Plant problems

Why Are My Achnatherum Leaves Curling?

Q: How do I grow achnatherum calamagrostis in the UK?

Achnatherum calamagrostis (silver spike grass) is one of the most elegant and reliable drought-tolerant ornamental grasses for a UK dry garden, gravel garden, or Mediterranean-style planting scheme. Grow in full sun in a very freely draining, poor to moderately fertile, sandy, gravelly, or stony soil; the combination of full sun and sharp drainage is the key to success with this grass in UK conditions; on a heavy, moisture-retentive, or clay soil it is likely to suffer from root rot, particularly in a wet UK winter. Avoid enriched, fertile, moisture-retentive garden soils; excessive fertility and moisture produce overly lush, floppy growth that is more prone to flopping and less characteristic of the naturally compact, silvery clump habit of the species in its natural dry-soil habitat. Drainage: sharp drainage is more important than any other factor for achnatherum; on clay or heavy loam, raise the planting area by incorporating significant quantities of horticultural grit or fine gravel (at least one third of the soil volume by bulk) to open up the drainage; a raised bed or gravel garden with a deep gravel mulch is ideal. Watering: water sparingly; once established (after one to two growing seasons), achnatherum is highly drought-tolerant and should be watered only in prolonged, severe drought; regular watering encourages root rot and soft, floppy, atypical growth. Cutting back: cut back to the base in spring (March to April) to encourage clean new growth; the dried flower panicles provide winter interest through autumn and winter and can be left until late winter before cutting back. Division: divide overgrown clumps in spring; achnatherum divisions establish readily in free-draining conditions.

Q: What is the difference between achnatherum and stipa?

Achnatherum and Stipa are closely related genera of ornamental grasses; the two genera have been treated differently by different taxonomic authorities, and the boundaries between them have shifted as molecular phylogenetic studies have clarified the evolutionary relationships among the feather grass group. The main ornamental feather grasses grown in UK gardens under both names include: Stipa gigantea (golden oats, giant feather grass): one of the most spectacular UK ornamental grasses, with massive, golden, open panicles of pendulous, oat-like spikelets reaching 1.8 to 2.5 m in height above a compact, evergreen basal rosette of narrow leaves; widely grown and available from UK garden centres; a genuine UK garden classic. Stipa tenuissima (Mexican feather grass, Nassella tenuissima): extremely fine, hair-like, green to cream-golden foliage and flower panicles that move with every breath of wind; often sold as Nassella tenuissima; spectacular in a sunny, free-draining position. Achnatherum calamagrostis (silver spike grass): arching, silvery, narrow-leaved clump with fine, open, silver-beige flower panicles in summer; more compact than Stipa gigantea; excellent in gravel gardens and dry borders. The practical horticultural distinction between Achnatherum and Stipa is less important for a UK gardener than the shared cultural requirement of all species in this group: full sun, sharp drainage, and a relatively poor, dry growing medium. All of the above are suitable for UK gravel garden, Mediterranean garden, and dry border planting.

Q: Is achnatherum suitable for a UK clay garden?

Achnatherum calamagrostis (silver spike grass) is not well-suited to a heavy, moisture-retentive UK clay garden in its unimproved state; the combination of cold, wet, anaerobic winter conditions that characterize a heavy UK clay soil is the primary cause of root rot and plant failure in achnatherum, which is native to dry, stony, free-draining, often calcareous habitats in Mediterranean Europe and central Asia. However, UK gardeners with a clay soil can grow achnatherum successfully with some soil modification and careful placement: raise the planting area significantly by building a raised bed or mound with free-draining imported topsoil mixed with significant quantities (at least one third by volume) of horticultural grit or coarse sand, ensuring the raised area has a substantial depth (at least 30 to 40 cm) of improved, free-draining material above the underlying clay; plant achnatherum into the raised, free-draining layer and ensure the root zone drains freely even in wet UK winters. A deep gravel mulch (5 to 8 cm of pea gravel or horticultural grit) around the plant base helps keep the crown dry in wet conditions, which is particularly important for winter survival on clay soils. Alternatively, grow in a large terracotta or stone container with a deep layer of drainage crocks and a free-draining compost mixed 50:50 with horticultural grit; container cultivation allows complete control of drainage and is a reliable approach for dry-loving grasses like achnatherum in a UK clay garden setting. The container should be moved under cover (a cold frame, garage, or polytunnel) in the wettest periods of winter to prevent the root zone from sitting in cold, wet, waterlogged growing medium.

Achnatherum calamagrostis (silver spike grass) is a drought-tolerant, cool-season ornamental grass for UK gravel gardens and dry borders, grown for its silvery, narrow, inwardly rolled leaves and fine silver-beige flower panicles in summer. The leaves curl from overwatering or poor drainage, summer heat, or nutrient deficiency. Sharp drainage is the single most important cultural requirement.

