Plant problems

Why Are My Pseudosasa Leaves Curling?

Q: Why are my pseudosasa leaves curling?

Pseudosasa (arrow bamboo or metake) leaves curl tightly along their length and develop brown tips most commonly in UK gardens because of cold, drying winter wind (pseudosasa japonica, the most commonly grown species, is an evergreen bamboo that retains its leaves through winter; in cold, windy UK winter conditions, the large, strap-like, dark green leaves curl inward tightly along their length and develop brown tips as a water-conservation response to wind desiccation; the leaves lose water through their surface faster than the frozen or cold roots can replace it), drought stress in summer (the leaves curl in the same way during hot, dry UK summers when the root zone dries out; the leaf curl in pseudosasa is a rapid, visible drought-stress response that reverses quickly when water is applied), or cold damage in very hard UK winters (pseudosasa japonica is one of the hardiest bamboos in UK cultivation, generally tolerating temperatures to approximately -15 to -18°C in the rhizome, but the leaves are more tender and will die back in a very hard winter below about -10 to -12°C sustained; in most UK winters the leaves survive but curl significantly during cold spells). Pseudosasa is a genus of around 10 to 12 species of medium to large bamboos (Poaceae, the grass family) from China, Japan, and Korea; Pseudosasa japonica (arrow bamboo or metake) is the most commonly grown species in UK gardens, forming dense, upright to arching clumps of canes to 3 to 6 m with very large, broad, strap-like, dark green leaves (to 30 cm long and 3 to 4 cm wide) and a running rhizome that spreads slowly to form an impenetrable thicket over time.

Q: Is pseudosasa invasive in the UK?

Pseudosasa japonica has a running (leptomorph) rhizome that spreads outward from the original planting position, sending up new canes at increasing distances from the clump centre over time; in the UK climate, the spread is relatively slow compared to the most invasive running bamboos (such as Phyllostachys), but it can become a persistent nuisance in a garden where it is not regularly controlled. In a medium-sized UK garden without a rhizome barrier, pseudosasa can spread 1 to 2 m over five to ten years and is difficult to eradicate once established because the rhizomes are tough and deeply embedded. Installing a rhizome barrier (a solid sheet of 60 to 80 cm deep, 2 mm thick polypropylene or HDPE barrier buried around the planting position) before planting is the most effective control measure; the barrier must overlap by at least 20 cm and be sealed to prevent the rhizomes finding a gap. Alternatively, grow in a large container (minimum 60 cm diameter, solid-sided with no drainage holes in the bottom if rhizome containment is the goal) sunk into the ground or stood above it; check annually for rhizomes escaping through the drainage holes. Regular division and removal of expanding sections of the rhizome in late autumn or early spring is the low-tech control method if barriers were not installed; use a sharp spade and be thorough.

Q: How do I grow pseudosasa in the UK?

Pseudosasa japonica is one of the most tolerant and adaptable bamboos for UK cultivation; it grows in a wide range of soil types (including clay), tolerates partial shade (and is one of the few bamboos that performs well in quite shaded positions, making it valuable for screening under trees or in dark corners), and is hardy in virtually all UK gardens. For best results, plant in a moderately fertile, moisture-retentive soil in a position sheltered from the most severe winter winds; the sheltered position significantly reduces winter leaf curl and browning. Incorporate generous quantities of well-rotted organic matter at planting to improve moisture retention; once established, the deep, spreading root system of pseudosasa makes it drought-tolerant except in the first year or two. Install a rhizome barrier before planting if unwanted spreading is a concern. Water generously through the first growing season to establish a deep root system; thereafter, water in prolonged dry spells in summer. Feed with a high-nitrogen fertiliser or lawn fertiliser in early spring to promote strong cane growth and healthy leaf production. In late winter or early spring, remove old and damaged canes from the base to allow light to reach new canes developing from the rhizome; this maintains a fresher, tidier appearance and improves the overall density of new growth. Thinning also reduces the brown-leaf clutter that accumulates inside dense, old pseudosasa clumps.

Q: Why does pseudosasa leaf curl in winter?

Pseudosasa japonica (and most other evergreen bamboos) roll their leaves inward tightly along the length in cold or dry winter conditions as a physiological response to reduce water loss; this is a normal, functional behaviour rather than a sign of disease or imminent death. The leaf curl occurs because the roots in cold or frozen soil cannot supply water to the leaves rapidly enough to replace what is lost through transpiration on cold, windy days; the leaf rolls to reduce the surface area exposed to drying wind and cold air. The rolling is reversible: when temperatures rise, the roots supply water again and the leaves uncurl within a few hours. Significant brown tip and margin development (where the rolled leaf tissue has actually died) indicates that the desiccation went beyond the reversible stage and some leaf tissue was killed; the brown sections do not recover, but the rest of the leaf often remains functional and the plant typically produces a flush of new leaves in spring from the same canes. Very severe winter leaf death (most or all leaves browning and dropping) is caused either by an unusually severe frost, extreme wind desiccation, or a combination of both; even in this case, if the canes are still green and the rhizomes are alive, the plant will typically produce new leaves from the cane nodes in spring.

