Plant problems

Why Are My Imperata Leaves Curling?

Q: Is imperata cylindrica rubra hardy in the UK?

Imperata cylindrica 'Rubra' (Japanese blood grass) is borderline hardy in the UK and its cold tolerance depends significantly on drainage, crown protection, and position. In freely draining, gritty soil in a sheltered, sunny position, it typically tolerates down to about -10 to -12°C and can be grown outdoors year-round in most UK gardens with some protection in severe winters. In cold, wet, poorly draining conditions, the damage threshold is much lower and even a moderately severe UK winter can damage or kill the plant. The crown is more vulnerable than the root system; the rhizomes (underground running stems) often survive UK winters even when all above-ground growth is killed, and the plant regenerates from the rhizomes in spring. This makes distinguishing between 'killed' and 'dormant in a cold winter' imperata difficult until May; if no new growth has appeared by mid-May, the plant has likely not survived. In the coldest UK gardens (northern Scotland, upland areas), growing in a container and moving under frost-free cover from November to March is the safest approach. In mild coastal and urban UK gardens, imperata 'Rubra' is reliably perennial outdoors with a dry bark or grit crown mulch from November. In most UK gardens, the reality is that the plant is perennial in mild winters and may partially or fully die back in hard ones, regenerating from rhizomes in spring.

Q: How do I grow imperata in the UK?

Imperata cylindrica 'Rubra' grows best in a warm, sunny position in moderately fertile, consistently moist, well-drained soil; full sun is important for the best and most vivid red leaf colour. Unlike many ornamental grasses, imperata prefers soil that retains reasonable moisture through summer rather than dry, free-draining conditions; it is not a drought-tolerant grass and benefits from a moisture-retentive growing medium, mulching with bark in spring to retain soil moisture, and supplementary watering in dry UK summers. Plant in spring (April to May) after the coldest weather has passed; autumn planting risks the plant going into winter without an established root system. Space at 30 to 40 cm apart; imperata spreads by underground rhizomes and will slowly expand its territory over several seasons. Apply a dry bark or grit crown mulch from November to insulate the root zone from the worst winter cold and wet. In cold UK gardens, the plant can be lifted, potted up, and overwintered under frost-free cover from November to March, then planted out again in spring. Cut back the previous year's dead leaves in late February or early March before new growth begins. The distinctive feature of imperata 'Rubra' — the progressive development of vivid blood-red pigmentation from the leaf tips downward through spring and summer — is best in full sun; in shade, the red develops less intensely and the plant loses its primary ornamental value.

Q: Why is my imperata turning green?

Imperata cylindrica 'Rubra' produces leaves that emerge with green tips in spring and develop vivid red pigmentation from the tips progressively through summer; the most intense all-red or blood-red colouring of the entire leaf develops from mid-summer through autumn, reaching its peak in September and October. If the leaves are greener than expected, the most common causes are: too little sun (the red anthocyanin pigmentation develops most intensely in full sun and is suppressed in shaded positions; moving to a sunnier position typically produces much more vivid red colouring over the following season), cold or cloudy UK summer weather (the red colour development is partly temperature-dependent and more intense in warm, sunny conditions than in a cold, cloudy UK summer; there is little that can be done about weather, but a sunnier position maximises the available sun), or reversion to the green species form (imperata cylindrica is a vigorous, spreading grass and the red cultivar 'Rubra' can occasionally produce all-green reverted shoots from the rhizomes; these should be removed immediately at the base as the green form is more vigorous than 'Rubra' and will gradually take over if left; the green form of I. cylindrica is also considered invasive in warm climates). If the plant has reverted to green and the reversion is widespread, the safest approach is to discard the entire clump and start again with a fresh 'Rubra' plant.

Imperata cylindrica 'Rubra' (Japanese blood grass) is grown almost entirely for its spectacular foliage: flat, upright leaves that emerge green in spring and develop vivid blood-red pigmentation from the tips downward through summer, reaching peak intensity in autumn. Borderline hardy in the UK; needs full sun for the best red colour. Drought, cold in wet conditions, and shade are the main causes of curl and colour loss.

