Plant problems

Why Are My Clerodendrum Leaves Curling?

Q: Is clerodendrum bungei invasive in the UK?

Clerodendrum bungei (glory bower or rose glory bower) can spread aggressively by underground suckers in UK garden conditions and is considered invasive or problematic in some contexts, particularly in mild, sheltered gardens in southern England where the plant has plenty of warmth and moisture. The suckering habit means the plant can spread well beyond its original planting position, sending up new shoots at some distance from the parent plant and establishing a dense colony over several years. This suckering behaviour is most pronounced in light, freely draining soils and in warm, sheltered positions; in heavier soils or exposed gardens the plant spreads less aggressively. Clerodendrum bungei is listed as a non-native invasive species of concern in some mild regions internationally, but it is not formally classified as invasive under UK legislation and is freely sold in UK garden centres. To manage suckering: remove suckers by cutting them off at or below soil level as they appear in spring and summer; use a sharp spade to sever the underground connection to the parent root. In a restricted space, install a root barrier (a sheet of heavy-duty polythene or proprietary root barrier membrane) around the planting position at a depth of 40 to 50 cm to contain spread. The plant is easily controlled by regular suckering removal; the effort involved depends on how warm and sheltered the growing position is.

Q: How do I prune clerodendrum?

Clerodendrum bungei and Clerodendrum trichotomum are pruned differently because of their different growth habits in UK conditions. Clerodendrum bungei dies back to the ground in most UK winters (surviving as a root system that regenerates each spring); in this case, pruning consists simply of cutting the dead stems to the ground in late winter or early spring (February to March) before new growth begins; there is no choice about timing as the top growth is typically dead by this stage anyway. In very mild coastal gardens where the top growth survives winter, cut back any dead or frost-damaged stems in April when the extent of damage is clear, and lightly trim the remaining growth to encourage a more compact, bushy habit. Clerodendrum trichotomum is more tree-like and develops a permanent framework of stems; prune to remove dead, damaged, or crossing branches in April after the risk of frost is past and growth is resuming; it is easier to assess which stems are alive versus dead once the plant is growing actively. Clerodendrum trichotomum needs minimal pruning beyond removing dead wood; hard pruning is unnecessary and can reduce flowering. On both species, flower heads should not be deadheaded if the decorative turquoise-blue berries in red calyces of Clerodendrum trichotomum are wanted (they develop from the flowers and are the plant's most ornamental feature in autumn). Deadheading is unnecessary on Clerodendrum bungei as the plant flowers late in the season.

Q: What are the turquoise berries on my clerodendrum?

The striking turquoise-blue berries on Clerodendrum trichotomum (harlequin glorybower) are the plant's fruit: small, round, brilliant turquoise-blue or bright blue-green berries about 8 to 10 mm across, held in the persistent star-shaped red calyces that surround the fragrant white late-summer flowers. The contrast between the turquoise berries and the deep red, star-shaped calyx is exceptionally ornamental and unlike almost any other fruit in the UK garden; the berries appear in September and October as the flowers finish, and the combination of old flowers still open, developing berries, and fully ripe turquoise berries all visible simultaneously makes the plant very striking in early autumn. The berries are not edible for humans; they are mildly toxic and should not be consumed. They are relished by some birds, which disperse the seeds; self-sown seedlings can occasionally appear in sheltered gardens where the plant has fruited well. To maximise berry production: choose a warm, sheltered position in full sun; the flowers need warm, settled conditions in August and September to be pollinated successfully and set fruit; plants in cool, exposed positions or in shade often flower but fail to set berries. Clerodendrum trichotomum var. fargesii is a more cold-hardy variety with smaller leaves, white stems, and the same turquoise berries; it is the form most widely recommended for UK gardens as it is slightly more reliable in cold winters than the species.

Clerodendrum (principally Clerodendrum bungei, glory bower, and Clerodendrum trichotomum, harlequin glorybower) are marginally hardy, exotic-looking shrubs grown in UK gardens for their late-summer flowers and, in the case of Clerodendrum trichotomum, the spectacular turquoise berries held in red star-shaped calyces in autumn. The large, soft leaves are prone to aphid damage in spring and cold damage in poor winters; Clerodendrum bungei typically dies to the ground in UK winters and regrows from the roots each spring.

Cold damage

Cold damage is the most common and most serious problem for clerodendrum in UK gardens. Clerodendrum bungei is root-hardy in most of southern England (surviving to about -10 to -15°C at the roots) but the top growth is killed in most UK winters; the shoots die back to the ground and the plant regrows from the base each spring. In mild winters or very sheltered positions, the top growth may survive, but it is generally best to treat the plant as effectively herbaceous and expect it to regenerate from the roots each year. Clerodendrum trichotomum is slightly less root-hardy and may not survive severe UK winters in colder, more inland positions; it is most reliable in mild coastal gardens and sheltered urban positions in the south and west of England.

