Repotting guide

When & how to repot Spider Iris (Ferraria crispa)

Also called Spider Iris, Starfish Iris, Starfish Lily, Black Flag.

More about spider iris

About Spider Iris

Ferraria crispa · also called Spider Iris, Starfish Iris · flowering

Ferraria crispa is a winter-growing South African corm from the Iridaceae family, producing unusual, deeply fringed, star-shaped flowers in shades of brown, purple, and cream with a distinctive spidery appearance and a strong, carrion-like scent that attracts fly pollinators. In the UK it requires frost-free glasshouse or conservatory conditions, as it cannot survive freezing temperatures; outdoors it is only permanent in the mildest frost-free climates. The most critical care point is to keep the corm completely dry during its summer dormancy, as summer moisture will cause rot. Ferraria crispa belongs to the Iridaceae family; iris plants are listed as toxic to cats and dogs on the ASPCA database, and this genus should be treated accordingly.

Mature size: 20–40 cm tall, 5–10 cm spread

Watch for — Summer corm rot: The most common failure: watering or damp compost during the summer dormancy causes the corm to rot. Store dry in the pot or lift and keep in dry sand from late spring until new shoot tips appear in early autumn.

How to tell spider iris needs repotting

Repotting on a calendar is less reliable than reading the plant. For spider iris, 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 spider iris) 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 spider iris

Only every 2–4 years, when genuinely crowded. Spider Iris 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. Winter-growing, summer-dormant cormous perennial.

What size pot to step spider iris up to

Go up only one pot size — roughly 2–3 cm (about an inch) wider in diameter, no more. Spider Iris 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 spider iris 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 spider iris

Early spring, just as new growth restarts, is the ideal window for spider iris. 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 spider iris

Confirm it actually needs it. Slide spider iris 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 spider iris 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 well-drained loam or sandy compost with added grit, 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 spider iris 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 spider iris

Spider Iris wants well-drained loam or sandy compost with added grit. Plant corms 15 cm deep in a peat-free, loam-based compost mixed with 20–30% coarse grit. Excellent drainage is non-negotiable — winter-wet compost will rot the corm during its dormant period. Always use fresh mix when you repot — reusing old, broken-down soil reintroduces the compaction and poor drainage you are repotting to fix.

Repotting spider iris — frequently asked questions

How often should you repot spider iris?

Only every 2–4 years, when genuinely crowded for spider iris. Only repot spider iris 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 well-drained loam or sandy compost with added grit. The key mistake is over-potting: a too-big pot stays wet and rots the roots.

What size pot does spider iris need?

When is the best time of year to repot spider iris?

Does spider iris like to be root-bound?

Yes — spider iris 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 spider iris after repotting?

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