Plant problems

Why Are My Cenchrus Leaves Curling?

Q: How do I grow cenchrus setaceus in the UK?

Cenchrus setaceus (purple fountaingrass) is grown in UK gardens as a tender annual or short-term tender perennial overwintered indoors, for its graceful, fountaining, arching growth habit and the characteristic soft, bottlebrush or foxtail flower heads in pink, red, or purple-pink produced abundantly from July through October. From seed: sow under glass in March to April at 20 to 25°C; germination typically takes two to three weeks; pot on into individual pots and grow on in a warm, bright greenhouse or conservatory until after last frost risk (late May in most of the UK); harden off for two weeks before planting outside. From young plants: many UK garden centres offer Cenchrus setaceus 'Rubrum' (purple fountaingrass) and related cultivars as young plants from May onwards; simply harden off and plant out after last frost risk. Position: full sun is essential for best leaf colour (especially for the burgundy-red forms), most compact growth, and best flower production; avoid shade. Soil and containers: a free-draining, moderately fertile, warm soil or container compost; in containers, use a peat-free potting compost with added perlite for drainage; ensure good drainage to avoid root problems. Watering: regular watering in summer, particularly in containers; the high leaf area of an actively growing fountaingrass in a sunny, warm position can result in rapid pot dry-out; water every one to two days in hot weather. Overwintering: bring indoors before the first autumn frosts (September to October); maintain in a frost-free conservatory, greenhouse, or windowsill above 5 to 8°C; reduce watering significantly in winter; resume watering and feeding in March as growth resumes.

Q: Is cenchrus the same as pennisetum?

Cenchrus and Pennisetum are closely related grass genera that have recently been merged by most botanical authorities following molecular phylogenetic studies that showed Pennisetum as classically defined is not a natural (monophyletic) group; the species traditionally placed in Pennisetum have been redistributed, with the majority transferred to the genus Cenchrus. The practical horticultural implication is that the ornamental grasses most UK gardeners and garden centres still call 'Pennisetum' are botanically now correctly named Cenchrus: Pennisetum setaceum (purple fountaingrass) is now Cenchrus setaceus. Pennisetum villosum (feathertop, white fountaingrass) is now Cenchrus longisetus. Pennisetum alopecuroides (Chinese fountaingrass, swamp foxtail) is now Cenchrus alopecuroides. Pennisetum orientale (oriental fountaingrass) is now Cenchrus orientalis. Many UK garden centres, nurseries, seed catalogues, and gardening books still use the old Pennisetum names; this is understandable given the long horticultural history of the genus under that name and the fact that the reclassification is relatively recent. When searching for these plants in a UK garden centre or nursery, searching for both Pennisetum and Cenchrus is advisable as labelling practice varies. The ornamental characteristics, cultivation requirements, and garden performance of all these plants are unaffected by the name change; they remain the same plants with the same beautiful, soft, bottlebrush or foxtail flower heads and the same graceful fountaining growth habit.

Cenchrus setaceus (purple fountaingrass, formerly Pennisetum setaceum) is one of the most popular tender annual ornamental grasses for UK summer containers and patio displays, grown for its graceful fountaining form, burgundy-red leaves, and soft bottlebrush flower heads in pink to purple from July to October. Not frost-hardy; grown as a tender annual in the UK. Leaves curl from cold damage, drought in containers, or insufficient light.

Cold damage and frost

Cenchrus setaceus is native to North Africa and the Middle East; killed by temperatures below 0 to minus 2°C. In the UK it is grown as a tender annual outdoors through summer and either discarded after the first autumn frosts or overwintered indoors. The characteristic burgundy leaf colour deepens as temperatures cool in autumn before the plant is killed by frost. Cenchrus longisetus (feathertop) is somewhat more cold-tolerant but not reliably hardy across the UK.

What to do

Do not plant outside until after the last frost risk (late May in most of the UK). Harden off for two weeks before planting out. Bring indoors before the first autumn frosts (September to October) if overwintering; maintain in a frost-free conservatory, greenhouse, or windowsill above 5 to 8°C; reduce watering significantly in winter; resume watering and feeding in March. If treating as an annual, discard after autumn frosts and raise new plants from seed or purchase young plants the following May.

