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Name: konjac, devil’s tongue (Amorphophallus konjac).
Height: tuber to 30cm in diameter; single leaf to 1.3m across and 60–75cm tall.
Foliage: single leaf on tall stem divides into three, each part with numerous leaflets.
Climate: sub-tropical to tropical; indoor plant in temperate or cooler areas.
Soil: rich loam that drains well; quality premium potting mix in pots.
Position: full sun or good ambient light; protection from winds (brittle stem).
Flowering: dark red, lily flower with pungent odour; rarely flowers.
Feeding: use a long-term controlled-release fertiliser.
Watering: keep consistently moist from the time the leaf shoot appears until it dies down in autumn.
Native to warm sub-tropical and tropical areas of eastern Asia, konjac grows from a large tuber up to 25cm or more across. In spring, it grows a single sturdy leaf stalk that opens out into three sections, creating an umbrella effect spanning more than a metre.
Like the titan arum, konjac flowers very rarely, but when it does, the flower will appear in late winter, before the leaf. The flower is very large, lily-like, deep maroon to purple-brown, and smells like rotting flesh to attract pollinating insects. If the smell really offends you, cut it off!
The leaf will turn brown in autumn and then die. Cut it off if it doesn’t fall off. The tuber will shrink during the growing season, then a new one will grow to replace it.
Devil’s tongue is grown in many Asian countries for its tubers, from which a flour and a jelly or setting agent are made. Konjac flour is often substituted for gelatine in vegan diets.
The flour, konnyaku in Japanese, is used in noodles known as shirataki and dishes such as oden. It has very little taste and is valued more for its texture and thickening than its flavour. It may also be made into yam cakes. Konjac is also used to make bath sponges that are very soft on the skin.
Devil’s tongue is grown from tubers and offsets or “pups” that develop around the outside of the parent tuber. You might find these when re-potting the main tuber. Detach and pot them up individually—friends and neighbours might enjoy growing them!
Young tubers may be available to buy in winter while they are dormant (without leaves).
Coming from the tropics and sub-tropics, konjac will only grow when it is given sufficient warmth. It will grow outdoors in the northern states, but in the south of the country a heated glasshouse or conservatory is usually required. A sunny, north-facing courtyard may trap enough warmth in cooler areas. While the plant prefers dappled sun to light shade, it can tolerate full sun.
Watering
Konjac needs to be kept consistently moist from the time the leaf or flower shoot starts to emerge until the leaf dies off in autumn. Don’t let the potting mix dry out at all during this period. Some growers have great success by standing pots in shallow ponds or bowls of water so the mix is always moist.
Stop watering as soon as the leaf starts to brown in autumn and, if possible, lie the pot on its side to keep it completely dry.
Fertilising
Devil’s tongue is a heavy feeder, so it’s important to fertilise it well through the growing period to keep it healthy and vigorous.
Always use a premium potting mix that includes a controlled-release fertiliser, and top this up in late winter and late summer with further applications of a six-month controlled-release plant food.
Fortnightly boosts of liquid or water-soluble fertiliser from early spring until the leaf starts to die off in autumn will keep it growing strongly.
Konjac may be susceptible to some fungal diseases, including a form of cinnamon fungus (Phytophthora). Use a general-purpose fungicide to control it.
If insect pests are problematic, apply a pyrethrum-based insecticide as directed on the label.
Garlic: a popular Asian ingredient readily grown from bulbs (cloves) planted in spring or autumn.
Moringa: a tree grown for its horseradish flavour and used in soups and curries; high in antioxidants.
Ginger: a sub-tropical and tropical plant grown for its flavoursome rhizomes, used in many cuisines.
Check out our huge range of plants now and get your garden growing!
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