant limits water loss through the vast leaf surface area. Young plants in their first season are considerably more vulnerable to drought than established specimens. Container-grown melianthus dries out rapidly in summer and can wilt dramatically in hot weather, losing much of its ornamental impact if not watered consistently.

Water newly planted melianthus consistently during its first two growing seasons. Container specimens need generous, regular watering in warm weather, as the large leaf canopy creates a high evaporative demand. Once established in a sunny, sheltered border, melianthus rarely needs supplementary watering except during prolonged drought. A bark mulch around the base of the plant helps retain soil moisture and insulates the crown through winter. The glaucous, waxy coating on the leaves, which gives melianthus its distinctive appearance, is itself an adaptation to the warm, sunny conditions of its South African habitat and provides some resistance to water loss in dry conditions.

Scale insects

Scale insects occasionally colonise melianthus, particularly plants growing in sheltered positions against warm walls where air circulation is limited. The insects attach to the undersides of leaflets and to the stems, appearing as small brown or grey bumps. They feed by sucking sap, causing localised yellowing and, in heavy infestations, general weakening and curling of the large leaves. Honeydew deposits create a sticky coating on the leaf surfaces below the insects, attracting sooty mould that further reduces the visual appeal of this ornamental plant. Scale on melianthus is most common on container specimens or on plants in very enclosed, warm planting positions.

Treat small scale infestations by wiping affected stems and leaflets with a cloth or cotton swab dampened with methylated spirit. For larger infestations, apply an insecticidal soap or plant oil spray to all leaf surfaces, with particular attention to the undersides. Improve air circulation around the plant by trimming back neighbouring plants that crowd the melianthus. Cutting the plant back in spring, which is standard practice after winter dieback, removes any scale colonies on the old stems along with the damaged growth, providing a clean start for the season.

Spider mites

Spider mites colonise melianthus during hot, dry conditions in summer, particularly on plants in very sheltered, south-facing positions against walls where temperatures are high and humidity is low. The mites feed on the undersides of the leaflets, causing the upper surface to develop a fine bronze stippling that dulls the glaucous blue-green colouring that makes melianthus so distinctive. Affected leaflets curl downward at the edges. Fine webbing between the leaflets and along the stems confirms mite infestation. Mite damage on melianthus in open garden positions is less common than on the same plant grown in enclosed courtyards or against very hot, dry walls.

Mist the foliage with water on hot afternoons to raise the humidity and discourage mite reproduction. Remove the most badly affected stems. Apply neem oil or a miticide spray to all leaf surfaces, concentrating on the undersides of the leaflets. Repeat at seven-day intervals for two applications. Moving container melianthus to a slightly less enclosed position provides a long-term solution in gardens where mites are a recurring problem. Melianthus's vigorous growth rate means it typically replaces mite-damaged growth quickly once the population is controlled and conditions become less favourable.

Root rot

Root rot is a significant risk for melianthus during winter in cold, wet soils. Although the roots are hardier than the stems, they are not immune to the combined effects of cold and waterlogged soil, and Phytophthora and other soil-borne pathogens become more active in saturated, poorly draining ground. The symptoms of root rot become apparent in spring when a plant that appeared to have survived winter fails to produce new growth from the base, or produces only weak, yellowing shoots that collapse before they can develop. Unlike frost damage, where the crown is firm and alive when pressed, a root-rotted crown feels soft and may smell sour.

Plant melianthus in well-draining soil in a sheltered position. Against a south-facing wall is ideal: the wall provides reflected warmth and the ground directly at the base of a wall tends to be drier and better draining than open border soil. Apply a thick dry mulch of bark or straw over the crown in autumn, which insulates against both cold and excessive moisture. Avoid mulching with wet materials that hold moisture against the crown through winter. Container melianthus should be moved under frost-free glass for winter in cold gardens, as this simultaneously protects both the stems and the root system from the conditions that cause rot.

Frequently asked questions

Why are my melianthus leaves curling?

Frost damage is the most common reason melianthus leaves curl and collapse in UK gardens, as honey bush is only marginally hardy and the stems are killed back in a hard winter. Aphids colonise the new growth in spring and summer. Drought stress causes the large, glaucous leaves to curl in hot, dry conditions.

Is melianthus hardy in the UK?

Melianthus major is marginally hardy in mild UK gardens, surviving winters in sheltered coastal and city gardens where temperatures rarely fall below about minus 5 Celsius. The stems are often killed back to the crown or to ground level in a hard winter, but the plant regrows strongly from the rootstock in spring. In colder gardens, protect the crown with a thick mulch of bark or straw.

Why is my honey bush dying back in winter?

Melianthus stems dying back in winter is caused by frost damage. The plant is semi-tender and the above-ground stems are vulnerable to hard frosts. The root system is hardier than the stems and the plant typically regrows vigorously from the base in spring. Do not cut the dead stems away until spring, as they provide some insulation for the crown.

When should I cut back melianthus?

Cut back frost-damaged melianthus stems in late spring, once new growth from the base confirms the plant is alive and the risk of further frost has passed. Cutting back in autumn or early winter removes the insulation provided by the dead stems and increases the risk of the crown being killed. In mild gardens where stems survive winter, cut back by a third in early spring to encourage fresh growth.