Plant problems

Why Are My Arthropodium Leaves Curling?

Q: Why are my arthropodium leaves curling?

Arthropodium (renga lily or rock lily) leaves curl and yellow most commonly in UK gardens because of cold damage in hard winters (arthropodium cirratum, the most commonly grown species in UK gardens, is of borderline hardiness in the UK, surviving to approximately -5°C without protection in a very sheltered, well-drained position; the strap-like, pale blue-green leaves curl and brown when frosted, and in a hard winter the above-ground growth may be killed to the ground while the tuberous roots survive), drought stress in summer (the leaves curl inward along their length when the tuberous roots dry out; consistent moisture is needed during the growing season from April to October), or waterlogging in winter (the tuberous roots rot in saturated soil, particularly in cold, wet conditions from October to March). Arthropodium cirratum (renga lily or New Zealand rock lily) is a striking, clump-forming, lily-family perennial from coastal cliff and rocky scrub habitats in New Zealand; it produces broad, strap-like, pale grey-green leaves (to 40 to 60 cm long) that are somewhat similar to those of a broad-leaved agapanthus, and delicate, branching sprays of small, white, star-shaped flowers on stems to 90 cm in early summer (May to June). It is widely grown in New Zealand gardens but has a more specialised role in the UK as a tender or borderline-hardy exotic perennial for very sheltered, mild positions in the south and west of England, Wales, and Ireland.

Q: Is arthropodium hardy in the UK?

Arthropodium cirratum (renga lily) is borderline hardy in the UK; it is generally reliable only in the mildest UK areas (coastal south and west England, Wales, and Ireland, particularly in sheltered town gardens and walled gardens) where frosts rarely fall below -3 to -5°C. In most UK gardens outside the mildest areas, arthropodium needs to be treated as a tender perennial: grown in a container, brought into a frost-free or cool greenhouse from November to April, and placed outdoors again after the last frost in May. Alternatively, in-ground plants can be protected with a very deep mulch (30 cm or more of dry bark, bracken, or straw) and wrapping the above-ground stems with fleece from November to April in mild areas. The tuberous roots are hardier than the leaves; plants whose leaves are killed by frost often resprout successfully from the root system if it has been adequately insulated from frost. Arthropodium candidum (lesser rengarenga) is much smaller (to about 15 cm) and slightly hardier, but it is rarely grown ornamentally. In the coldest UK gardens, arthropodium is best grown as a container plant brought indoors for winter or as a single-season exotic plant replaced annually.

Q: How do I grow arthropodium in the UK?

In milder UK areas (coastal, south, west, or very sheltered urban gardens), arthropodium cirratum can be grown in a very sheltered, south or south-west facing position against a warm wall in freely draining, moderately fertile, neutral to slightly acid soil. Plant in a generous amount of well-prepared, gritty, free-draining soil; do not plant in heavy clay or low-lying, frost-pocket positions. Mulch deeply in autumn (from early November) with at least 20 to 30 cm of dry bark, straw, or bracken; wrap the above-ground growth in two to three layers of horticultural fleece for added protection. Remove the mulch and fleece gradually in April to May when the risk of hard frost has passed. In colder UK areas, grow in a large container (at least 30 cm diameter) in a freely draining, loam-based compost such as John Innes No. 2 with 20 to 25% additional grit; bring into a frost-free cool greenhouse or conservatory from November to April, then place outdoors in full sun from May. Water regularly during the growing season; reduce watering from October as the plant enters a semi-dormant state. The white flower sprays in May to June are delicate and attractive; the broad, grey-green leaves provide strong foliage impact year-round in mild areas.

Arthropodium cirratum (renga lily or New Zealand rock lily) is a striking, broad-leaved, lily-family perennial from New Zealand coastal cliffs, grown in mild UK gardens for its pale grey-green, strap-like leaves and delicate white flower sprays in early summer. Borderline hardy in the UK (reliable only in the mildest areas without protection), it needs a sheltered, sunny position and frost protection in most UK gardens. Cold damage and drought are the main causes of leaf curl.

