Plant problems

Why Are My Semiarundinaria Leaves Curling?

Q: How does the culm colour develop in semiarundinaria fastuosa?

One of the most attractive features of Semiarundinaria fastuosa (Narihira bamboo) is the development of warm reddish-brown to purple-brown colouration on the culm surface in their second and third year of growth, giving the plant a bicolour culm effect when grown in full or half sun; the culm colour development in semiarundinaria is driven by several factors. First-year culms: newly emerged culms in their first growing season are mid to bright green, covered with culm sheaths (the protective overlapping scales that enclose the newly growing internode and drop as the internode elongates and matures); the culm sheaths of semiarundinaria are attractive in their own right, being mottled and streaked with a pinkish or brownish tinge; first-year culms are entirely green after the culm sheaths have dropped. Second and third-year culm colour development: from the second year onward, the exposed, sun-facing side of the culm (the side most exposed to direct sunlight) begins to develop a warm reddish-brown to purple-brown pigmentation; this pigmentation is a UV and light-triggered response in the outer culm tissue and develops most intensely and attractively in a full-sun or half-sun position; in full shade, the bicolour reddish-green culm effect does not develop and culms remain green throughout their life; the intensity of the reddish-brown colour varies from plant to plant and between different growing conditions, with more sun, warmth, and possibly some water stress contributing to more intense colouration. Maintaining good culm colour: remove older culms that have begun to lose their vigorous reddish colour (typically after four to five years) to encourage a fresh supply of vigorous new culms; thin overcrowded culm stands so that all culms receive good light penetration; position the planting in full sun or half sun to maximise the bicolour culm colour effect.

Q: Is semiarundinaria different from phyllostachys?

Semiarundinaria fastuosa (Narihira bamboo) and the Phyllostachys species (golden bamboo, black bamboo, etc.) are both running or semi-running bamboos that can reach considerable height in UK gardens, but they differ in several important ways that affect their use in UK gardens. Habit and culm arrangement: Semiarundinaria fastuosa has a very distinctive, rigidly upright, narrow, columnar habit; the culms grow very straight and vertical and the branches emerge at a very acute, upright angle to the culm, maintaining the narrow, columnar silhouette throughout the plant's life; this gives a very architectural, formal, almost architectural effect; by contrast, most Phyllostachys species have culms that arch outward at the top, giving a more open, fountain-like or arching silhouette. Culm colour and appearance: Semiarundinaria culms are initially green and develop reddish-brown to purple-brown colouration on the sun-exposed side in their second to third year; the most distinctive phyllostachys species have their own culm colour effects: Ph. nigra (jet black in years 2 to 3), Ph. aurea (golden yellow, with compressed nodes at base), Ph. aureosulcata (yellow groove on green, or vice versa). Rhizome habit: both semiarundinaria and phyllostachys have running or moderately running rhizomes; phyllostachys are generally more strongly and rapidly running than semiarundinaria; both require active management (root barrier or annual rhizome cutting) to prevent invasive spread. Height: both can reach very tall in favourable UK conditions; semiarundinaria is among the tallest bamboos that can be grown in UK gardens, comparable to the tallest phyllostachys species in similarly favourable conditions. Best use: Semiarundinaria for a very upright, formal, columnar tall screen or specimen; Phyllostachys nigra for the black culm colour effect; Phyllostachys aurea for golden culms; both need containment management in gardens where spread is problematic.

Semiarundinaria fastuosa (Narihira bamboo) is one of the tallest bamboos for UK gardens, reaching 6 to 10 m with a uniquely rigid, upright, columnar habit quite unlike the arching form of most bamboos. Culms develop attractive reddish-brown colouration on the sun-facing side in their second and third year. Leaves curl from drought, cold and wind desiccation (the tall culms are particularly wind-exposed), or normal late-winter leaf replacement.

Drought and water stress

More drought-tolerant than fargesia thanks to its moderately running rhizome system accessing moisture from a wider soil area; however, young recently planted specimens not yet with a full rhizome system are vulnerable in thin, freely draining soils in July and August. Established, older in-ground plants are rarely badly drought-stressed in typical UK conditions.

What to do

Water thoroughly in the first two growing seasons after planting while the rhizome system establishes; apply a deep mulch (10 to 15 cm) of composted bark around the base; in subsequent years water only in the most severe, prolonged UK droughts; a moisture-retentive, humus-rich soil in a sheltered position requires no supplemental irrigation once the plant is established except in exceptional conditions.

