Plant problems

Why Are My Stipa Leaves Curling?

Q: Why are my stipa leaves curling?

Stipa (feather grass or needle grass) leaves curl, tangle, and brown most commonly in UK gardens because of waterlogging in heavy, poorly draining UK winter soil (stipa comes from dry, rocky, or sandy grassland habitats across Europe, Asia, and the Americas; the fine, needle-like or hair-like leaves curl, yellow, and the plant collapses when the crown and root system sit in saturated, poorly aerated soil over winter; Stipa gigantea and S. tenuissima are the most commonly affected species in wet UK winters), too much shade (stipa is a sun-loving genus; in a shaded position the clump becomes loose, the fine leaves elongate and flop, losing the tight, upright or arching form that makes them ornamentally valuable; the leaves curl and the clump loses its architectural character in insufficient light), or natural seasonal die-back (Stipa tenuissima and similar species are technically semi-evergreen perennials that go through a phase of leaf browning and matting in winter; this is normal and the plant refreshes itself with new growth in spring). Stipa (in the broad sense that UK gardeners commonly use the name) includes Stipa gigantea (giant feather grass or giant oat grass, one of the most dramatic ornamental grasses available, producing enormous, oat-like, golden seed heads to 2 m on wiry stems above a low evergreen clump), Stipa tenuissima (Mexican feather grass or pony tails, now correctly named Nassella tenuissima, producing extraordinarily fine, silky, hair-like green leaves and flower heads that move in the slightest breeze and bleach to straw gold by midsummer), and the needle grasses of the true Stipa genus.

Q: Is stipa gigantea hardy in the UK?

Stipa gigantea (giant feather grass) is hardy throughout most of the UK in freely draining conditions; it tolerates temperatures to approximately -12 to -15°C in a well-drained, open, full-sun position and is reliably perennial in virtually all UK gardens except the most exposed upland sites. The low, evergreen basal clump of narrow, arching, dark green leaves typically survives UK winters undamaged; it is the root system, not the foliage, that is sensitive, and wet, cold, poorly draining soil in winter is the primary risk factor. In a well-drained sunny position, S. gigantea is essentially maintenance-free: the dead flower stems can be cut back in late winter or left for winter interest, and the basal clump refreshes itself annually. Stipa tenuissima (Mexican feather grass, Nassella tenuissima) is also generally hardy in most of the UK to approximately -8 to -10°C; it may brown significantly in a cold, wet winter in heavy soil but typically regrows from the crown in spring. The most critical requirement for both species is sharp drainage; a plant in poor drainage that would thrive indefinitely in freely draining soil can be killed by a single wet UK winter.

Q: How do I grow stipa in the UK?

Stipa grows best in a warm, open, full-sun position in poor to moderately fertile, very freely draining, neutral to slightly alkaline soil; the type of thin, dry, chalky or sandy soil that most garden plants struggle in is often ideal for stipa. Rich, fertile soil produces lush, floppy growth that is less ornamentally effective and more susceptible to winter wet problems. For Stipa gigantea: plant in a prominent, open position where the extraordinary flower stems (to 2 m) can be seen against the sky; a single specimen in a gravel garden or at the back of a sunny border is one of the most dramatic plantings possible in a UK garden. The flower stems appear from June and the golden, oat-like seed heads catch the light from July to October; the dead stems can be left for winter interest and cut back to the basal clump in late February. For Stipa tenuissima (Nassella tenuissima): plant in a sunny, dry position in sharply draining soil or gravel; the fine, silky leaves move in the slightest breeze and the effect in a drift or mass planting is uniquely beautiful. Both species self-seed freely in suitable dry, open conditions; deadhead if you do not want seedlings. Divide congested clumps of S. tenuissima every three to four years in spring.

Q: Is stipa tenuissima invasive in the UK?

Stipa tenuissima (Mexican feather grass, Nassella tenuissima) self-seeds prolifically in suitable dry, sunny, open conditions and is classified as an invasive species in some parts of the world, particularly California and parts of Australia where it has established in native grassland habitats. In UK garden conditions, self-seeding is common in gravel gardens, paved areas, dry walls, and open, sunny borders; seedlings are easily pulled out when small but can establish quickly in dry, disturbed ground. The risk of environmental invasiveness from UK gardens is considered low in most assessments because the UK climate is marginal for the prolific seeding behaviour seen in warmer, drier climates; however, the Royal Horticultural Society and some garden authorities have raised concerns about its potential. In practice, UK gardeners who grow S. tenuissima should deadhead the fluffy, self-seeding flower heads before they shed seed if they want to prevent spread beyond their intended planting area. Named cultivars that are less prolific seeders than the species are available and may be a better choice for gardeners concerned about self-seeding. The self-seeding tendency is less marked in heavy, wet UK soils where establishment of seedlings is less reliable than in the dry, open conditions the plant prefers.

