Repotting guide

When & how to repot Purple Moor Grass (Molinia caerulea 'Moorhexe')

Also called purple moor grass, moor grass, Moorhexe moor grass.

More about purple moor grass

About Purple Moor Grass

Molinia caerulea 'Moorhexe' · also called purple moor grass, moor grass · flowering

Molinia caerulea 'Moorhexe' is a compact, upright deciduous grass prized for its narrow, tufted habit and rich purple-tinged flower spikes in late summer. It thrives in moist, acidic soils and is exceptionally well suited to rain gardens, bog edges, and naturalistic plantings. Autumn colour turns golden-yellow before the foliage collapses cleanly to the ground.

Mature size: 60–75 cm tall (flower spikes to ~75 cm), 40–50 cm wide

Watch for — Slow establishment: Molinia grasses are slow to establish and may appear weak in their first season. Do not over-feed or over-water in response — patience is required. Growth accelerates in years two and three once the root system is settled.

How to tell purple moor grass needs repotting

Repotting on a calendar is less reliable than reading the plant. For purple moor 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 purple moor 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 purple moor grass

Only every 2–4 years, when genuinely crowded. Purple Moor 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, upright clump-forming deciduous grass with narrow arching leaves and erect purple flower spikes.

What size pot to step purple moor grass up to

Go up only one pot size — roughly 2–3 cm (about an inch) wider in diameter, no more. Purple Moor 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 purple moor 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 purple moor grass

Early spring, just as new growth restarts, is the ideal window for purple moor 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 purple moor grass

Confirm it actually needs it. Slide purple moor 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 purple moor 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 moist, acidic, humus-rich loam or sandy loam; ph 4.5–6.5, 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 purple moor 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 purple moor grass

Purple Moor Grass wants moist, acidic, humus-rich loam or sandy loam; ph 4.5–6.5. Thrives in acidic to neutral, moisture-retentive soils. Performs poorly on alkaline or chalky soils. Amend heavy clay with grit to improve drainage while retaining moisture. Avoid highly fertile or nitrogen-rich soils, which promote lush but floppy growth over the ornamental upright form. Always use fresh mix when you repot — reusing old, broken-down soil reintroduces the compaction and poor drainage you are repotting to fix.

Repotting purple moor grass — frequently asked questions

How often should you repot purple moor grass?

Only every 2–4 years, when genuinely crowded for purple moor grass. Only repot purple moor 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 moist, acidic, humus-rich loam or sandy loam; ph 4.5–6.5. The key mistake is over-potting: a too-big pot stays wet and rots the roots.

What size pot does purple moor grass need?

When is the best time of year to repot purple moor grass?

Does purple moor grass like to be root-bound?

Yes — purple moor 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 purple moor grass after repotting?

Not immediately. Wait about 4 weeks after repotting purple moor 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.