How to Propagate Konjac: Offsets, Runner Corms, Division and Seed

The plant does most of the work for you

You very rarely need to buy konjac twice. Through each growing season the corm produces small offsets around its shoulder and base, and many plants also push out one or more slender underground runners that swell into a new corm a hand's width from the parent. Lift a healthy third-season plant and you will typically find between two and six new corms attached or lying loose in the soil around it.

Those offsets are genetically identical to the parent, they carry no seed-borne variation, and they establish faster than anything you could raise from seed. For a plant that already takes three to four seasons to reach usable size, that head start matters enormously. Treat the autumn lift as your propagation event and you will never be short of plants.

Do not go looking for offsets during the growing season. Digging around a growing corm tears surface roots and opens the wounds that soft rot enters, and it gains you nothing you would not see clearly in autumn.

Separating offsets at the autumn lift

Work in gloves. Broken konjac tissue releases sap loaded with calcium oxalate crystals, and separating offsets means making fresh wounds on both parent and offspring, so bare hands will be itching and burning within minutes.

The rule for separating is simple: if an offset comes away under light thumb pressure, it is ready and the break is clean. If it resists, leave it attached over winter and it will part easily in spring. Twisting or cutting a firmly attached offset leaves two raw surfaces going into storage, which is exactly how a tray of corms turns to rot by midwinter.

1. Lift the whole clump intact Fork in a foot (30 cm) out and raise parent and offsets together. Sift the loosened soil afterwards by hand for detached runner corms, which are easy to miss and easy to slice.
2. Dry before separating Let the clump sit somewhere airy and shaded for a day so soil crumbles away and the skins firm up. Separating wet, soil-covered corms tears skin.
3. Take only what parts easily Thumb pressure at the join. Clean break means take it; resistance means leave it on the parent until spring.
4. Cure the wounds for a week Spread offsets in a single layer at 68 to 77F (20 to 25C) for seven to ten days so every break seals over. Discard anything that softens in that time.
5. Grade and label by size Store walnut-sized and larger offsets separately, since planting depth and spacing next spring both follow corm size. Note which parent each came from.

Growing the offsets on

Store offsets exactly as you store the parent corm: dry, packed apart in sand or shavings, at 50 to 59F (10 to 15C) and never below 45F (7C). Small offsets have a higher surface-to-volume ratio and shrivel more readily than big corms, so bury them a little deeper in barely damp sand and check them monthly.

In spring, plant them once the soil at four inches (10 cm) is steadily above 59F (15C), covering each to two to three times its own height and spacing them 12 to 18 inches (30 to 45 cm) apart. A walnut-sized offset only needs about two inches (5 cm) of cover. In their first season they make a small, single leaf on a short stalk, sometimes only a foot (30 cm) tall, and that is entirely normal.

Very small offsets, under an inch (2.5 cm) across, are better grown in a nursery bed or in deep pots for their first year, where you can control moisture precisely and lift them without hunting through open ground. Move them into the main bed for their second season.

Expect roughly three to five times the planting weight per good season. On that basis a walnut-sized offset is four seasons away from a usable corm, and a hen's-egg offset is three.

Dividing a large corm, and the two long routes

Cutting a large corm into pieces works, but it is a poorer option than it looks. Each piece must carry a live bud from the crown, and konjac corms usually have only one strong growth point plus a few weak secondary ones, so you cannot get many viable pieces out of one corm. The cut surfaces also rot readily. If you do it, cut in spring shortly before planting, use a clean sharp knife, dust the surfaces with sulfur or wood ash, and cure the pieces for a full week somewhere warm and dry before they go in the ground.

Some plants produce small bulbils at the junctions of the leaflets in late summer. Where they form, they detach when ripe and can be potted into free-draining compost and overwintered frost free, but they are tiny and add at least a season compared with an offset. Treat them as a curiosity rather than a plan.

Seed is the longest route of all. A mature corm may throw a single enormous flower in spring instead of a leaf, a dark maroon hood around a tall central spike that reeks of rotting meat for a day or two to attract carrion flies. If pollinated it sets a spike of bright red berries by autumn, and the seed inside will germinate in warm, moist compost. But the flower costs the corm most of a season of growth, and seedlings need five years or more to reach usable size, so this is worth doing for interest and nothing else.

Wear gloves for all of this, including handling seed berries. The sap and the fruit both contain the same irritant crystals as the corm.

Keeping track across the years

Because konjac runs on a three to four year cycle, memory is not good enough. Give each corm or family of corms a number, and each autumn record its lifted weight, how many offsets it produced, the size of those offsets, and where in the garden it grew that season.

Two or three years of that record answers questions no general guide can. It shows which parent corms are consistently productive and worth keeping as mothers, which corner of the garden bulks corms fastest, and which storage conditions came through winter without shriveling. It also shows you, plainly, whether an offset is on schedule to be usable next year or the year after, so you can decide what to plant where in spring instead of guessing.