Drought stress

Unlike many ornamental grasses, imperata prefers consistently moist soil and is not drought-adapted; the flat, upright leaves curl inward when the root zone dries out. Container plants and first-year specimens without established root systems are most vulnerable in dry UK summers. Drought also fades the vivid red colouring as the stressed plant diverts resources away from pigment production.

What to do

Grow in moderately fertile, moisture-retentive soil and mulch with bark in spring to retain moisture through summer. Water in dry spells; imperata in the ground needs checking weekly in a hot UK summer and containers need daily attention. Unlike festuca or helictotrichon, imperata should not be grown in lean, dry conditions; it needs reliable summer moisture to perform well and maintain its characteristic colour.

Cold damage

Imperata 'Rubra' is borderline hardy in the UK; cold combined with wet soil is more damaging than cold alone. In a hard UK winter, above-ground leaves die back completely; the rhizomes often survive and regenerate new growth in spring. The crown is the most vulnerable point; protecting it with a dry bark or grit mulch from November significantly improves winter survival. Late spring emergence (May) means the plant may appear dead through March and April.

What to do

Apply a dry bark or grit crown mulch from November. In cold UK gardens, lift and pot up in autumn and overwinter under frost-free cover from November to March; replant in spring after frost risk has passed. Do not cut back dead leaves until late February as they provide some crown insulation. If no new growth appears by mid-May, check for live rhizomes; even sparse rhizome survival can produce a full plant by summer.

Too little sun

The blood-red leaf pigmentation of imperata 'Rubra' develops most intensely in full sun; in partial or dappled shade, the red colour is less vivid and more of the leaf surface remains green, with red restricted to the upper leaf tips rather than spreading down the full leaf length. In deep shade, the characteristic colouring is largely absent and the plant loses its primary ornamental value entirely.

What to do

Grow in full sun for the most vivid blood-red colouring; a warm, south-facing position maximises both sun exposure and the heat that intensifies the red pigmentation through summer. Moving a greening, shaded imperata to a sunny position in spring typically produces significantly more vivid red development within one growing season. If growing in a container, position it where it receives at least six to eight hours of direct sun per day.

Frequently asked questions

Why are my imperata leaves curling?

Imperata leaves curl most commonly because of drought stress (curl inward when roots dry; unlike most ornamental grasses, imperata needs consistent moisture; mulch and water regularly), cold damage in hard or wet UK winters (leaves die back completely; rhizomes often survive; dry crown mulch from November; overwinter under cover in cold gardens; expect late emergence in May), or too little sun (red colour fades; develop most intensely in full sun; move to sunniest position available). Moisture-retentive soil, full sun, and crown protection in winter are the keys.

Is imperata cylindrica rubra hardy in the UK?

Borderline hardy; tolerates approximately -10 to -12°C in freely draining conditions with crown protection. Cold combined with wet is more damaging than cold alone. Leaves die back in hard winters; the rhizomes often survive and regenerate in spring. In mild coastal or urban UK gardens, reliably perennial outdoors with a dry crown mulch. In cold UK gardens, overwinter under frost-free cover. No new growth by mid-May suggests the plant has not survived. The rhizomes are hardier than the crown; check for surviving rhizomes before discarding.

How do I grow imperata in the UK?

Grow in full sun in moderately fertile, consistently moist, well-drained soil. Unlike most ornamental grasses, imperata needs reliable summer moisture; mulch with bark in spring and water in dry spells. Plant in spring after frost risk passes. Space 30 to 40 cm apart; spreads by rhizomes. Apply dry crown mulch from November. Cut back dead leaves in late February. The blood-red colour develops from the leaf tips downward through summer, peaking in September and October in full sun. Remove any all-green reverted shoots immediately.

Why is my imperata turning green?

Likely insufficient sun (the red pigmentation develops most intensely in full sun; move to a sunnier position for more vivid colour), cool or cloudy UK summer weather (the colour is partly temperature-dependent; a warm, sunny summer produces the best red), or reversion to the all-green species form (more vigorous green shoots from rhizomes; remove these immediately at the base or the green form will gradually take over; widespread reversion means discarding the clump and starting with a fresh 'Rubra' plant).