Spiraea Leaves Curling

Aphids

Aphids are the most common cause of curling leaves on spiraea and affect virtually all spiraea species and cultivars at some point during the growing season. Dense colonies of pale green, yellowish, or blackish aphids colonise the soft new shoots, particularly on the summer-flowering Spiraea japonica types whose cultivars such as Gold Flame, Anthony Waterer, and Little Princess produce repeated flushes of tender new growth through the growing season. Each flush of new growth is attractive to aphids, and populations can build rapidly. The youngest leaves at the shoot tips curl tightly around the colonies, and the stems become distorted and sticky with honeydew. Sooty mould develops on honeydew-covered stems and older leaves. On the golden and orange-leaved cultivars of Spiraea japonica, which are commonly grown for their foliage colour rather than primarily for their flowers, aphid damage to the coloured new growth is particularly visible and damaging to the plant's ornamental effect.

Treat aphid colonies on spiraea with a strong jet of water directed at the affected shoot tips, which dislodges most insects effectively. Insecticidal soap spray applied to all affected growth provides chemical control. Natural predators including ladybirds, lacewings, and aphid parasitic wasps are usually effective at controlling spiraea aphid populations from early summer once predator numbers have built up. For the golden-leaved forms that are particularly attractive to aphids, the practice of pruning the plant hard in spring and then cutting back the first flush of growth again when it colours up (to provoke a second flush) provides two opportunities to remove any aphid colonies from the plant before they develop into large populations.

Powdery mildew

Powdery mildew is a common problem on spiraea, particularly on Spiraea japonica and its cultivars in late summer. The disease produces a white or pale grey floury coating on the upper surfaces of the leaves and causes the leaf margins to curl upward and inward. It develops most readily in the warm, dry at the leaf surface combined with humid air conditions typical of UK late summers, and is most severe on plants in crowded positions where air cannot circulate freely through the stems. Dense, mature clumps of summer-flowering spiraea with multiple stems growing closely together are more susceptible than young, open-grown plants or those pruned hard each spring to maintain a more open habit. The mildew is rarely fatal on otherwise healthy spiraea but significantly reduces the ornamental value of the plant in late summer and can weaken it enough to reduce the following year's flowering if recurrent.

Prune summer-flowering spiraea hard in early spring, cutting the stems back close to the base. This produces vigorous, well-spaced new stems with better air circulation than a mature, multi-stemmed shrub. Avoid planting spiraea in positions where there is no airflow. Water at the base rather than overhead. Remove affected leaves and apply a potassium bicarbonate fungicide or bicarbonate of soda solution at the first sign of white coating. The Chelsea chop approach of cutting the plant back by a third in late spring, which delays and compacts flowering, also results in a somewhat shorter, more open plant that is less prone to mildew in late summer.

Drought and leaf scorch

Drought stress and heat scorch cause spiraea leaves to curl inward, develop brown tips and margins, and eventually drop. Spiraea is more drought-tolerant than many garden shrubs once established, but the small, fine-textured leaves of species such as Spiraea arguta and the compact cultivars of Spiraea japonica can curl and scorch in prolonged hot, dry conditions. Plants in very free-draining, sandy soils or in positions receiving full midday sun on a south-facing slope are most susceptible. Container spiraea is particularly vulnerable to drought because the restricted root volume dries rapidly in warm weather. Young plants in their first summer before a deep root system has developed are also more susceptible than established plants.

Water spiraea during prolonged dry spells, particularly young plants and those in containers or sandy soil. Apply a mulch of bark chips or garden compost annually to retain moisture in the root zone. Established spiraea in reasonably moisture-retentive soil rarely requires supplementary watering in a normal UK summer. In very exposed, sunny positions, ensuring good moisture retention through mulching is more effective than reactive watering after the plant is already showing stress. The natural tolerance of spiraea for a range of conditions is one of its great assets as a garden shrub, and established plants in suitable positions are generally very self-sufficient.

Waterlogging

Waterlogging causes yellowing, leaf curl, and root rot on spiraea. Although spiraea tolerates a wide range of soil conditions, it does not thrive in genuinely waterlogged or permanently wet soil. Plants in heavy clay, in low-lying positions where water accumulates, or in compacted soil where drainage is impeded develop progressively yellowing foliage that curls and drops as the root system is damaged. The problem is often gradual, with the plant declining over two or more seasons as root damage accumulates. Waterlogging in winter, when deciduous spiraea has no leaves, can damage the root system significantly enough that the plant produces weak, distorted growth when it leafs out in spring.