Pseudosasa japonica (arrow bamboo or metake) is one of the most commonly grown and most cold-tolerant bamboos in UK gardens, forming dense, upright clumps of canes to 3 to 6 m with large, broad, dark green leaves. A running bamboo requiring a rhizome barrier to prevent unwanted spread. Leaf curling in winter is a normal water-conservation response to cold and wind; drought curl in summer is also common and reverses quickly when watered.

Cold wind and winter desiccation

The most common cause of pseudosasa leaf curl in the UK is cold, drying winter wind; the large, strap-like leaves roll inward tightly along their length on cold, windy days as a normal physiological response to reduce water loss through the leaf surface. The roots in cold or frozen soil cannot supply water to the leaves fast enough to replace what is lost on a windy day, so the plant rolls its leaves to reduce the exposed surface area. This curl is entirely normal and fully reversible when temperatures rise.

What to do

Plant in a position sheltered from prevailing winter winds; even a modest windbreak (a wall, fence, or hedge) significantly reduces the severity and duration of winter leaf curl. If significant brown tip development accompanies the curl (indicating that tissue has actually died from desiccation rather than just temporarily conserving water), move the plant to a more sheltered position or install a temporary windbreak screen from November to March. The brown sections do not recover, but the plant produces new leaves from the same canes in spring.

Drought stress

Drought causes the same inward leaf curl as cold wind but in summer conditions; the large leaf surface loses water rapidly in a hot, sunny UK July, and the leaves roll up within hours of the root zone drying out. Unlike the winter curl (which can involve tissue damage if prolonged), summer drought curl in pseudosasa is almost always fully reversible within a few hours of thorough watering. The rapid, visible drought response is one of the most useful plant-health indicators in the garden.

What to do

Water thoroughly during prolonged dry spells; established pseudosasa in the ground rarely needs irrigation in a normal UK summer, but in a very hot, dry July or August, water deeply at the base every five to seven days. Newly planted pseudosasa needs consistent watering through its first full growing season to develop a deep, drought-resilient root system. Apply a 5 to 8 cm bark mulch over the root zone every spring to reduce moisture loss from the soil surface.

Hard frost damage

In a very hard UK winter with sustained temperatures below -10 to -12°C, pseudosasa leaves die back significantly or completely; the leaves go from curled to brown and papery, hanging on the canes or dropping. Even in this case, if the canes are still green when scratched (indicating the vascular tissue is alive) and the rhizomes have not frozen solid, the plant typically produces new leaves from the cane nodes in spring. Very severe winters can kill entire canes back to the rhizome, but new canes then emerge from surviving rhizomes in May or June.

What to do

Do not cut back brown canes until May; check for green tissue under the brown outer layer by scratching with a fingernail before removing. Apply a generous bark mulch (10 to 15 cm) over the root zone from November to insulate the rhizomes. In the most severely affected plants, remove all dead foliage and let the plant regenerate from cane nodes or rhizome shoots. In most UK winters, even plants that lose most of their leaves in December and January recover completely by June.

Frequently asked questions

Why are my pseudosasa leaves curling?

Pseudosasa leaves curl most commonly because of cold wind and winter desiccation (normal, reversible water-conservation response; plant in a sheltered position; brown tips indicate tissue death but the plant recovers in spring), drought stress in summer (rapid, fully reversible curl; water deeply during prolonged dry spells; mulch the root zone), or very hard frost (leaves die back; check canes for green tissue before removing; new leaves from cane nodes or rhizome shoots in spring). Winter curl is normal behaviour for all evergreen bamboos in cold conditions.

Is pseudosasa invasive in the UK?

Pseudosasa japonica has a running rhizome that spreads slowly; in a UK garden without containment, it can spread 1 to 2 m over five to ten years. Install a 60 to 80 cm deep rhizome barrier before planting, or grow in a large container. Control established spread by cutting the rhizomes around the perimeter with a sharp spade in late autumn or early spring. Removal of an established colony without barriers requires persistent effort over two to three years.

How do I grow pseudosasa in the UK?

Grow in any moderately fertile, moisture-retentive soil in full sun to quite deep shade; one of the most shade-tolerant bamboos available, making it valuable for dark corners and under trees. Plant with a rhizome barrier installed first. Shelter from severe winter winds significantly reduces leaf curl and browning. Water through the first growing season; thereafter drought-tolerant. Feed with high-nitrogen or lawn fertiliser in early spring. Thin old and damaged canes from the base each spring.

Why does pseudosasa leaf curl in winter?

Winter leaf curl is a normal, functional response: the roots in cold soil cannot replace water lost through the leaf surface on windy days, so the leaves roll inward to reduce exposed surface area and limit water loss. This is reversible and the plant uncurls when temperatures rise. Brown tip development indicates the desiccation went beyond the reversible stage and some tissue died; the brown sections do not recover, but the plant produces new leaves from cane nodes in spring. This behaviour is common to all evergreen bamboos in cold conditions.