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leaves of amorphopallus konjac also known as devils tongue
Devil’s tongue is the commonly used name for the konjac plant, which is related to the titan arum (Amorphophallus titanum), the world's largest (and smelliest!) flower. Other common names include voodoo lily and elephant yam. It is grown for its starchy tubers, which are used in Japanese and Chinese cuisine.

What you need to know about konjac

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.

Appearance and characteristics of devil’s tongue

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.

close up of the Asian root vegetable, konjac noodles

Uses for devil’s tongue

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.

How to grow devil’s tongue

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).

Planting konjac

  • Konjac’s spreading root system grows from the neck, rather than the base of the tuber, so make sure to plant deeply to provide some stability for the plant—at least the tuber’s width below soil level. For example, if a tuber is 15cm wide, then it should be covered by at least 15cm of soil or potting mix.
  • In Australia, konjac is best grown in a large pot, rather than in the garden. If it is being grown purely for its ornamental value, it may remain in the same pot for a number of years, or at least until it decides to flower.
  • Choose a container that is big enough to hold a tuber up to 30cm in size. Top up the potting mix every year. If re-potting is needed, do this in winter, while the tuber is dormant.
  • Place the pot where it will be sheltered from strong winds that may snap the main leaf stalk.

Climate

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.

Caring for devil’s tongue

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.

Diseases and pests

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.

If you like this then try

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.

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Health & Safety

Asbestos, lead-based paints and copper chromium arsenic (CCA) treated timber are health hazards you need to look out for when renovating older homes. These substances can easily be disturbed when renovating and exposure to them can cause a range of life-threatening diseases and conditions including cancer. For information on the dangers of asbestos, lead-based paint and CCA treated timber and tips for dealing with these materials contact your local council's Environmental Health Officer or visit our Health & Safety page.

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