Overwatering and poor drainage

Achnatherum is native to dry, stony, free-draining Mediterranean and steppe habitats; on a heavy, moisture-retentive, or waterlogged UK soil, particularly in a wet winter, the roots sit in cold, wet, anaerobic conditions that cause root rot. The leaf tips yellow, the clump collapses, and the plant paradoxically shows drought-like symptoms because the damaged root system cannot supply water to the leaf surface. This is the most common cause of achnatherum failure in UK gardens.

What to do

Grow in full sun in very freely draining, poor to moderately fertile, sandy, gravelly, or stony soil; incorporate large quantities of horticultural grit (at least one third by volume) into heavy soils before planting. A raised bed or gravel garden is ideal. Apply a deep gravel mulch (5 to 8 cm) around the crown. On clay soils, grow in a large container with free-draining compost and keep under cover in the wettest winter periods. Water sparingly; once established, water only in prolonged severe drought.

Summer heat and dormancy

A cool-season grass, achnatherum grows most actively in UK spring and early summer; in the hottest part of a warm, dry UK summer the outer leaf tips may brown and curl slightly as the plant enters partial semi-dormancy. This is a normal stress response and does not indicate a serious cultural problem in an otherwise well-grown plant; the clump recovers and resumes growth as temperatures cool in autumn. The dried flower panicles provide winter interest through the dormant period.

What to do

In a well-drained, full-sun position, summer semi-dormancy requires no intervention; the clump recovers naturally in autumn. Do not increase watering in response to summer leaf tip curl in an established plant in a free-draining position; additional moisture on an already well-established plant in summer is rarely beneficial and may increase the risk of root problems. Cut back in spring (March to April) to encourage clean new growth; the dried winter panicles can be left until late winter.

Nutrient deficiency

In extremely nutrient-poor or strongly acidic conditions, achnatherum may show leaf tip yellowing and marginal curl alongside a general pallor and reduced vigour. This is uncommon in most UK garden soils; the species is adapted to grow well in naturally poor soils and does not require or benefit from regular feeding. Excessive feeding (particularly excess nitrogen) produces lush, floppy, untypical growth that is counterproductive to the naturally compact, silvery habit of the plant.

What to do

If genuine nutrient deficiency is suspected (strongly acidic soil below pH 5.5, or a severely leached sandy soil or exhausted container medium), a light top-dressing of a balanced slow-release fertiliser in spring resolves the deficiency without promoting excessive soft growth. Avoid annual feeding as a routine; the species grows best in relatively poor conditions. In containers, refresh the growing medium every two to three years rather than feeding regularly.

Frequently asked questions

Why are my achnatherum leaves curling?

Achnatherum leaves curl most commonly because of overwatering and poor drainage (native to dry, stony, free-draining habitats; root rot in cold, wet, anaerobic winter soil is the most common UK failure cause; grow in full sun in very freely draining, gritty soil; raised bed or gravel garden; deep gravel mulch; water only in severe drought), summer heat and dormancy (cool-season grass; normal partial semi-dormancy in hot UK summers; resolves in autumn; do not increase watering; cut back in spring), or nutrient deficiency (uncommon; light balanced fertiliser in spring if suspected; avoid routine feeding; refresh container medium every two to three years). Sharp drainage is the single most important requirement.

How do I grow achnatherum calamagrostis in the UK?

Full sun in very freely draining, poor to moderately fertile, sandy, gravelly, or stony soil; sharp drainage is the key requirement. On heavy soils, incorporate significant quantities of horticultural grit (at least one third by volume) and apply a deep gravel mulch. Water sparingly; once established, only in prolonged severe drought. Cut back to the base in March to April; leave dried panicles for winter interest through autumn and winter. Divide overgrown clumps in spring. Particularly effective in gravel gardens, Mediterranean-style borders, and dry garden planting schemes.

What is the difference between achnatherum and stipa?

Closely related genera; boundaries have shifted with molecular phylogenetic studies. Key ornamental species: Stipa gigantea (golden oats) -- massive golden open panicles to 2.5 m above compact evergreen rosette; UK garden classic. Stipa tenuissima (Mexican feather grass, also sold as Nassella tenuissima) -- extremely fine, hair-like, wind-responsive foliage. Achnatherum calamagrostis (silver spike grass) -- arching, silvery, compact clump; fine silver-beige panicles in summer. All share the same cultural requirements: full sun, sharp drainage, and relatively poor, dry growing medium.

Is achnatherum suitable for a UK clay garden?

Not in unimproved clay; cold, wet, anaerobic winter conditions cause root rot. Successful on clay with modification: build a raised bed or mound with free-draining imported topsoil mixed with at least one third horticultural grit by volume (at least 30 to 40 cm depth of improved material above the clay); apply a deep gravel mulch (5 to 8 cm) around the crown to keep it dry in winter. Alternatively, grow in a large terracotta or stone container with drainage crocks and 50:50 compost and horticultural grit; move under cover (cold frame, garage, polytunnel) in the wettest winter periods to prevent root zone waterlogging.