Stipa (feather grass) includes some of the most beautiful ornamental grasses for UK gardens: Stipa gigantea (giant feather grass) with its extraordinary golden oat-like flower stems to 2 m, and Stipa tenuissima (Mexican feather grass) with its silky, hair-fine leaves that dance in any breeze. Both need a warm, open, full-sun position in very free-draining, lean soil. Waterlogging in winter is the main killer; shade produces limp, ineffective growth.

Waterlogging

Waterlogging is the most common cause of stipa decline and death in UK gardens; the plants come from dry, rocky, or sandy grassland habitats and have no adaptation to saturated soil. The crown and roots rot in cold, waterlogged conditions over winter, and the plant either fails to emerge in spring or produces a sparse, yellowing clump of curling leaves from a rotting root system. Heavy UK clay soils in low-lying positions are the most dangerous environment for both S. gigantea and S. tenuissima.

What to do

Plant only in very freely draining conditions; a raised position in lean, gritty soil, a gravel garden, or a sunny dry border with 30 to 50% coarse grit incorporated into the planting area is ideal. In clay gardens, grow in a raised bed or large container in very free-draining, lean compost. Apply a coarse gravel mulch around the crown from November to deflect winter rain from the root zone and reduce the risk of the crown sitting in standing water.

Too much shade

Shade causes stipa to produce elongated, floppy leaves that curl and tangle rather than the tight, upright, architectural form seen in a sunny position; the clump loses its characteristic density and the leaves droop without the tension they develop in full sun. Stipa gigantea in shade fails to produce its extraordinary flower stems in any useful quantity; S. tenuissima in shade loses the fine-textured, rippling movement effect that makes it ornamentally distinctive.

What to do

Grow in a position receiving at least six hours of direct sun per day; full sun for the entire day is better. Both S. gigantea and S. tenuissima perform poorly in partial shade and should not be planted under tree canopy, against north-facing walls, or in positions shaded by neighbouring shrubs for any significant part of the day. Move container-grown specimens to full sun if floppy, curling growth is developing.

Winter die-back

Stipa tenuissima goes through a natural cycle of browning and matting in winter; the fine, silky leaves bleach to straw gold by late summer and may mat and brown further through a wet UK winter. This is normal and not a cause for concern. Stipa gigantea's basal clump may develop some browning of older leaves in winter, also normal. The annual spring tidy (cutting back to the base in late February or early March) removes the accumulated winter material and allows fresh new growth to emerge unimpeded.

What to do

Cut S. tenuissima back hard to 5 to 8 cm from the base in late February or early March before new growth begins; the clump regenerates quickly and looks fresh within four to six weeks. Cut back dead flower stems of S. gigantea to the basal clump in late winter but do not cut into the evergreen basal rosette itself. Leave the straw-coloured dead stems of both species through winter if possible; they provide effective winter interest, catch frost, and protect the crown from the worst of the cold.

Frequently asked questions

Why are my stipa leaves curling?

Stipa leaves curl and the clump declines most commonly because of waterlogging in wet UK winter soil (crown and roots rot; grow in very freely draining, lean soil; a raised position or gravel garden is ideal; apply gravel mulch around the crown from November), too much shade (elongated, floppy, curling leaves; grow in full sun for at least six hours per day), or normal winter die-back in S. tenuissima (straw-coloured, matted leaves in winter are normal; cut back hard to 5 to 8 cm in late February). Free drainage is the single most critical requirement.

Is stipa gigantea hardy in the UK?

Stipa gigantea is hardy throughout most of the UK in freely draining conditions, tolerating approximately -12 to -15°C; the evergreen basal clump survives UK winters reliably and the extraordinary flower stems (to 2 m with golden, oat-like seed heads) appear from June. The primary risk is wet, cold, poorly draining soil in winter rather than cold itself. In a well-drained sunny position, S. gigantea is essentially maintenance-free and long-lived.

How do I grow stipa in the UK?

Grow in a warm, open, full-sun position in poor to moderately fertile, very freely draining, neutral to slightly alkaline, gritty or sandy soil. Rich soil produces floppy growth susceptible to winter wet. Position S. gigantea prominently where the extraordinary 2 m flower stems can be seen against the sky from June to October. Cut dead flower stems back to the basal clump in late winter. Cut S. tenuissima back hard to 5 to 8 cm in late February. Both self-seed in dry, open conditions.

Is stipa tenuissima invasive in the UK?

Stipa tenuissima self-seeds prolifically in suitable dry, sunny, open conditions (gravel gardens, paving gaps, dry walls) and is classified as invasive in some warmer countries. UK invasiveness risk is considered lower because the UK climate is marginal for prolific seeding. Deadhead the fluffy flower heads before they shed seed to prevent spread beyond the intended area. Named cultivars that seed less freely are available and are a prudent choice for gardeners concerned about self-seeding in open, dry garden areas.