Plant problems

Why Are My Rostraria Leaves Curling?

Q: How do I grow rostraria cristata in a UK garden?

Rostraria cristata (Mediterranean hair grass) can be grown as an ornamental annual grass in a UK garden for its distinctive, compact, cylindrical, cat's-tail-like or phleum-like flower spikes, produced in June to August and valued as cut and dried flowers. From seed: sow directly in a prepared, well-drained seedbed in late March to April (after the risk of hard frosts has passed) in a warm, sheltered, sunny position; alternatively, sow in modules of well-drained, gritty compost under glass in March and plant out in late April to May after the last frost; the seed is small and should be surface sown and kept moist until germination; thin or plant out at 15 to 20 cm spacing. Position: a warm, sheltered, sunny, south- or west-facing position in a freely draining, light, well-drained soil; rostraria does not perform well in heavy, wet, cold soils or in a cool, shaded, north-facing border; the warmest, sunniest, most sheltered border position in the garden is ideal; a south-facing wall base or a gravel border in full sun gives the best results. Soil: light, well-drained, moderately fertile soil; add grit to heavy soil to improve drainage; do not add heavy organic mulch that retains too much moisture around the base of the plant. Watering: water in dry spells in May and June before the flower spikes are fully developed; once the flower spikes are well formed, drought stress is less likely to significantly affect the ornamental quality of the spike; avoid overwatering which causes yellowing and poor development. Harvest: for cut flowers, harvest when the cylindrical flower spikes are at the freshest, most compact, fully developed green stage; the spikes dry very well and retain their cylindrical, compact shape and velvety texture in a dried arrangement.

Q: What is the difference between rostraria and koeleria?

Rostraria cristata was formerly included in the genus Koeleria (as Koeleria phleoides) and is still sometimes listed under this name in older references; the relationship between Rostraria and Koeleria reflects a broader taxonomic discussion about the boundaries of these two grass genera in the family Poaceae. Key differences: Annual versus perennial: Rostraria cristata is an annual; native UK Koeleria species (particularly Koeleria macrantha, crested hair grass) are perennial; this is one of the most reliable practical distinctions for UK observers. Flower spike character: both genera produce dense, cylindrical, spike-like or narrowly paniculate flower heads; the spike of Rostraria cristata is very dense and compact, somewhat similar to a small Phleum (cat's-tail) head; the spikes of Koeleria macrantha are also compact and cylindrical but typically have a more lobed or interrupted base, and the spikes often contract and narrow slightly at the base giving them a slightly characteristic waist; these characters can be subtle and require careful examination. Origin: Rostraria is primarily a genus of Mediterranean and subtropical annual grasses (centred in the Mediterranean region, the Middle East, and parts of Africa); Koeleria is a genus of perennial grassland grasses with a broader Northern Hemisphere distribution; UK Koeleria macrantha is a native perennial of chalk and limestone grassland throughout the UK. Taxonomic treatment: some taxonomists continue to include Rostraria within an expanded Koeleria; others treat Rostraria as a separate genus; the treatment adopted in BSBI Flora of the British Isles and the New Flora of the British Isles (Stace) treats Rostraria as distinct from Koeleria. In a UK garden context, the most practically useful distinction is annual (Rostraria cristata) versus perennial (Koeleria macrantha), and southern casual or ornamental (Rostraria) versus native chalk and limestone grassland perennial (Koeleria macrantha).

Q: Is rostraria cristata invasive in the UK?

Rostraria cristata (Mediterranean hair grass) is not an established invasive species in the UK and is not considered a threat to UK native plant communities; it occurs in the UK as an occasional casual introduction, typically found in disturbed, open, sandy or gravelly ground in coastal southern England, in ports and docks (introduced with ballast or imported goods), and as a garden escape or seed introduction; it rarely persists for more than one or two seasons in any UK location before being outcompeted by native vegetation or eliminated by the cold and wet of UK winters. Reasons why it does not become invasive in the UK: UK climate mismatch: rostraria is a warm-season Mediterranean annual adapted to hot, dry Mediterranean summers and mild winters; UK summers are too cool and wet and UK winters are too cold and persistently wet for the species to become established and spreading; the conditions that would allow it to naturalise and become invasive (hot, dry summers followed by mild, dry winters) are not reliably present in the UK. Competition: in the UK, native annual and perennial grasses and wildflowers are well-adapted to UK conditions and generally outcompete Mediterranean casual annuals in established vegetation; rostraria only persists in very open, disturbed, bare ground where UK competition is low, and such habitats are usually only transiently available. Climate change: as UK summers become warmer and drier and UK winters become milder, Mediterranean annual grasses may begin to establish more successfully; rostraria cristata is one of several Mediterranean annual grass species that may expand its UK casual occurrence in the coming decades, though a full invasive status remains unlikely in a climate as variable and wet as the UK's. It is safe to grow rostraria cristata as an ornamental annual in a UK garden; the plant does not self-seed persistently in UK garden conditions and is very unlikely to spread or cause any harm to native vegetation.

Rostraria cristata (Mediterranean hair grass, crested hair grass) is an annual grass native to the Mediterranean region, occurring in the UK as an occasional coastal casual and sometimes grown ornamentally for its compact, cylindrical, cat's-tail-like flower spikes valued as cut and dried flowers. Leaves curl from drought in its preferred sandy substrates, rapid summer ripening as an annual in warm conditions, or cold and damp stress in cool UK summers that are unlike its native Mediterranean climate.

