Silver spike grass
Achnatherum calamagrostis. Family Poaceae. Ornamental grass.
Also called Silver spear grass, Stipa calamagrostis.
Silver spike grass is a clump-forming perennial grass with arching green leaves and loose, silvery flower plumes that turn buff as they mature. Grow it for movement and long-lasting seed heads in sunny borders and gravel gardens, especially where the soil drains freely.
At a glance
- Sun
- Full sun; flowering and stem strength are best in an open position.
- Water
- Water regularly while establishing, then mainly during prolonged dry spells. Established clumps tolerate drought.
- Soil
- Well-drained sandy, gravelly or loamy soil, including chalky soil; tolerates poor fertility. Avoid ground that stays wet in winter.
- Height
- 2-4 ft (0.6-1.2 m), including flower stems
- Spread
- 1.5-3 ft (0.45-0.9 m)
- Flowers
- Summer into early autumn; loose, silvery-green plumes mature to straw-coloured seed heads.
- Foliage
- Narrow, arching, medium-green leaves form a fountain-shaped mound and turn straw-coloured in autumn; some foliage may persist in mild winters.
- Growth
- Moderate
- Care needed
- Low
The year at a glance
- Deer resistant
- Drought tolerant
- Wildlife
| J | F | M | A | M | J | J | A | S | O | N | D | |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Flowers | ||||||||||||
| Fruit | ||||||||||||
| Plant |
Northern hemisphere months. In Australia, shift them by six.
- Difficulty
- Easy
- Toughness
- Very tough
Will it suit your garden?
Pick your zone, country, light and whether you have pets.
Full sun is 6 or more hours of direct sun a day, part shade 3 to 6 hours, shade under 3.
How it grows
Planting day
1.1 ft (0.35 m)
After 1 year
1.9 ft (0.57 m)
After 2 years
2.6 ft (0.79 m)
After 3 years
3 ft (0.9 m)
full size
About 3 years to full size.
From a typical nursery size, at an average rate for this kind of plant. A rule of thumb, not a forecast.
Good to know
- Cats and dogs
- Safe
- Growth rate
- Moderate
Care
Jobs through the year
- WaterApr–May, Sep, every 28 days
- Water established clumps only during prolonged drought; water first-year plants more often.
- WaterJun–Aug, every 21 days
- Water established clumps deeply only if prolonged dry weather dries the root zone; avoid light sprinkling.
- PruneFeb–Mar
- Cut old stems and foliage to 10-15 cm before new shoots lengthen.
- DivideMar–Apr
- Divide only if the centre becomes bare or the clump outgrows its space.
- MulchApr
- If mulch is needed, use a light gravel layer and leave the crown uncovered.
- DeadheadAug–Sep
- Optionally remove ripening seed heads to limit seedlings; otherwise retain them for winter texture.
- ProtectJan–Feb, Nov–Dec
- Keep crowns free of wet debris and ensure winter rain drains away.
- Plant in spring in colder areas, keeping the crown level with the soil surface.
- Water deeply during the first growing season, allowing the surface soil to dry between waterings.
- Avoid routine feeding; rich soil can produce floppy growth.
- Use a light gravel mulch rather than piling moisture-retentive mulch against the crown.
- Divide in spring if the centre becomes bare or the clump outgrows its space.
Pruning. Leave the dry stems standing through winter if they remain upright. In late winter or early spring, cut old foliage and stems to 4-6 in (10-15 cm) above ground before new growth lengthens.
Through the year
- Late winter
- Cut back old growth before fresh shoots extend.
- Spring
- Plant new clumps or divide established plants as growth resumes.
- Late spring
- Remove unwanted seedlings while they are small.
- Summer
- Water newly planted clumps during dry weather; avoid frequent light sprinkling.
- Autumn
- Leave sound seed heads for winter structure, or remove them before seed falls if seedlings are unwanted.
- Winter
- Check that water is not collecting around the crown and improve drainage where necessary.
In the garden
- Repeat clumps through a sunny perennial border to link flowering plants with fine texture.
- Combine with lavender, catmint and yarrow in a gravel garden.
- Place beside a path where the arching plumes can be seen against darker planting, allowing room for their spread.
- Plant in loose groups with late-flowering perennials for an informal meadow-like effect.
Garden styles it suits: New perennial (prairie), Dry gravel garden, Mediterranean, Low-maintenance, Modern rustic and Winter interest.
Plant it with: Lavender, Catmint, Yarrow, Russian sage, Purple coneflower and Autumn stonecrop.
Wildlife: Dense clumps provide shelter for small invertebrates. The flowers are wind-pollinated rather than a significant nectar source for bees.
Problems to watch for
- Crown rot in wet ground: improve drainage or move the plant to a raised, freely draining site.
- Flopping stems: provide full sun, stop feeding and avoid excessive watering.
- Unwanted seedlings in gravel or open soil: remove young plants promptly or cut flower heads before seed ripens.
- Bare centre in an ageing clump: lift and divide in spring, replanting vigorous outer sections.
Safety
Not known to be toxic to people, cats, dogs or horses. Dry, pointed grass material can cause mechanical irritation if chewed or caught in eyes or ears; this is not poisoning.
Easily confused with
Feather reed grass (Calamagrostis × acutiflora)
Usually has a more upright habit and narrower, denser flower heads; silver spike grass has looser, spreading plumes and more arching stems.
Mexican feather grass (Nassella tenuissima)
Has much finer, hair-like foliage and silky flower heads that merge into a soft haze; silver spike grass has broader leaves and more distinct, open plumes.
The name
The common name refers to the silvery appearance of its flowering plumes.
Propagation. Divide established clumps in spring. Sow seed in spring in a freely draining seed compost; seedlings may vary, so divide named cultivars to preserve their characteristics.
Checked against open data
- Accepted botanical name
- Achnatherum calamagrostis
- Native range
- Albania, Altay, Austria, Bulgaria, Buryatiya, China North-Central, China South-Central, Chita, France, Germany, Greece and Inner Mongolia.
- Measured
- Average height 12 cm.
Sources: GBIF (CC BY 4.0) and Trefle (CC BY 4.0). The care and design notes are LandscapeGPT's own.
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