Cold damage

Cold damage is the most common cause of arthropodium leaf curl and browning in UK gardens; the broad, strap-like leaves are frost-tender and curl, brown, and collapse after exposure to temperatures below about -2 to -3°C. In a hard UK winter, the above-ground growth may be killed entirely while the tuberous roots survive to resprout in spring. Even in mild UK winters, a sharp frost in November or March can damage leaves that are actively growing. The combination of cold and wet is more damaging than cold alone.

What to do

In mild UK areas, apply a deep mulch (20 to 30 cm of dry bark, straw, or bracken) from early November and wrap exposed above-ground growth in two or three layers of horticultural fleece; remove gradually from April. In colder areas, grow in a large container and bring into a frost-free greenhouse or conservatory from November to May. Frosted leaves rarely recover but the tuberous roots often survive; remove damaged leaves and wait for new growth from the base in spring.

Drought stress

The broad, blue-green leaves of arthropodium curl inward along their length when the tuberous roots dry out in summer; the plants come from coastal cliff habitats where some summer moisture from sea mist and seasonal rain is usually available. Container-grown plants and those in very sandy soils are most susceptible to drought stress. Drought stress also reduces the number and quality of the white flower sprays produced in early summer.

What to do

Water regularly during the growing season (April to October); allow the growing medium to approach dryness between waterings but do not allow to dry out completely. Container plants need more frequent watering than in-ground plants in summer; a large container dries out less rapidly than a small one. Mulch the base of in-ground plants with composted bark to retain soil moisture and moderate root temperature in summer.

Waterlogging

The tuberous roots of arthropodium rot rapidly in waterlogged, poorly drained soil, particularly in cold, wet conditions from October to March; the leaves yellow, curl, and collapse as the root system fails. Waterlogging combined with frost is the most reliably lethal combination for arthropodium in UK gardens. Heavy clay soil without improvement, low-lying positions with poor drainage, and container plants left in saucers of water are all high-risk conditions.

What to do

Plant in freely draining soil; incorporate grit and organic matter in heavy clay soils. In containers, use a freely draining compost (John Innes No. 2 plus 25% grit) and ensure drainage holes are unobstructed; never leave containers standing in saucers of water. Reduce watering from October onwards as the plant enters a less active growth phase; in a greenhouse, water only when the compost is almost dry from November to April.

Frequently asked questions

Why are my arthropodium leaves curling?

Arthropodium leaves curl most commonly because of cold damage (leaves curl and brown after frost below -2 to -3°C; tuberous roots usually survive; mulch deeply in November and wrap with fleece in milder areas; use a greenhouse in colder areas), drought stress in summer (leaves curl inward; water regularly and mulch the base), or waterlogging rotting the tuberous roots (leaves yellow and collapse; ensure perfect drainage; reduce watering from October). Cold combined with wet is the most lethal UK condition.

Is arthropodium hardy in the UK?

Arthropodium cirratum is borderline hardy, reliable without protection only in the mildest UK areas (coastal south and west, sheltered urban gardens) where frosts rarely fall below -3 to -5°C. In most UK gardens, treat as a tender perennial: grow in a container, bring into a frost-free greenhouse from November to May, and place outdoors after the last frost. In mild areas, deep mulch (30 cm) and fleece from November provide adequate protection. Tuberous roots are hardier than leaves and often survive hard winters to resprout in spring.

How do I grow arthropodium in the UK?

In mild UK areas, plant in a very sheltered, south-facing position against a warm wall in freely draining, moderately fertile soil; mulch deeply from November and fleece the above-ground growth. In colder areas, grow in a large container (30 cm minimum) in John Innes No. 2 plus 25% grit; bring under cover from November to May. Water regularly from April to October; reduce from October. White flower sprays appear on 90 cm stems in May to June. The broad, grey-green leaves provide striking foliage impact year-round in mild gardens.

What is arthropodium cirratum?

Arthropodium cirratum (renga lily) is a New Zealand coastal cliff perennial with broad, pale grey-green leaves similar to a wide-leaved agapanthus and delicate white flowers in May to June; in New Zealand it is a robust, widely grown garden plant. In the UK it is a specialist's plant for mild, sheltered gardens or greenhouse growing. The closest UK garden relatives in style are agapanthus (for the foliage) and libertia (for the white flowers and New Zealand provenance). It is outstandingly beautiful in the right position.