Plant spiraea in well-drained to moderately moisture-retentive soil. In heavy clay, incorporate coarse grit and organic matter and create a slightly raised planting position. Avoid low-lying areas where water stands after heavy rain. If waterlogging has caused decline and the plant still shows some live growth, lift it in early spring, remove any rotted root tissue, and replant in improved, better-draining soil. The wide adaptability of spiraea to different soil types makes it an excellent choice for most garden positions, but consistently waterlogged soil is the one condition it cannot thrive in.

Spider mites

Spider mites occasionally affect spiraea during hot, dry summer conditions, particularly on plants in very exposed, sunny positions. The mites feed on the undersides of the small leaves, causing the upper surface to develop a speckled, dulled, or bronzed appearance and the leaf margins to curl downward. Fine webbing on the undersides of affected leaves confirms the presence of mites. Spider mites on spiraea are most common during prolonged heatwaves and less of a recurring annual problem than aphids or powdery mildew. The smaller-leaved spiraea species are somewhat more resistant to mite colonisation than broader-leaved plants, as the fine leaf texture provides less sheltered habitat for mite populations.

Mist the foliage of affected spiraea with water in the morning to raise humidity and discourage mite reproduction. Apply neem oil or a miticide spray to all leaf surfaces, paying particular attention to the undersides. Repeat at seven-day intervals for two applications. Maintaining consistent soil moisture around spiraea reduces drought stress that makes the plant more susceptible to mite attack. Natural predators including predatory mites and ladybird larvae feed on spider mites and usually keep populations below damaging levels in a garden environment with diverse plantings.

Nutrient deficiency

Nutrient deficiency causes pale, yellowing foliage and sometimes marginal leaf curl on spiraea, particularly on plants in very poor, sandy soils where nutrients leach quickly or in containers where the compost has become exhausted. Nitrogen deficiency is most common, producing uniformly pale or yellow foliage and weak, sparse growth. Iron or manganese deficiency causes interveinal chlorosis, where the leaf tissue between the veins yellows while the veins remain green; this is most likely on spiraea planted in very alkaline or chalky soil. The golden and orange-leaved forms of Spiraea japonica have naturally coloured foliage, and distinguishing the natural colouring from nutrient deficiency requires looking at the vigour of the growth: deficient plants produce small, weak leaves even in their coloured forms, whereas well-fed plants produce full-sized leaves with the expected colour intensity.

Feed spiraea with a balanced general-purpose fertiliser in spring as growth begins. A second feed in early summer maintains vigour through the growing season. In very poor soils, incorporate well-rotted garden compost or a slow-release fertiliser at planting. Container spiraea should be fed monthly through the growing season with a balanced liquid fertiliser. If iron chlorosis on alkaline soil is suspected, apply a sequestered iron product to the root zone in spring. Spiraea is not a heavy feeder and grows well in average garden soil without intensive fertilising, but plants in genuinely nutrient-poor conditions benefit noticeably from regular feeding.

Frequently asked questions

Why are my spiraea leaves curling?

Aphids are the most common reason spiraea leaves curl, with dense colonies colonising the soft new shoots and causing the youngest leaves to curl around the insects in spring and early summer. Powdery mildew causes white coating and leaf margin curling in late summer. Drought stress causes the small leaves to curl, brown at the tips, and drop in prolonged dry conditions.

Why are spiraea leaves turning yellow?

Yellowing on spiraea most often indicates waterlogging or root rot, particularly on plants in heavy clay or poorly draining soil. Nutrient deficiency in very poor or very alkaline soil also causes yellowing. Some yellowing of the older, lower leaves in late summer is normal on deciduous spiraea and is followed by the natural autumn colour change. Heavy aphid infestations weaken the plant and can cause yellowing of the affected shoots.

Does spiraea get powdery mildew?

Yes, powdery mildew is common on spiraea, particularly on the summer-flowering Japanese spiraea types including Spiraea japonica and its named cultivars. The disease appears as a white, floury coating on the upper leaf surfaces in late summer and causes leaf margin curling. Improve air circulation by pruning to maintain an open shape, water at the base rather than overhead, and apply a potassium bicarbonate spray at the first sign.

When should I prune spiraea?

The timing of spiraea pruning depends on when it flowers. Spring-flowering spiraea (such as Spiraea arguta or S. thunbergii) should be pruned immediately after flowering, cutting back the flowered stems to strong new growth. Summer-flowering spiraea (such as Spiraea japonica cultivars) should be pruned hard in early spring before new growth begins, cutting stems back close to the base to promote vigorous new shoots and flowers.