Plant problems

Why Are My Indigofera Leaves Curling?

Q: Why are my indigofera leaves curling?

Indigofera (indigo shrub) leaves curl and drop most commonly in UK gardens because of cold damage killing the top growth in hard winters, drought stress on the pinnate leaves in hot or dry conditions, or waterlogging in heavy or poorly drained soils. Several species of indigofera are grown in UK gardens; the most commonly cultivated is Indigofera heterantha (also known as Indigofera gerardiana), a deciduous shrub from the Himalayas that produces arching stems clothed with small, pinnate leaves and racemes of pink-purple, pea-like flowers from July to September; it is hardy to about -10 to -15°C in a sheltered, well-drained position. Indigofera amblyantha and Indigofera decora are also grown in milder UK gardens. All UK-grown indigofera species behave as deciduous or semi-evergreen shrubs in UK conditions, and the top growth may be partially or fully killed in hard winters; the plants typically regrow vigorously from the base or from living buds on the lower stems in spring. The pinnate leaves are small and numerous; the whole plant looks light and airy in leaf. Leaf curl in actively growing indigofera is most commonly caused by drought stress (the pinnate leaves roll inward along their length when the plant is dry), or by aphid infestation on the soft, new growing tips. In winter and early spring, browning and collapse of stems and buds is caused by cold damage; the plant may look dead but will usually regrow from the base.

Q: Is indigofera hardy in the UK?

Indigofera heterantha (the most commonly grown UK species) is hardy to approximately -10 to -15°C in a sheltered, well-drained position; it is reliably perennial throughout most of England and Wales and in sheltered Scottish gardens. The plant typically behaves as a deciduous shrub in UK conditions, losing its leaves in autumn and regrowing in spring; the top growth may be killed back to the ground in hard winters, but the plant regrows vigorously from the rootstock, often producing flowering shoots by July or August even after being cut to the ground by frost. Indigofera decora is slightly less hardy and is best grown in sheltered, warm positions in milder UK areas. All indigofera species perform best in a sheltered, south or west-facing position, against a wall or fence, or in the shelter of other large shrubs; a freely draining, moderately fertile soil is essential for reliable overwintering. The wall-shrub position against a warm brick wall is ideal: the wall provides both shelter from cold north and east winds and thermal mass that prevents hard freezes at the root level. In such a position, indigofera will often retain some living stems through all but the hardest UK winters and flower from June onwards on the current season's growth; even when top growth is killed, the plant reliably regrows and still flowers on new growth later in the season.

Q: How do I grow indigofera in the UK?

Indigofera grows best in a sunny, sheltered position in full sun, in well-drained, moderately fertile, neutral to slightly alkaline soil. It is well-suited to growing against a warm wall (south or west-facing) where it benefits from the shelter and warmth; it also grows well as a freestanding shrub in the open garden in milder UK areas. Plant in spring after the last hard frost; water in well and mulch the root zone in the first winter. Indigofera is a member of the pea family (Fabaceae) and, like other legumes, is capable of fixing atmospheric nitrogen through root nodule bacteria; it does not benefit from high-nitrogen fertiliser and generally performs better in leaner soils. Feed with a low-nitrogen, potassium-rich fertiliser (tomato feed) in April to encourage flowering rather than lush, soft vegetative growth. Indigofera flowers on the current season's growth, which means that even if the top growth is killed back hard by winter frost, the plant will produce new flowering shoots from the base or from the rootstock that will flower in the same season. Prune in April: cut back all dead or winter-damaged stems to healthy buds or to ground level (where the plant has been fully cut back by frost). If winter damage is minimal, cut back the previous year's flowered shoots to strong buds in April; this encourages vigorous new growth that flowers from July onwards. Hard annual pruning to about 30 cm from the ground in April (as one might do with buddleja) produces the most vigorous, floriferous growth even when the plant has not been cut back by frost; the arching new shoots with abundant pink-purple flower racemes are most attractive on hard-pruned plants.

Q: When does indigofera flower in the UK?

Indigofera heterantha flowers in the UK from approximately late June or July to September or October, depending on the season and the severity of the previous winter. The plant flowers on the current season's growth; the arching stems produced from spring onwards carry elongated racemes of small, pea-like flowers in shades of pink-purple (more accurately described as rose-purple or pinkish-mauve) from the leaf axils. The flowering display is produced over a long period; new racemes open progressively from the base to the tip of each arching stem over many weeks, extending the flowering season considerably. In mild autumns, indigofera can continue to produce new flowers until the first hard frost, making it one of the later-flowering shrubs in the garden. Plants that have been hard-pruned in April, or that were cut back by winter frost and are regowing from the base, typically begin flowering 2 to 4 weeks later than unpruned plants that have retained their stems; the flowering season of hard-pruned plants therefore runs from approximately late July to October. In a warm, sheltered position where the previous year's stems survive intact and only need light pruning, flowering can begin from late June. The flower colour of Indigofera heterantha is typically described as pink-purple but is variable; a rich, deep rose-purple is the most attractive form. The cultivar 'Silk Road' reportedly produces longer flower racemes and a more prolific display than the typical species.

