Fertilising guide
How to fertilise Duku (Lansium domesticum)— schedule & NPK
Also called Langsat, Longkong, Duku Langsat.
More about duku
About Duku
Lansium domesticum · also called Langsat, Longkong · edible
Duku is a prized Southeast Asian fruit tree producing clusters of pale yellow, grape-like fruits with translucent sweet flesh and a mild, tangy flavour. It requires humid tropical conditions and a distinct dry season to trigger flowering. A slow-growing, long-lived tree that demands patience but rewards with delicious harvests. Toxicity to pets is not established; treat with caution.
Growth habit: Slow-growing evergreen tree with a spreading crown
Watch for — Leaf chlorosis: Yellowing may indicate iron or magnesium deficiency; apply chelated micronutrients and maintain acidic soil pH.
What fertiliser duku actually wants — and why
Duku feeds in two distinct phases — balanced to build the plant, then high-potassium the moment flowering starts to set and fill a heavy crop.
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.
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 duku: 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 duku, 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 duku:
Apply a complete slow-release fertiliser with micronutrients in spring and early summer. Supplement with fortnightly liquid feeds (balanced NPK) during the vegetative flush. Phosphorus and potassium are important for flowering induction and fruit quality. 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 duku 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 duku
Follow the crop-feed label rate for duku — 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 duku first if the soil is dry, then apply the diluted feed. The companion question is when to water at all, covered in the duku watering schedule.
Signs you are over-feeding duku
Over-feeding is far more common — and more damaging — than under-feeding for most plants. The classic tells for duku:
- 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 duku
- 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 duku 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 duku 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 duku
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 duku — frequently asked questions
What fertiliser does duku 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. Duku 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 duku?
Apply a complete slow-release fertiliser with micronutrients in spring and early summer. Supplement with fortnightly liquid feeds (balanced NPK) during the vegetative flush. Phosphorus and potassium are important for flowering induction and fruit quality. Apply a complete slow-release fertiliser with micronutrients in spring and early summer. Supplement with fortnightly liquid feeds (balanced NPK) during the vegetative flush. Phosphorus and potassium are important for flowering induction and fruit quality. 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 duku?
Follow the crop-feed label rate for duku — these are calibrated for hungry vegetables. Consistency through fruiting matters more than strength; erratic feeding causes problems like blossom-end rot.
What does over-feeding duku 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 duku 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 duku?
In containers, fertiliser salts build up fast — water duku 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.
Keep reading
- Duku care — the full brief (light, soil, humidity, problems, pet safety)
- How often to water duku — the watering schedule
- The houseplant fertiliser schedule — feeding through the year
- NPK ratio explained — what the three numbers on the bottle mean
- How to fertilise black mission fig
- How to fertilise plantain
- How to fertilise lady finger banana
- All 11687 fertilising guides in the Growli library