Gadenin日本語Get the appWhat you are actually planting, and what it is not
Konjac is grown for its underground corm, and that corm is not a vegetable you can cook and eat. It is packed with needle-shaped calcium oxalate crystals that burn the mouth and throat, so the raw corm is inedible and genuinely unsafe. It only becomes food after it is ground, mixed with an alkali such as slaked lime and set into the firm gray block sold in shops. That is the reason you buy konjac as a prepared block and never as a fresh root on a market stall.
Plant it knowing that. The gardening reward here is a striking foliage plant that happens to bank a usable corm underneath, not a quick crop for the kitchen. A corm planted at seed size will need three to four growing seasons of being lifted, stored frost free and replanted before it reaches the two to four pounds (1 to 2 kg) that processing is worth doing on. Set your expectations for that timescale on the day you plant.
Wear gloves whenever you handle cut or damaged corms. The sap carries the same crystals as the flesh and will leave hands itching and stinging for hours.
Wait for the soil, not for a date
Konjac breaks dormancy on warmth. A corm sitting in cold, wet spring ground does not sprout, it rots, and soft rot in the first fortnight is the single commonest way beginners lose a corm. The trigger to plant is soil at four inches (10 cm) deep holding steadily above 59F (15C), which in most temperate gardens falls two to three weeks after the last frost. Air temperature is a poor guide; push a soil thermometer in at mid morning and read it on three consecutive days.
Give the plant a run of at least 150 frost-free days. The leaf will not survive even a light frost, and a season cut short in autumn means the corm goes into storage having banked less than it could. If your season is shorter than that, start corms in deep pots under cover four weeks early and move them out with the shoot already showing.
Corms you lifted last autumn should feel firm, dry-skinned and heavy for their size, with a visible flat bud or a pale pointed shoot at the crown. Anything soft, weeping or sour-smelling goes on the bonfire rather than into the bed, because bacterial rot travels with planting material.
| Soil at 4 in (10 cm) | What a planted corm does | Verdict |
|---|---|---|
| Below 54F (12C) | Sits inert, skin softens, rot organisms move in | Do not plant |
| 54 to 59F (12 to 15C) | Slow, uneven emergence over five to six weeks | Too early, wait |
| 59 to 68F (15 to 20C) | Shoot breaks the surface in three to four weeks | The right window |
| Above 68F (20C) | Emergence in two to three weeks | Fine, but less season left ahead |
Choosing a site: shade, shelter and moisture
Konjac grows on woodland edges and shaded slopes, and it wants those conditions reproduced. Bright dappled shade, or morning sun with shade from mid afternoon, gives the best leaf. Full open sun bleaches and scorches the blade by midsummer, and a scorched leaf stops feeding the corm months before it should.
Shelter matters even more than light. The whole season above ground rests on one upright stalk, and a gust that snaps it ends growth for the year with no second flush to follow. Choose ground with a hedge, wall or shrub group on the windward side, and avoid open corners and gaps between buildings where wind funnels.
Soil should be deep, humus-rich, moisture-retentive and yet free-draining, at a pH between 6.0 and 7.0. Fork a bucket of well-rotted compost or leaf mold into every square yard (square meter) before planting. Heavy clay that lies wet all winter is the one soil to refuse outright, though it becomes workable if you plant into a raised ridge and lift the corms every autumn anyway.
| Site | Suitability | What to do about it |
|---|---|---|
| Dappled shade under high, open tree canopy | Excellent | Plant as is, add leaf mold yearly |
| Morning sun, afternoon shade beside a wall | Excellent | Ideal shelter as well as light |
| Open, full sun, all day | Poor | Rig 30 to 40 percent shade cloth from midsummer |
| Windy open ground | Poor | Move it, or build a temporary windbreak screen |
| Dense dry shade under conifers | Poor | Too dark and too dry, choose elsewhere |
Depth, spacing and putting the corm in
Depth follows corm size rather than a fixed number. Cover a corm with soil to roughly two to three times its own height, which works out at three to four inches (8 to 10 cm) for a corm the size of an egg and five to six inches (13 to 15 cm) for one the size of a fist. Planted too shallow, the corm heaves upward as it swells and the exposed shoulder scalds; planted much deeper than that, emergence is slow and the shoot exhausts itself reaching the light.
Spacing follows the leaf, and the leaf is far larger than a new grower expects. A third-season plant throws a blade two to three feet (60 to 90 cm) across on a stalk of similar height, so allow 18 inches (45 cm) between small corms and 24 to 30 inches (60 to 75 cm) between large ones. Crowded plants shade one another, hold damp air around the stalk bases and invite leaf spot.
- Find the bud and plant that side up The crown carries a flattened growth point or a short pink shoot; the base is scarred where roots and offsets grew. Set the bud upward. A corm planted upside down will usually still grow, but it loses two to three weeks turning itself around.
- Dig wide, not just deep Loosen a hole a foot (30 cm) across so the new season roots, which come from the top of the corm rather than the bottom, spread sideways into soft ground.
- Set the corm on firmed soil Press the base of the hole flat so the corm sits level and no air pocket remains beneath it. A corm resting over a void sits in trapped water and rots at the base.
- Backfill and water once, then leave it Fill in, water enough to settle the soil, and then keep the bed only just moist until the shoot appears. Repeated watering of bare ground with nothing growing in it is the classic cause of early rot.
- Mark the position clearly Three to four weeks of bare soil is long enough to forget where the corm is and drive a fork through it. Push in a labeled cane the moment you backfill.
The first weeks above ground, and what to write down
What comes up is not a stem and it is worth recognizing it as such. A single thick stalk pushes through, mottled and blotched in green, cream and dull purple in a pattern that looks convincingly like snakeskin, and at the top it unfurls into one huge horizontally held blade divided into many leaflets. The whole structure is one leaf: the mottled column is its petiole. There is no second leaf coming this year, so protecting this one is the entire job of the season.
Slugs find the emerging tip irresistible, and a shoot grazed through at ground level in its first week is a season lost. Ring each planting position at emergence and check it every evening for the first fortnight.
Keep a note of the planting date, the exact position of each corm, the weight of each corm as it went in, and the date the shoot broke the surface. Weighing corms in and out each year is the only way to see whether a site is actually feeding them, and knowing that a corm went from 5 ounces (140 g) to 14 ounces (400 g) in one season tells you far more than any general advice can. Record the emergence date too; a corm that comes up markedly later than its neighbors is usually one that was stored too cold.