Indigofera heterantha (indigo shrub or Himalayan indigo) is a graceful, deciduous shrub with arching stems, fine pinnate foliage, and long racemes of pink-purple, pea-like flowers from July to October. Hardy to about -15°C in a sheltered, well-drained position, it flowers reliably on the current season's growth even after being cut to the ground by winter frost. Drought stress and cold damage are the most common causes of leaf curl; the plant regrows vigorously from hard annual pruning.

Cold damage

Cold damage is the most common cause of stem die-back and leaf loss in indigofera in UK gardens; hard frosts below about -10 to -12°C (or cold winds in exposed positions) kill the previous year's stems, which turn brown and brittle. The pinnate leaves wither and fall; in spring the plant may look entirely dead. However, indigofera is almost always alive at the base; the rootstock is significantly hardier than the top growth, and vigorous new shoots typically emerge from ground level by April or May.

What to do

Do not cut back after winter until April; new growth from the base or from low buds on the stems confirms the plant is alive. In April, cut all dead stems to ground level or to the first healthy bud. Apply a deep bark mulch over the root zone in November to insulate the rootstock. In subsequent seasons, consider hard annual pruning to about 30 cm in April even without frost damage; this produces the most vigorous, floriferous new growth and maintains a neat, arching habit.

Drought stress

Indigofera is moderately drought-tolerant once established in well-drained soil, but the small pinnate leaves roll inward along their length during active growth when the root zone dries out significantly in summer. This leaf curl is a reversible stress response; the leaves unfurl when the soil moisture is restored. Newly planted specimens in their first summer are most vulnerable, as the root system is still establishing and cannot access deep soil moisture. Container-grown plants can suffer more severe drought stress as the restricted root volume dries out quickly.

What to do

Water newly planted indigofera in its first summer whenever the top 10 cm of soil is dry; established plants (after two seasons) rarely need supplementary watering in average UK conditions. Mulch the root zone in spring to reduce moisture loss. Container-grown plants need regular watering throughout the growing season. If leaves curl during a hot dry spell, water thoroughly and the leaves will unfurl within a day or two.

Aphids

Aphids colonise the growing tips of indigofera in spring and early summer, causing the soft young shoot tips and the developing leaves to curl and distort; heavy infestations stunt the developing racemes. The problem is typically self-limiting as the season progresses and natural predators (ladybirds, parasitic wasps, hoverfly larvae) build up. The vigorous growth habit of indigofera means it quickly outgrows light to moderate aphid damage.

What to do

No treatment is usually needed for light infestations; natural predators arrive and reduce populations within a few weeks. Pinch out heavily colonised shoot tips. Apply insecticidal soap spray to young growth if the infestation is heavy or if the developing flower racemes are being damaged. Avoid high-nitrogen feeds that promote soft, lush growth most attractive to aphids.

Waterlogging

Indigofera does not tolerate waterlogged or consistently wet soils; the roots rot in saturated, poorly aerated conditions, causing the leaves to yellow and drop progressively and the stems to die back from the tips. Waterlogging is most damaging in winter when the plant is dormant and the soil remains saturated for extended periods. Heavy clay soils and low-lying positions are most at risk. The problem is sometimes confused with cold damage as both cause stem die-back in winter and spring, but waterlogging-caused die-back typically affects the whole plant progressively rather than primarily the outer tips exposed to frost.

What to do

Plant only in freely draining soil; add grit to clay soils before planting. Raised beds with a gritty growing medium provide the best winter drainage in wet UK areas. Do not position in low-lying spots where water pools after rain. A wall base (where the wall overhang deflects some rainfall from the root zone) is an ideal position.

Frequently asked questions

Why are my indigofera leaves curling?

Indigofera leaves curl most commonly because of drought stress (the small pinnate leaves roll inward along their length when dry; reversible with watering), cold damage (top growth killed by frost; plant regrows from base), aphids on soft young shoot tips (distorted curled growth at the tips; typically self-limiting), or waterlogging (progressive yellowing and die-back in saturated soils). In a well-drained, sheltered, sunny position it is a reliable, long-flowering shrub.

Is indigofera hardy in the UK?

Indigofera heterantha is hardy to about -10 to -15°C in sheltered, well-drained conditions and is reliably perennial throughout most of England and Wales. The top growth may be killed in hard winters but the rootstock survives and regrows vigorously. Even after complete top-growth kill, plants flower on the current season's new growth from July onwards. A sheltered south or west-facing wall position in well-drained soil maximises cold hardiness.

How do I grow indigofera in the UK?

Grow in full sun in a sheltered position against a south or west-facing wall, in well-drained, moderately fertile soil. Feed with low-nitrogen tomato feed in April. Prune in April: cut all dead or damaged stems to the ground or to the first healthy bud; hard pruning to 30 cm even on undamaged plants produces the most vigorous, floriferous new growth. Flowers appear from late June to October on the current season's arching stems.

When does indigofera flower in the UK?

Indigofera heterantha flowers from approximately late June or July (on unpruned or lightly pruned plants) to September or October. Hard-pruned plants or those that were cut back by frost typically begin flowering from late July. The flowers are small, pea-like, pink-purple racemes produced progressively over many weeks on the arching current-season shoots; mild autumns can extend the season until the first hard frost.