Fertilising guide

How to fertilise Konjac (Amorphophallus konjac)— schedule & NPK

Also called konjac, devil's tongue, elephant yam, konnyaku.

More about konjac

About Konjac

Amorphophallus konjac · also called konjac, devil's tongue · edible

Amorphophallus konjac is a tropical aroid grown for its large starchy corm, the source of glucomannan used to make konnyaku and shirataki noodles. It produces a single tall, dramatically dissected leaf in summer and a dark, foul-smelling spathe in spring. The corm is edible only after thorough processing; raw tissue is acrid.

Growth habit: Tuberous perennial aroid producing one large, deeply divided umbrella-like leaf per season from an underground corm, with a malodorous inflorescence before leaf in spring.

Watch for — Leaf scorch: Direct midday sun burns the single leaf. Site in dappled shade or bright indirect light.

What fertiliser konjac actually wants — and why

Konjac feeds in two distinct phases — balanced to build the plant, then high-potassium the moment flowering starts to set and fill a heavy crop.

For the language behind the three numbers on the bottle — what nitrogen, phosphorus and potassium each do — see the NPK ratio explained entry. The short version for konjac: match the feed to the job the plant is doing right now, not to a generic “plant food” on the shelf.

How often to feed konjac, and which months

Feeding only earns its keep while the plant is in active growth and can use the nutrients — pour feed into a dormant or low-light plant and it simply builds up as root-burning salt. For konjac:

Feed every 2-3 weeks through the growing season with a balanced liquid fertiliser to build a large corm; a high-potash feed late in the season helps corm development. Stop feeding as the leaf dies down for dormancy. So: a balanced feed or compost at planting, then a high-potash liquid every 1-2 weeks from first flower through harvest across the main season (spring through early autumn).

The dormant-season rule matters more than the exact interval: skip feeding entirely when konjac is resting. For the wider context on indoor feeding rhythms across the seasons, the houseplant fertiliser schedule walks through the year month by month.

What strength to mix for konjac

Follow the crop-feed label rate for konjac — these are calibrated for hungry vegetables. Consistency through fruiting matters more than strength; erratic feeding causes problems like blossom-end rot.

Feeding always goes onto already-damp soil, never dry roots — water konjac first if the soil is dry, then apply the diluted feed. The companion question is when to water at all, covered in the konjac watering schedule.

Signs you are over-feeding konjac

Over-feeding is far more common — and more damaging — than under-feeding for most plants. The classic tells for konjac:

Vigorous dark-green leafy growth but few flowers or fruit (excess nitrogen).
Lush foliage hiding the crop; soft growth prone to pests and disease.
Salt crust on the soil and scorched leaf edges in containers.

Signs you are under-feeding konjac

Pale, yellowing lower leaves and stunted growth.
Small fruit, poor set, and a quickly exhausted plant.
Blossom-end rot and weak cropping from erratic or insufficient feeding.

If the symptoms point at watering, light or roots rather than nutrition, the full konjac care brief covers soil, humidity and the common problems for this species.

Flushing and leaching the salts

In containers, fertiliser salts build up fast — water konjac thoroughly so excess drains from the base each time, and flush pots with plain water every few weeks to prevent a damaging salt build-up.

Organic vs synthetic feeds for konjac

Organic options

Garden compost or well-rotted manure dug in before planting, plus a liquid comfrey or seaweed feed once fruiting starts. UK: comfrey feed or organic Tomorite; US: Espoma Tomato-tone or Neptune's Harvest. Builds soil and feeds in one.

Synthetic / liquid feeds

A balanced feed at planting then a high-potash tomato feed in fruiting — UK: Growmore at planting then Tomorite (Levington) or Phostrogen; US: a balanced 10-10-10 then Miracle-Gro Tomato or a bloom booster.

Brand names are examples, not endorsements, and UK and US ranges differ — check the label’s own NPK and dilution rate, since formulations change.

Fertilising konjac — frequently asked questions

What fertiliser does konjac need?

Balanced (even N-P-K) at planting for roots and frame, then switch to a high-potassium ("high-potash") tomato-style feed once the first flowers open — potassium is what sizes and ripens fruit, not nitrogen. Konjac feeds in two distinct phases — balanced to build the plant, then high-potassium the moment flowering starts to set and fill a heavy crop.

How often should I feed konjac?

Feed every 2-3 weeks through the growing season with a balanced liquid fertiliser to build a large corm; a high-potash feed late in the season helps corm development. Stop feeding as the leaf dies down for dormancy. Feed every 2-3 weeks through the growing season with a balanced liquid fertiliser to build a large corm; a high-potash feed late in the season helps corm development. Stop feeding as the leaf dies down for dormancy. So: a balanced feed or compost at planting, then a high-potash liquid every 1-2 weeks from first flower through harvest across the main season (spring through early autumn).

What strength of feed for konjac?

What does over-feeding konjac look like?

Vigorous dark-green leafy growth but few flowers or fruit (excess nitrogen). Lush foliage hiding the crop; soft growth prone to pests and disease. Salt crust on the soil and scorched leaf edges in containers. Staying on a high-nitrogen feed once konjac starts flowering is the classic error — you get a huge leafy plant and a disappointing crop. Switch to high-potash the moment flowers appear.

Should I flush the soil of konjac?