Repotting guide

When & how to repot Snake's Head Iris (Hermodactylus tuberosus)

Also called Snake's head iris, Widow iris, Velvet flower-de-luce, Black iris.

More about snake's head iris

About Snake's Head Iris

Hermodactylus tuberosus · also called Snake's head iris, Widow iris · flowering

Hermodactylus tuberosus (also treated by some authorities as Iris tuberosa) is a tuberous perennial native to the eastern Mediterranean — from southern France and Italy through the Balkans to Turkey and Israel — where it grows in dry, rocky hillsides and olive groves. It produces distinctive late-winter to early-spring flowers with velvety, deep purple-black falls and pale yellow-green standards, lending it a striking, near-monochrome appearance. The most important care fact is to give it a hot, dry summer dormancy in alkaline, sharply drained soil; poorly drained or acidic conditions quickly cause the tubers to rot. All parts of the plant are harmful if eaten and are toxic to pets.

Mature size: 20–40 cm tall in flower; tubers slowly multiply to form small clumps 10–15 cm across.

How to tell snake's head iris needs repotting

Repotting on a calendar is less reliable than reading the plant. For snake's head iris, watch for these signs:

Flowering has tailed off year on year and the clump has become congested and overcrowded.
Lots of leaf and few flowers — a classic sign that snake's head iris bulbs or tubers need lifting and dividing.
Bulbs visibly bursting the pot or pushing each other to the surface.
It is the natural dormancy window (foliage yellowed and died back) — the only safe time to lift and split.

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 snake's head iris

Lift and divide every 3–4 years once clumps congest. Rather than a true repot, snake's head iris is lifted and divided once the clump congests and flowering drops off. Tuberous perennial with narrow, four-angled, rush-like leaves forming small fans; solitary flowers appear in late winter to early spring on stems 20–40 cm tall..

What size pot to step snake's head iris up to

Pot size matters less than depth and spacing here. When you replant snake's head iris, set the bulbs or tubers at the correct depth (a rough guide: two to three times their own height of soil over the top) and space them so they are not touching. A wide, shallow pot suits a clump better than a tall narrow one.

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 snake's head iris

The only safe window is dormancy: wait until the foliage has yellowed and died back naturally, lift and divide then, and replant before or at the start of the next growing season. Disturbing snake's head iris in full growth or flower sets it back badly.

Step-by-step: repotting snake's head iris

Wait for dormancy. Let snake's head iris foliage yellow and die back completely. Lifting while it is in growth wastes the energy it is storing for next year.
Lift carefully. Loosen the soil well away from the bulbs/tubers with a fork and ease the whole clump out without spearing them.
Separate the offsets. Gently pull the clump apart into individual bulbs or tubers. Keep only firm, healthy, blemish-free ones.
Replant at the right depth. Reset them in fresh alkaline, sharply drained gritty loam or chalk at the correct depth and spacing — not touching — so each has room to bulk up.
Water in and rest. Water once to settle them, then keep on the dry side until growth resumes. Do not feed until leaves are actively growing.

Aftercare

After replanting snake's head iris, keep the soil barely moist — not wet — until shoots appear; bulbs and tubers rot in cold, saturated soil. Once leaves are growing strongly, resume normal watering. Hold off feeding until the plant is in active growth again.

The right soil mix for snake's head iris

Snake's Head Iris wants alkaline, sharply drained gritty loam or chalk. Prefers alkaline to neutral pH on chalk, limestone, or gritty loam; add lime to acid soils and improve drainage with coarse grit before planting the finger-like tubers. Always use fresh mix when you repot — reusing old, broken-down soil reintroduces the compaction and poor drainage you are repotting to fix.

Repotting snake's head iris — frequently asked questions

How often should you repot snake's head iris?

Lift and divide every 3–4 years once clumps congest for snake's head iris. Snake's Head Iris is lifted and divided, not "repotted". Every 3–4 years, once the foliage has died back and it is dormant, lift the clump, separate the offsets, and replant at the correct depth in alkaline, sharply drained gritty loam or chalk. Crowding, not pot size, is what reduces flowering over time.

What size pot does snake's head iris need?

When is the best time of year to repot snake's head iris?

Do you "repot" snake's head iris, or lift and divide it?

You lift and divide it. Snake's Head Iris grows from bulbs or tubers, so instead of repotting you wait for dormancy, lift the congested clump, separate the healthy offsets, and replant them at the right depth and spacing. Doing this every 3–4 years restores flowering.

Should you fertilise snake's head iris after repotting?

Hold off feeding snake's head iris until it is in active growth again. Fresh soil already carries enough nutrients to get it re-established, and feeding disturbed roots too soon does more harm than good.