Drought in sandy and gravelly substrates

More drought-adapted than most UK annual grasses, rostraria nonetheless rolls its flat, narrow, hairy leaves inward and tip-browns in prolonged dry spells in thin, freely draining sandy substrates; its fine annual root system does not store large reserves of moisture. In a particularly hot UK summer or a thin south-facing coastal substrate, drought stress can cause the plant to curtail leaf growth and move toward early ripening before its full ornamental potential is reached.

What to do

Water in dry spells in May and June before the flower spikes are fully developed; once the cylindrical spikes are well formed, drought stress is less likely to significantly affect ornamental quality. Avoid overwatering; rostraria prefers the lean, dry, warm conditions of a south-facing gravel or sandy border in full sun. In a cutting garden, consistently moist (but not waterlogged) conditions through May and June give the biggest, best-formed, most compact spikes.

Summer heat and rapid annual ripening

Rostraria germinates in spring and can complete its entire annual lifecycle in as little as two to three months in a warm, dry summer; the dense, cylindrical flower spikes ripen quickly from green to straw-coloured and the whole plant dries and dies after seed set. In warm UK summers this summer ripening can be rapid and the ornamental spike quality declines quickly if harvest is delayed.

What to do

For the best cut or dried flower material, harvest the cylindrical spikes when they are at the freshest, most compact, fully developed, soft green stage; the spikes dry beautifully at this stage and retain their compact, cylindrical shape and velvety texture; fully ripened, straw-coloured spikes have less attractive dried flower quality. Make successive small sowings (late March, then late April) to extend the harvest window through summer.

Cold and damp stress in cool UK summers

A Mediterranean species adapted to hot, dry summers, rostraria performs less well in cool, wet, dull UK summers; in such conditions it grows slowly, produces fewer or less well-developed flower spikes, and the leaves yellow and curl from cold and damp stress. This is the opposite of drought stress but produces similar leaf symptoms; the tell is in the conditions (persistently cool, wet, and overcast) and in the poorly developed, fewer, or stunted spikes.

What to do

Choose the warmest, sunniest, most sheltered position available; a south- or west-facing wall base, a gravel bed in front of a heat-retaining stone wall, or a sunny south-facing raised bed in freely draining gritty compost provide better heat accumulation than an open, exposed, north-facing or shaded border. In a cool, wet UK summer, rostraria may be a poor performer; Koeleria macrantha (crested hair grass, a perennial native) is better suited to cooler, more exposed UK conditions and produces similar cylindrical spikes in June to July.

Frequently asked questions

Why are my rostraria leaves curling?

Rostraria leaves curl most commonly because of drought in sandy and gravelly substrates (fine annual roots; leaves roll and tip-brown in dry spells; water in May to June before spikes fully form; avoid overwatering; south-facing gravel or sandy border in full sun; consistent moisture gives biggest spikes), summer heat and rapid annual ripening (annual; completes lifecycle in 2 to 3 months in warm summers; spikes ripen green to straw-coloured quickly; harvest at fresh compact green stage for best dried flower; make successive sowings for extended harvest window), or cold and damp stress in cool UK summers (Mediterranean annual; performs poorly in cool wet dull summers; fewer or stunted spikes; choose warmest sunniest most sheltered position; south or west facing wall base or gravel bed; Koeleria macrantha is a better choice for cooler exposed UK conditions). Warmth and full sun are the key requirements.

How do I grow rostraria cristata in a UK garden?

Sow directly in well-drained seedbed in late March to April in warm sheltered sunny position; or sow in modules under glass in March and plant out after last frost; thin or plant at 15 to 20 cm spacing. Warm, sheltered, sunny south or west-facing position; freely draining, light, moderately fertile soil; add grit to heavy soil; no heavy organic mulch. Water in dry spells in May and June before spikes develop; avoid overwatering. For cut flowers, harvest when spikes are at the fresh compact fully developed green stage; spikes dry well and retain compact cylindrical shape and velvety texture. Make successive sowings to extend the harvest season.

What is the difference between rostraria and koeleria?

Formerly included in Koeleria (as K. phleoides); now treated as a separate genus in Stace Flora. Key practical differences: Annual versus perennial: Rostraria cristata is annual; Koeleria macrantha (crested hair grass) is perennial; the most reliable practical distinction in the UK. Flower spike: both dense, cylindrical, cat's-tail-like; Koeleria macrantha spikes typically have a slightly lobed or interrupted base and may narrow slightly below giving a characteristic waist; Rostraria spikes are very dense and compact throughout. Origin: Rostraria is a Mediterranean and subtropical annual genus; Koeleria is a perennial genus with a broader Northern Hemisphere distribution; UK Koeleria macrantha is a native perennial of chalk and limestone grassland throughout the UK. In UK garden context: Rostraria = annual ornamental or coastal casual; Koeleria macrantha = native perennial chalk grassland grass.

Is rostraria cristata invasive in the UK?

No. Not an established invasive species; not a threat to UK native plant communities. Occurs as an occasional casual in disturbed, open, sandy or gravelly ground in coastal southern England; rarely persists more than one to two seasons before outcompeted or eliminated by UK winters. UK climate mismatch: adapted to hot dry Mediterranean summers and mild winters; UK summers are too cool and wet; UK winters too cold and wet for persistent naturalisation. Safe to grow as an ornamental annual in a UK garden; does not self-seed persistently and will not spread into native vegetation. Climate change may increase casual occurrence frequency as UK summers warm, but full invasive status is very unlikely given the persistent variability and wetness of the UK climate.