Repotting an Elephant Ear: Pot Size, Mix and Handling the Corm

Why this plant needs repotting more often than most

A colocasia in a warm summer roughly doubles its root mass in a season and can fill a pot from top to bottom in ten weeks. That is not a plant you pot up every second or third year. Expect to move it up at least once a year, and in a hot climate a vigorous plant will want it twice, once at the start of the season and again at midsummer.

A pot-bound colocasia tells you clearly. The compost dries out within hours no matter how much you water, the plant wilts every warm afternoon and stands back up every evening, new leaves come out visibly smaller than the ones before them, and roots show at the drainage holes and push the surface of the compost upward. Any two of those together mean it needed a bigger pot a fortnight ago.

When to do it, and when not to

The best time is early spring, as the plant is waking and just before it starts pushing leaves. Roots move into fresh compost fastest at that point and there is a whole season ahead to use it. A midsummer repot is entirely possible too, because the plant is growing so hard that it barely notices, as long as you keep the water up for the week afterward.

Do not repot in autumn as it is slowing, and never repot a dormant, leafless corm into a large pot of fresh wet compost. A big volume of damp mix with no roots working through it is exactly the condition that rots a corm. If a dormant plant must be moved, move it into barely damp compost in the smallest pot that holds it and leave it dry until growth restarts.

If the plant is already wilting from being pot bound, water it thoroughly a few hours before you repot. Repotting a plant with dry roots multiplies the check, and colocasia roots snap easily when they are dry and brittle.

Pot size and material

Be generous. The usual advice about going up one size at a time is written for plants that rot in excess compost, and colocasia is not one of them. Go up two sizes, from a six inch (15 cm) to a ten inch (25 cm) pot for example, and you will not have to do it again mid-season. Depth matters as much as width because the corm sends roots straight down.

Plastic and glazed pots are better than terracotta here. Terracotta breathes and dries, which is the whole point for most houseplants and precisely the wrong property for this one. A heavy outer pot is still worth having for stability, so the usual arrangement is a plastic pot inside a heavy glazed one, which also doubles as the water reservoir.

The mix: heavier than you would use for anything else

Forget free-draining. A colocasia wants a mix that holds water and stays wet, and the airy bark-and-perlite blends that suit most large-leaved houseplants will leave you watering three times a day. A good starting point is roughly two parts of a standard peat-free or loam-based potting mix, one part loam or garden soil for weight and water holding, and a modest handful of grit only to stop the whole thing setting solid.

Add slow-release fertilizer at potting time at the rate on the packet, and expect to supplement it with liquid feed anyway. Because water moves through the pot constantly during the season, nutrients leach fast, and a plant this size at this speed is genuinely hungry. Pale, yellowing older leaves in midsummer on a well-watered plant are a feed problem, not a water one.

1. Mix the compost before you start Combine the compost, loam and grit in a tub and wet it lightly so it is damp throughout. Potting into dry mix means the first watering runs down the sides and leaves dry pockets against the roots.
2. Slide the plant out, do not pull the stalks Lay the pot on its side, squeeze it and ease the rootball out. The stalks are soft and juicy and will tear off the corm if you lift the plant by them, which opens a wound right where rot starts.
3. Tease the base only Loosen the bottom inch (3 cm) of circling roots with your fingers and leave the rest of the ball intact. There is no need to wash the roots or comb them out unless you are dividing.
4. Set the depth by the corm shoulder Position the plant so the top of the corm, where the stalks emerge, sits just at the surface of the new compost. Burying the shoulder deeply in a wet mix invites rot. Leaving it high and exposed dries the growing point.
5. Firm and water heavily Fill around the ball, firm it with your fingers rather than ramming it, then water until it runs freely from the base. Top up any settling with more mix rather than pressing the surface down.
6. Shade it for a few days Keep it out of hard direct sun for three or four days afterward while the roots take hold, then return it to its normal position and set the water tray back up.

What to inspect while it is out of the pot

Repotting is the only regular chance you get to look at the corm, and it is worth two minutes. Feel it. A healthy corm is hard, like an unripe potato, with clean pale roots coming from the lower half and dry papery scale where old stalks have fallen away. Any part that gives under thumb pressure, or is brown and wet inside, is rot and must be cut back to clean firm tissue with a sharp blade.

You will often find small offsets around the base, and sometimes thin runners with young plantlets on the end. Leave them attached if you want a fuller clump, or take this opportunity to remove them as new plants. Either way, note down what you found. Recording the corm's size and firmness at each repot, and the number of offsets, tells you within two seasons whether your regime is actually building the plant up or slowly wearing it down.

Colocasia sap irritates skin and is genuinely unpleasant in the eyes or mouth, because of needle-shaped crystals in the tissue. Wear gloves when cutting into a corm, keep your hands away from your face, and wash tools and hands afterward.