What to do

Plant Clerodendrum bungei in a warm, sheltered position against a south or west-facing wall to maximise survival of the root system through cold winters. Apply a deep mulch (10 to 15 cm) of composted bark, straw, or other insulating material over the root zone in late autumn to protect the roots from hard frosts; remove the mulch in March when the risk of severe frost is past. Do not cut the dead top growth until spring, as it provides some protection to the root zone and crown. For Clerodendrum trichotomum, choose the most sheltered, warmest position available and provide the same root zone protection.

Aphids

The large, soft leaves of Clerodendrum bungei are particularly attractive to aphids (typically greenfly and blackfly) as they emerge and expand in spring; aphid colonies on the young shoot tips cause the new leaves to curl, pucker, and distort. The infestation is usually most visible in May and June as the plant puts on its main flush of growth; heavy aphid pressure on the soft young growth can slow expansion and give the plant an untidy appearance in early summer. Natural predators (ladybirds, lacewings, parasitic wasps) typically arrive in numbers by June and reduce colonies significantly; the plant's vigorous growth rapidly outpaces the damage.

What to do

On most occasions no treatment is needed; natural predator pressure and plant vigour mean aphid colonies decline within 3 to 4 weeks. Pinch off the most heavily infested shoot tips or apply a jet of water to dislodge colonies without using insecticide. If the infestation is heavy and the plant is in a key visible position, apply insecticidal soap spray directly to the aphid colonies on young shoots, taking care to cover all surfaces; repeat after 10 days. Avoiding high-nitrogen fertilisers reduces the flush of very soft growth that is most attractive to aphids.

Drought stress

The large, tropical-looking leaves of Clerodendrum bungei lose moisture rapidly in hot, sunny, windy conditions; in drought conditions or in a very exposed, dry position, the leaves curl inward and the margins may brown. Clerodendrum bungei comes from the warm, moist woodlands of China and is not particularly drought-tolerant; in UK conditions it grows most vigorously and produces its best foliage and flowers in a position with adequate moisture. In dry soils the plant may fail to produce the vigorous growth needed to flower well in late summer.

What to do

Apply a mulch of composted bark or leaf mould around the root zone (keeping it away from the crown) to retain moisture and reduce evaporation. Water during extended dry spells; the large leaves are a guide to moisture needs: they wilt visibly when the plant is thirsty. A position with some afternoon shade reduces moisture demand and protects the large leaves from sun scorch. Avoid exposed, windy positions where moisture loss from the large leaf surface is most rapid.

Verticillium wilt

Verticillium wilt (Verticillium dahliae) is a soilborne fungal disease that can infect clerodendrum through the roots; the fungus colonises the water-conducting vessels of the stem and causes individual branches to wilt, curl, and die back, even though the rest of the plant may appear healthy. Cutting an affected stem reveals dark staining in the outer sapwood at the point where the stem changes from healthy to wilted. There is no fungicide treatment for verticillium wilt; the disease persists in the soil for many years.

What to do

Cut affected branches back to healthy wood well below the point of wilting; disinfect pruning tools with a bleach solution between cuts. If the whole plant is affected, remove and destroy it (do not compost affected material); do not replant with susceptible species (clerodendrum, dahlia, chrysanthemum, potato, tomato) in the same spot for at least 5 years. Improve soil drainage and organic matter content; the disease is less severe in well-aerated, free-draining soils. On Clerodendrum bungei, which regenerates from the roots each year, pruning affected shoots to the ground and waiting to see if new root growth is healthy can sometimes resolve the problem if the root system itself has not been infected.

Frequently asked questions

Why are my clerodendrum leaves curling?

Clerodendrum leaves curl most commonly in the UK because of cold damage (the top growth dies back to the ground in most UK winters on Clerodendrum bungei), aphids on the large, soft young leaves in spring (causing curling and puckering of new growth), drought stress (the large leaf surface loses moisture rapidly), or verticillium wilt (a soilborne fungal disease causing branch dieback with dark stem staining). Both UK clerodendrum species are marginally hardy; most problems relate to their tropical origin meeting UK winter conditions.

Is clerodendrum bungei invasive in the UK?

Clerodendrum bungei can spread by underground suckers and form a spreading colony over time, particularly in warm, sheltered gardens. It is not formally classified as invasive under UK legislation but can become persistent and widely spreading in mild conditions. Control suckers by severing them at or below soil level as they appear. Installing a root barrier 40 to 50 cm deep around the planting position limits spread in restricted spaces.

How do I prune clerodendrum?

Clerodendrum bungei typically dies to the ground in UK winters; cut dead stems to the ground in late winter before new growth begins. In mild gardens where top growth survives, cut back dead and frost-damaged stems in April. Clerodendrum trichotomum needs minimal pruning; remove dead, crossing, or damaged branches in April. Do not deadhead Clerodendrum trichotomum if the turquoise berries are wanted; they develop from the flowers in September and October.

What are the turquoise berries on my clerodendrum?

The turquoise-blue berries on Clerodendrum trichotomum (harlequin glorybower) are the plant's fruits, held in persistent star-shaped red calyces in September and October; the contrast of turquoise berry against red calyx is one of the most distinctive ornamental effects in the UK garden. They are mildly toxic and not edible. Clerodendrum trichotomum var. fargesii is slightly more cold-hardy and widely recommended for UK gardens.