Cold and wind desiccation

Hardy to approximately -15 to -20°C in sheltered positions; normal cold-triggered leaf rolling reverses as temperatures rise. The tall, upright culms are particularly susceptible to heavy snow loading (which can bend or break culms) and to persistent cold drying easterly winds in February and March that cause tip and margin browning. The tallest culm tips can die back in very cold, exposed positions.

What to do

Choose a sheltered position away from prevailing north and north-east winds; a south-facing or west-facing wall or building on the windward side significantly reduces cold wind damage to the tall culms; after a severe cold spell or snow loading event, inspect culms for damage in spring; cut back any dead culm tips to a healthy node; dead culms to ground level; the plant regenerates vigorously from the rhizome system in spring. Shake snow off the upright culms after heavy snowfall to reduce the risk of bending or breaking from snow loading.

Normal leaf replacement

As with all bamboos, semiarundinaria drops a proportion of old leaves in late winter and spring as part of the annual renewal cycle; old leaves on lower culm sections and inner parts of the planting yellow, some curl, and drop in February to April before fresh new leaves follow. This is entirely normal and not a sign of stress.

What to do

No action needed. Check that new green buds and emerging leaves are visible on branch tips in spring to confirm that the dropping is part of the normal renewal cycle and not a sign of plant stress or culm death. Old leaves can be raked from beneath the planting and composted.

Frequently asked questions

Why are my semiarundinaria leaves curling?

Semiarundinaria leaves curl most commonly because of drought and water stress (more tolerant than fargesia due to moderately running rhizomes; young recently planted specimens most vulnerable; water thoroughly in first two growing seasons; deep mulch; established in-ground plants rarely drought-stressed in typical UK conditions), cold and wind desiccation (hardy to -15 to -20°C; normal cold-triggered rolling reverses as temperatures rise; tall upright culms particularly susceptible to heavy snow loading and persistent cold easterly winds; sheltered position essential; cut back dead culm tips in spring), or normal late-winter leaf replacement (old leaves yellow, curl, and drop in February to April; fresh new leaves follow in spring; entirely normal; check for new green buds on branch tips to confirm renewal rather than stress). Tall culms need a sheltered position more urgently than most shorter bamboos.

How does the culm colour develop in semiarundinaria fastuosa?

First-year culms emerge mid to bright green with mottled pinkish-tinged culm sheaths that drop as internodes mature; culms are entirely green after sheath drop. Second and third year: sun-facing side develops warm reddish-brown to purple-brown pigmentation triggered by UV and light exposure; intensity greatest in full or half sun; culms in full shade remain green. Maintaining good colour: remove older culms losing their reddish colour after 4 to 5 years; thin overcrowded stands for good light penetration throughout; full or half sun position for the best bicolour effect.

How much space does semiarundinaria fastuosa need in a UK garden?

Height: 6 m plus in sheltered warm positions; occasionally 8 to 10 m in most favourable long-established sites; very upright narrow habit uses relatively little horizontal space for its height. Horizontal spread: moderately running rhizomes; typically 30 to 60 cm per year outward in good UK conditions; root barrier (60 to 70 cm deep solid HDPE) strongly recommended before planting; or annual spade cut around perimeter in autumn or spring. Best use: very upright narrow columnar tall screening along a fence or boundary (minimal bed space, maximum height); tall specimen where striking culm colour and upright habit can be appreciated; avoid very exposed wind-swept positions where tall culms are susceptible to wind damage and desiccation.

Is semiarundinaria different from phyllostachys?

Habit: Semiarundinaria has a rigidly upright, narrow, columnar habit; branches emerge at a very acute upright angle; very formal architectural effect. Phyllostachys culms arch outward at the top giving a more open fountain-like silhouette. Culm colour: Semiarundinaria green maturing to reddish-brown on sun side in years 2 to 3. Ph. nigra: jet black in years 2 to 3. Ph. aurea: golden yellow, compressed base nodes. Ph. aureosulcata: yellow groove on green. Rhizome habit: both moderately to strongly running; Phyllostachys generally more rapidly spreading; both require containment management. Best use: Semiarundinaria for very upright formal columnar tall screen or specimen; Phyllostachys nigra for black culm effect; Ph. aurea for golden culms.