Gadenin日本語Get the appRepot in the warm half of the year
Repotting always breaks roots, and the plant can only replace them while it is warm and actively growing. In a cold room, cut root surfaces sit wet and rot rather than healing, and the corm follows them. That is why the season matters far more than convenience.
| Season | Repot? | Reason |
|---|---|---|
| Late spring | Best time | Warmth rising, new roots form within days |
| Summer | Yes, with shade afterwards | It transpires heavily, so cut roots mean wilting |
| Early autumn | Only if urgent | Little time to root before the cold |
| Late autumn to early spring | Avoid | Roots do not move and cut surfaces rot |
The one exception is an emergency. If the compost smells sour or the plant is collapsing, unpot it immediately whatever the season, cut away the rot and repot into dry fresh compost. Waiting for spring means losing the corm.
When it actually needs repotting
An alocasia does not need annual repotting, and it flowers and grows perfectly well slightly snug. Look for the signs rather than working to a calendar.
- Water runs straight through Water appears at the drainage holes within a couple of seconds and the compost barely darkens. The roots have filled the pot and the water is running down channels instead of soaking in.
- Roots out of the drainage holes Roots pushing out of the bottom, or the whole rootball lifting out as one solid block, means there is no room left.
- Offsets crowding the surface Small plantlets appearing around the base are competing with the parent for the same pot. Repotting is your chance to separate them.
- New leaves smaller than the last If light, warmth and feeding are all adequate and each new leaf is still smaller than the one before, the roots cannot supply it any more.
- The compost has collapsed Compost that has become dense and slow to drain after two or three years needs replacing whether or not the pot is full of roots.
One size up, and no more
The temptation is to move a plant into a much larger pot and save yourself a job. With an alocasia this is the single fastest way to kill one. A wide pot holds a mass of compost with no roots in it, that compost stays wet for weeks, and the corm sits in the middle of it.
| Current pot | Next pot | Typical plant |
|---|---|---|
| 4 in (10 cm) | 5 in (13 cm) | A young plant with three or four leaves |
| 5 in (13 cm) | 6 in (15 cm) | Leaves around 8 in (20 cm) long |
| 6 in (15 cm) | 8 in (21 cm) | Leaves around 12 in (30 cm) long |
| 8 in (21 cm) and up | Same pot, fresh compost | When you do not want it any larger |
A deep narrow pot suits an alocasia better than a wide shallow one. The corm is upright and the roots go down, so depth is used and width mostly just holds wet compost.
A mix that drains
The mix has to hold enough moisture for thin roots that dislike drying out completely, while still letting air to the corm. Standard bagged houseplant compost on its own is too dense and stays wet in the middle for far too long.
A workable blend is roughly half houseplant compost, a quarter perlite or coarse grit, and a quarter fine orchid bark or coir chips. The bark is what keeps the mix open as it ages. If your room is cool or you tend to water generously, push the perlite proportion higher.
- Keep the corm at its old depth Mark where the compost line sat on the stem before you lifted it and set it back at exactly that height. Burying the corm deeper to make the plant more stable is one of the commonest causes of rot.
- Do not press the compost down hard Firm it just enough that the plant stands. Compacted compost around the corm holds water against it and excludes the air the roots need.
- Leave old compost off the roots Tease away as much of the old collapsed mix as comes off easily, but do not scrub the roots clean. Anything clinging hard can stay.
- Do not use crocks instead of drainage A layer of stones in the bottom does not prevent rot. What prevents rot is an open mix and emptying the saucer.
Check the corm while it is out
Repotting is the one moment each year when you can actually see the part of the plant that decides its fate. Brush the compost off the base and look at the corm and the roots before you put anything back in a pot.
A sound corm is firm, pale to light brown, and hard when you squeeze it gently. Roots should be pale and springy. Anything brown, soft, hollow or smelling sour is rot, and it spreads. Cut it away with a clean blade until every cut surface is clean and firm, dust the cuts with cinnamon or a fungicide, and let them dry in the air for a few hours before repotting into fresh dry compost.
Wear gloves for all of this. Cut corm surfaces release sap loaded with calcium oxalate, which stings badly on broken skin and is far worse in an eye.
The first two weeks afterwards
How you treat the plant immediately after repotting decides whether it carries straight on or sulks for a month. Water it in thoroughly once, then let the top of the pot dry properly before the next watering. A freshly repotted plant has fewer roots than it did and uses less water, not more.
Keep it out of direct sun and somewhere warm and steady for a fortnight, and expect one older leaf to yellow and go. That is the plant balancing its top against the roots it has left and it is not a failure. Do not feed for about a month: fresh compost carries enough, and fertilizer on freshly cut roots does harm.
Never repot and divide on the same day as cutting the plant back hard. One operation at a time, with a few weeks between them.