Harvesting Konjac Corms: When to Lift, How to Cure and Store Them

The signal to lift is the leaf giving up

Konjac tells you when it is finished. In autumn the blade yellows evenly, the leaflets go limp, and within a week or two the whole stalk collapses onto the ground and the base turns papery. That collapse is the plant withdrawing everything usable from the leaf down into the corm, and lifting before it happens means throwing away the last and largest installment of the season's growth.

Wait until the stalk has fallen and dried, then lift within a week or two. Do not leave the corm in cold wet ground indefinitely once the leaf has gone, because a dormant corm sitting in saturated soil is exactly the target soft rot looks for, and a hard frost reaching down to it will ruin it outright. In a mild, well-drained garden you can leave corms in place under a thick mulch, but lifting is more reliable and it is the only way to weigh, sort and inspect what you have.

Choose a dry day. Soil that crumbles off the corm saves you washing, and a corm that goes into storage dry keeps far better than one that was scrubbed.

Lifting without wounding the corm

Every cut, graze or fork puncture is an entry point for storage rot, and a corm you intend to replant for two more seasons has to survive two more winters in store. Work slowly and treat the corm as if it were a thin-skinned pear.

Wear gloves throughout. Bruised or cut konjac tissue releases sap loaded with calcium oxalate crystals, and the itching and burning it causes on hands and forearms lasts for hours. Never handle it with broken skin bare, and keep your hands away from your eyes while you work.

1. Start well outside the plant Push the fork in a foot (30 cm) from where the stalk stood and lever gently. The corm sits shallower and wider than you expect, and offsets sit wider still.
2. Lift the whole clump by hand Loosen all round, then get your fingers underneath and raise the main corm with its offsets attached. Do not pull on the collapsed stalk; it tears away and takes a strip of skin with it.
3. Search the hole afterwards Small offsets and runner corms break away easily and are simple to miss. Sift the loosened soil by hand; every one you find is a future plant.
4. Rub off soil, do not wash Let the corms dry an hour, then brush the soil away by hand. Washing forces water into the skin and roughly doubles storage losses.
5. Separate offsets only when they part easily Offsets that detach with light thumb pressure come away now. Any that resist stay attached until spring; forcing them leaves a raw wound on both pieces.

Curing and sorting what you have lifted

Spread the corms in a single layer somewhere dry, airy and shaded at around 68 to 77F (20 to 25C) for seven to ten days. Curing dries the skin, seals minor grazes and lets any hidden rot declare itself while you can still isolate it. Turn them once halfway through and pull out anything that has developed a soft brown patch or a sour smell.

Then weigh and sort. This is the point at which the multi-season nature of the crop becomes obvious: a corm planted at egg size will typically come out at three to five times its planting weight, so a 3 ounce (85 g) corm becomes 10 to 14 ounces (280 to 400 g) in one good season, and only in its third or fourth season does it pass the two pounds (900 g) that makes processing worthwhile.

Small offsets the size of a walnut are next year's plants. Keep the best-shaped ones with clean skins and undamaged crowns, and discard anything misshapen, scarred or scabbed, because those keep badly and grow unevenly.

Storing corms frost free until spring

Konjac corms overwinter dry and dormant. Pack the cured corms in open trays or net bags with dry sand, vermiculite or wood shavings around them, so they are not touching one another and air can move between them, and keep them at 50 to 59F (10 to 15C). An unheated spare room, a frost-free garage or a cellar all work; a domestic fridge does not, since below 45F (7C) the tissue suffers chilling injury and the corm either fails to sprout or comes up weeks late.

Check the store every three or four weeks through winter. Squeeze each corm gently: a sound one is rock firm. Any that has gone soft, wet or gray comes out at once, because one rotting corm in a closed tray takes its neighbors with it. Shriveling is a different problem and means the air is too dry, which is fixed by burying the corms more deeply in barely damp sand.

Note the lifted weight of every corm, its number of offsets, and where it grew. Kept over three or four years, that record shows exactly which positions in the garden bulk corms fastest, and it is the only way to know when a particular corm is finally ready.

What happens to a corm that is big enough

A corm of two pounds (900 g) or more is finally worth using, and this is where konjac parts company with every other root crop in the garden. You cannot boil, roast or fry it. The raw flesh is dense with sharp calcium oxalate crystals and eating it in that state burns the mouth and throat badly, which is why konjac reaches shops only as the prepared gray block, never as a fresh root.

Turning a corm into that block means peeling and grinding it, then working the pulp with a food-grade alkali such as calcium hydroxide, which neutralizes the irritant and sets the glucomannan into a firm gel that is then boiled. It is a genuine kitchen process with real safety requirements around handling the alkali, and it should be followed from a reliable, detailed recipe rather than improvised. If that is not something you want to take on, the corm still earns its place: divide the offsets, replant the mother, and keep growing it for the leaf.

Never taste raw konjac corm, and do not offer trimmings to pets or livestock. The crystals cause immediate painful irritation and swelling of the mouth and throat.