Drought in containers

Container-grown cenchrus in a hot, sunny UK summer position can dry out rapidly, particularly in small terracotta pots; the narrow, arching leaves curl inward and the tips brown and dry when the root zone dries out. The warm-season C4 growth rate in summer means high water use relative to pot size; consistent watering is needed for best ornamental appearance through the main growing season from June to October.

What to do

Water every one to two days in hot weather; check containers daily in July and August and water before the compost has dried completely. Use a peat-free potting compost with added perlite for drainage; choose a container large enough for the root system (minimum 30 to 40 cm diameter for a mature fountaingrass plant). A saucer under the container retains some moisture but should not be allowed to keep the compost waterlogged; empty the saucer after heavy rain to prevent root problems.

Insufficient light

Cenchrus is a sun-loving warm-season grass; in partial shade the leaf colour (especially the deep burgundy-red of cultivars like 'Rubrum') is less vivid, the flower production is reduced, and the characteristic compact mounding to arching growth habit becomes looser and floppier. Full sun is essential for the best leaf colour, most compact growth, and most abundant flower head production through the long summer and autumn display period.

What to do

Position in full sun for the entire day; a south or west-facing position in a container or border gives the best results. Avoid planting near walls, buildings, or other plants that cast shade through the main summer growing season. The dark burgundy-red leaf colour of 'Rubrum' and similar cultivars requires maximum sunlight to develop fully; plants in shade produce greener, less dramatic foliage and far fewer of the characteristic soft, fluffy flower heads.

Frequently asked questions

Why are my cenchrus leaves curling?

Cenchrus leaves curl most commonly because of cold damage and frost (not frost-hardy; killed below 0 to minus 2°C; do not plant outside until after last frost in late May; harden off for two weeks; bring indoors before autumn frosts in September to October; overwinter above 5 to 8°C frost-free), drought in containers (high water use in hot sun; water every one to two days in July to August; check daily; peat-free compost with perlite; large container; empty saucer after heavy rain), or insufficient light (full sun required for best burgundy-red leaf colour, compact growth, and abundant flower production; partial shade produces greener, looser, fewer-flowered plants). Full sun plus consistent moisture are the two key summer requirements.

How do I grow cenchrus setaceus in the UK?

From seed: sow under glass March to April at 20 to 25°C; pot on in warm bright greenhouse; harden off for two weeks before planting outside after last frost in late May. From young plants: available at UK garden centres from May; harden off and plant out after last frost. Full sun is essential. Free-draining, moderately fertile compost with perlite in containers; water every one to two days in hot weather. Overwintering: bring indoors before first autumn frosts; frost-free above 5 to 8°C; reduce watering; resume watering and feeding in March. Main ornamental period July to October (soft bottlebrush flower heads in pink to purple-red).

Is cenchrus the same as pennisetum?

Cenchrus and Pennisetum have been merged following molecular phylogenetic studies showing Pennisetum as classically defined is not a natural group. Most Pennisetum species are now correctly named Cenchrus: P. setaceum (purple fountaingrass) is now C. setaceus; P. villosum (feathertop) is now C. longisetus; P. alopecuroides (Chinese fountaingrass) is now C. alopecuroides; P. orientale is now C. orientalis. Many UK garden centres, nurseries, and books still use the old Pennisetum names; searching for both Pennisetum and Cenchrus is advisable. Garden performance and cultural requirements are unaffected by the name change.

Is cenchrus setaceus invasive?

A significant invasive species in Hawaii, California, parts of the southwestern USA, Australia, South Africa, and the Canary Islands; dry biomass increases fire risk in these warm climates. In the UK, too cool and wet for outdoor naturalization; killed by UK frosts; low invasive risk in UK conditions. However, avoid disposing of plants or seeds in natural areas, especially in very mild southwestern UK gardens. Note: Cenchrus longisetus (feathertop grass, formerly Pennisetum villosum) IS on Schedule 9 of the Wildlife and Countryside Act in the UK; do not plant where it may spread to the wild; dispose of responsibly.