Plant problems

Why Are My Thamnocalamus Leaves Curling?

Q: Is thamnocalamus the same as fargesia?

Thamnocalamus crassinodus and Fargesia species are both high-altitude Himalayan clumping bamboos in the tribe Arundinarieae within the grass family Poaceae, and they share a number of important characteristics that UK gardeners find valuable; however, they are distinct genera with some important differences. Similarities: both are clumping (non-invasive) bamboos that form slowly expanding, contained clumps without producing the strongly running rhizomes of Phyllostachys; both are cold-hardy to well below typical UK winter minimum temperatures; both are native to high-altitude Himalayan and Chinese montane environments; both have narrow, lance-shaped, small to medium-sized leaves; both prefer a sheltered, moist, partially shaded position in UK gardens for the best performance. Key differences: The most visually distinctive difference is the extraordinary blue waxy (pruinose) culm bloom on newly emerged Thamnocalamus crassinodus culms in spring, which is not seen to the same degree in any Fargesia species commonly grown in UK gardens. Position preference: Thamnocalamus crassinodus is somewhat more sensitive to summer heat and drought stress than many Fargesia species and more specifically prefers the cooler, partially shaded position that reflects its higher-altitude native habitat; Fargesia 'Rufa' and F. murieliae are somewhat more tolerant of summer heat and sun exposure. Growth habit and scale: Thamnocalamus crassinodus in the UK is typically 3 to 5 m; Fargesia species range from the compact 1.5 m of F. 'Rufa' to the 4 m of a well-established F. murieliae. Availability: Fargesia species, particularly F. murieliae, F. nitida, and F. 'Rufa', are very widely available in UK garden centres; Thamnocalamus crassinodus is primarily available from bamboo specialist nurseries and is less commonly found in general garden centres.

Thamnocalamus crassinodus (mountain bamboo; cultivars 'Kew Beauty', 'Lang Tang', 'Merlyn') is a beautiful, high-altitude Himalayan clumping bamboo prized for its extraordinarily striking silver-blue to blue-grey waxy bloom on newly emerged culms in spring, and its small, delicate, gracefully fluttering leaves. Leaves curl from drought and heat stress in hot dry UK summers, cold and frost, or sun scorch in very hot, sunny, south-facing positions.

Drought and heat stress

A high-altitude Himalayan bamboo adapted to cool, moist, cloudy montane conditions; the small, delicate leaves have a high surface-area to volume ratio and lose water rapidly in hot, dry, sunny UK conditions. Leaves roll inward and the plant may drop significant numbers of leaves in the hottest, driest July and August periods. More sensitive to summer heat and drought than fargesia murieliae or fargesia 'Rufa'.

What to do

Plant in a cool, sheltered, partially shaded or dappled-shade position in moisture-retentive, humus-rich soil; a north-facing or east-facing position, or dappled shade under established deciduous trees, is ideal; incorporate large quantities of well-rotted compost and leaf mould into the planting hole; apply a deep mulch (10 to 15 cm) of composted bark or leaf mould each autumn; water thoroughly in dry periods from June to August; avoid very hot, dry, south-facing, exposed positions where the combination of heat and drought stress rapidly causes leaf rolling and leaf drop in this high-altitude species.

Cold and frost stress

The cultivars 'Kew Beauty' and 'Lang Tang' are cold-hardy to approximately -18 to -20°C in well-established, sheltered positions, making them among the most cold-hardy Himalayan clumping bamboos. Normal cold-triggered leaf rolling reverses as temperatures rise. Severe frost combined with persistent drying wind in February and March can cause tip browning and leaf drop beyond the normal protective response.

What to do

No action needed for normal cold-triggered leaf curl; the leaves uncurl as temperatures rise. Provide a sheltered position away from persistent cold easterly winds to reduce winter desiccation damage to the small, delicate leaves. Cut back any dead or damaged culms to ground level in spring after the risk of further frost has passed; the plant regenerates vigorously from the clumping root system.

