Repotting guide

When & how to repot silky thread grass (Nasella tenuissima)

Also called silky thread grass, Mexican feather grass, fine-leaved nassella, needle grass.

More about silky thread grass

About silky thread grass

Nasella tenuissima · also called silky thread grass, Mexican feather grass · flowering

Silky thread grass is a fine-textured, hair-like ornamental grass producing soft, billowing mounds of thread-thin leaves and feathery seed heads that shimmer in the slightest breeze. Extremely drought-tolerant and sun-loving, it thrives in poor, well-drained soil with minimal care. Self-seeds prolifically — treat as short-lived perennial in cooler zones.

Mature size: 30–45 cm tall; clump spread 30–45 cm

Watch for — Crown rot in wet or humid conditions: Poorly drained or consistently moist soil around the crown causes rapid dieback, especially in winter. Plant in raised beds or sharply drained gravel soil; avoid watering overhead. In wetter climates, treat as an annual or raise plants in pots with excellent drainage.

How to tell silky thread grass needs repotting

Repotting on a calendar is less reliable than reading the plant. For silky thread grass, watch for these signs:

Roots spiralling thickly out of the drainage holes or pushing the whole plant up out of the pot.
The pot is so packed that water runs straight through in seconds and barely wets the soil.
It has split a plastic pot, or the rootball is a solid mass with almost no soil left when you slide it out.
Growth and (for silky thread grass) flowering have clearly stalled despite good light and feeding — but remember this plant likes being snug, so a little crowding alone is not a reason to repot.

For the underlying biology of a pot-bound root system and why it stalls a plant, see our guide to spotting and fixing a root-bound plant.

How often to repot silky thread grass

Only every 2–4 years, when genuinely crowded. silky thread grass is one of the plants that genuinely prefers a snug pot — it grows and flowers better with its roots a little restricted, so resist the urge to repot it on schedule. Compact, clump-forming, warm-season perennial grass forming a dense, arching fountain of extremely fine, thread-like green to golden-green leaves. Produces masses of feathery, silvery-green to straw-coloured panicles from late spring through autumn that sway in the slightest breeze..

What size pot to step silky thread grass up to

Go up only one pot size — roughly 2–3 cm (about an inch) wider in diameter, no more. silky thread grass positively prefers a snug pot: it flowers and grows better when the roots are a little restricted. The single biggest repotting mistake here is over-potting — dropping silky thread grass into a pot two or three sizes up. All that surplus soil holds water the small root system cannot use, stays cold and wet, and rots the roots within weeks. When in doubt, choose the smaller pot.

Not sure of the exact diameter? Our pot size calculator takes the current pot and root spread and tells you the right next size — it deliberately recommends a single step up, never a big jump.

The best time of year to repot silky thread grass

Early spring, just as new growth restarts, is the ideal window for silky thread grass. The plant is moving into its strongest growth phase and re-roots into fresh soil quickly. Avoid repotting in winter dormancy or, for flowering plants, while it is in bud or bloom — recovery is slowest then and you risk dropping the flowers.

Step-by-step: repotting silky thread grass

Confirm it actually needs it. Slide silky thread grass out and check the roots. Only continue if it is genuinely packed — this plant prefers a snug pot, so if there is still soil and room, put it straight back.
Pick a pot only one size up. Choose a pot just 2–3 cm wider with good drainage. Resist anything bigger; over-potting is the main killer here.
Ease it out gently. Water lightly the day before, then tip silky thread grass out, supporting the base. Tease the outer roots free only enough to stop them circling.
Repot at the same depth. Add a layer of fresh poor to moderately fertile, sharply well-drained loam, sandy loam, or gravelly soil; ph 6.0–8.0, set the plant so the soil line sits exactly where it did before, and backfill around the sides, firming lightly.
Settle it in. Water once to settle the soil, then let it sit. Hold off on more water until the top of the soil dries — fresh soil around a small root system stays wet for a while.

Aftercare

Because the new soil holds more water than the old crammed rootball did, ease right back on watering — let the top of the soil dry before you water silky thread grass again, or you will rot the roots in the very pot you just moved it to. Keep it out of harsh direct sun for a fortnight. Do not fertilise for about 4 weeks — fresh mix already carries nutrients and feeding freshly disturbed roots scorches them.

The right soil mix for silky thread grass

silky thread grass wants poor to moderately fertile, sharply well-drained loam, sandy loam, or gravelly soil; ph 6.0–8.0. Thrives in poor, low-fertility soils where richer competitors are suppressed. Excellent for gravel gardens, dry slopes, and rocky sites. Does not tolerate heavy clay or waterlogged soil at any season. Sandy or gravelly soils with no supplemental fertility suit it perfectly. Always use fresh mix when you repot — reusing old, broken-down soil reintroduces the compaction and poor drainage you are repotting to fix.

Repotting silky thread grass — frequently asked questions

How often should you repot silky thread grass?

Only every 2–4 years, when genuinely crowded for silky thread grass. Only repot silky thread grass every 2–4 years, and only when it is genuinely root-bound — it flowers and grows best slightly crowded. Step up just one pot size in spring using poor to moderately fertile, sharply well-drained loam, sandy loam, or gravelly soil; ph 6.0–8.0. The key mistake is over-potting: a too-big pot stays wet and rots the roots.

What size pot does silky thread grass need?

When is the best time of year to repot silky thread grass?

Does silky thread grass like to be root-bound?

Yes — silky thread grass genuinely flowers and grows best when slightly pot-bound, so do not rush to repot it. The mistake to avoid is over-potting into a much larger pot: the excess soil stays wet, the roots cannot use it, and the plant rots. Only repot every few years and only one snug size up.

Should you fertilise silky thread grass after repotting?

Not immediately. Wait about 4 weeks after repotting silky thread grass. Fresh mix already contains nutrients, and feeding freshly cut or disturbed roots burns them. Resume your normal feeding routine once you see new growth.