Sun scorch in exposed positions

In a very hot, sunny, south-facing, exposed UK position in July and August, the small delicate leaves may suffer sun scorch in addition to drought stress, showing bleached, whitish to pale-brown patches on the most sun-exposed leaf surfaces alongside rolling and drooping. This combination is particularly damaging and the primary reason this species is consistently recommended for partial shade in the UK.

What to do

Relocate if possible to a cooler, more sheltered, partially shaded position; if relocation is not possible, provide temporary summer shading using a 30 to 50% shade cloth on the most sun-exposed side of the plant during the hottest July to August period, combined with thorough, consistent watering; remove summer shading in autumn once temperatures cool. The combination of afternoon shade and consistent moisture in summer eliminates both sun scorch and drought stress simultaneously.

Frequently asked questions

Why are my thamnocalamus leaves curling?

Thamnocalamus leaves curl most commonly because of drought and heat stress (high-altitude Himalayan species adapted to cool moist cloudy montane conditions; small delicate leaves lose water rapidly in hot dry sunny UK summers; cool sheltered partially shaded or dappled-shade position in moisture-retentive humus-rich soil; deep mulch; water thoroughly June to August; avoid hot dry south-facing exposed positions), cold and frost stress ('Kew Beauty' and 'Lang Tang' hardy to -18 to -20°C; normal cold-triggered rolling reverses as temperatures rise; sheltered position away from cold easterly winds; cut back dead culms in spring), or sun scorch in exposed positions (bleached whitish to pale-brown patches on sun-exposed leaf surfaces alongside rolling; provide afternoon shade and consistent moisture in summer; 30 to 50% shade cloth on most exposed side in July to August if relocation not possible). This species needs cool shade more than almost any other commonly grown UK garden bamboo.

What is the blue waxy culm coating on thamnocalamus?

A natural pruinose (waxy bloom) coating of epicuticular wax on newly emerged culm internodes; produced by the outer culm epidermis; provides UV protection and moisture resistance; reflects the high-altitude UV exposure of the native Himalayan habitat. Appears: spring as new culms grow rapidly from the soil; most intense and visually beautiful from April to June; 'Kew Beauty' has a particularly beautiful and intense blue bloom. Duration: not permanent; gradually wears away through the first summer; conventional pale green to olive-green by end of year 1; bloom does not return to the same culm in subsequent years; fresh blue bloom on each spring's new culms. Maximising the effect: cut back older culms that have lost their bloom to encourage vigorous new culm production; fresh new culms display the best blue waxy bloom.

What is the best position for thamnocalamus in a UK garden?

Temperature and shade: cool, sheltered, partially shaded or dappled-shade position; dislikes combination of full sun and summer heat; north or east facing or morning sun sheltered from hottest afternoon sun; dappled shade under established deciduous trees ideal. Soil and moisture: deep moisture-retentive humus-rich soil that does not dry out rapidly; more sensitive to summer drought than many UK garden bamboos; large quantities of well-rotted compost and leaf mould at planting; deep mulch (10 to 15 cm) each autumn. Shelter: sheltered from persistent cold easterly winds; sheltered position also helps appreciate the blue culm bloom. Avoid: very hot dry south-facing fully exposed positions; thin freely draining sandy soils; positions exposed to persistent cold easterly winds; full overhead shade that prevents good light from reaching the plant.

Is thamnocalamus the same as fargesia?

Similarities: both clumping non-invasive Himalayan bamboos; both cold-hardy to well below typical UK winter minimum temperatures; both prefer sheltered moist partially shaded positions; both have narrow lance-shaped leaves. Key differences: the extraordinary blue waxy pruinose culm bloom on newly emerged Thamnocalamus culms is the most visually distinctive difference; not seen in any Fargesia species commonly grown in UK gardens. Position preference: Thamnocalamus more sensitive to summer heat and drought stress; more specifically prefers cooler partially shaded position; Fargesia 'Rufa' and F. murieliae somewhat more tolerant of summer heat and sun. Availability: Fargesia widely available in UK garden centres; Thamnocalamus primarily from bamboo specialist nurseries.