Kratky Hydroponics for Beginners - Grow Leafy Greens Without a Pump

The Kratky method is the simplest way to grow vegetables in water: a seedling sits in a net cup in the lid of a covered, opaque container filled with nutrient solution. You don't need a pump or electricity. As the plant drinks, the water level falls and leaves an air gap; the upper roots breathe in that moist air while the lower roots keep drinking. It works best for fast leafy greens such as pak choy, lettuce, kangkung and choy sum. In Malaysia, one rule matters above all: cover every opening completely, so mosquitoes cannot breed in the water.

How Kratky works

In normal hydroponics a pump moves water or bubbles air through it, because roots sitting in still water can run short of oxygen. Kratky solves this by letting the water level drop. At the start, the bottom of the net cup just touches the solution. As the plant grows and drinks, the level sinks; the roots follow it down, and the roots above the waterline - in the humid air gap - take up oxygen. By the time the plant is ready to harvest, much of the solution is used up.

This means you should not keep refilling the container to the top. If the air gap fills with water again, the roots that grew to breathe in air can drown and rot.

What you need

1. An opaque container with a lid. Any food-grade box or bucket that blocks light. Light reaching the solution grows algae. If your container is clear, wrap it in black plastic or paint it, then wrap the outside in something reflective or light-coloured to keep it cool.
2. Net cups that fit snugly in holes cut in the lid.
3. Growing medium: sponge cubes or rockwool for sowing; some growers add clay pebbles around the cube to hold it steady.
4. AB hydroponic nutrient - two concentrates (A and B) that must be diluted separately into water.
5. An EC/TDS meter and pH test - optional for a first try if you follow the label exactly, but very useful.
6. Seeds of a fast leafy green.

A kit puts the container, cups and medium together:

Step-by-step

1. Sow. Soak sponge or rockwool cubes in plain water, place 1–2 seeds in each, and keep them damp in a shaded place. Pak choy and choy sum usually sprout in a few days; lettuce can be slower in hot weather (see FAQ).
2. Give light at once. As soon as seedlings appear, move them to bright light so they don't stretch.
3. Mix the nutrient. Fill the container with clean water. Add part A and stir well, then add part B and stir again. Never pour A and B concentrates together; some of the nutrients react and become unavailable to plants. Use the dilution on the label.
4. Check EC and pH (if you have a meter). For leafy greens, aim roughly for EC 1.0–1.8 mS/cm (lower for young seedlings) and pH 5.5–6.5. Use the calculator below to convert between EC and TDS and plan your mix.
5. Transplant. When seedlings have 1–2 true leaves and roots poke out of the cube, set each cube into a net cup.
6. Set the water level. Fill the solution so it touches only the bottom 1–2 cm of the net cup. The rest of the cup and the cube should stay above water.
7. Seal everything. Make sure the lid fits, every net cup sits tightly, and any unused hole is covered (with an empty cup and tape, or a plug).
8. Place it. Put it where leaves get good light but the container is shaded from hot afternoon sun.
9. Wait and watch. Check every few days: leaf colour, water level, and any gaps in the lid.
10. Harvest. Most leafy greens are ready around 4–8 weeks after sowing, depending on crop and weather.

CalculatorsHydroponic AB Nutrient EC / TDS CalculatorHydroponic AB nutrient calculator: how many ml of stock A and B to reach your target EC, with TDS in ppm on the 500 and 700 scales, plus a top-up mode.

Topping up

With a large enough container, a fast crop like pak choy can finish on one fill. If the level drops too fast (often in hot weather or with bigger plants), top up with diluted nutrient solution - but only to a level below the main mass of roots, leaving the air gap in place. Topping up with plain water only dilutes the nutrients; topping up with full-strength solution several times can make it too concentrated. An EC reading tells you which way it is going.

As a rough guide, allow several litres of solution per plant. A bigger volume gives a more stable solution and fewer top-ups.

Mosquitoes: the Malaysian rule

Any still water can breed Aedes mosquitoes, which spread dengue. A Kratky container is perfect for them if there is even a small gap.

• Every opening must be fully covered: net cup holes, unused holes, gaps at the lid edge, the filling hole.
• A net cup with a thin seedling can still leave a gap around the stem at the start; packing clay pebbles or sponge around it helps.
• Look into the solution with a torch every week. If you see wrigglers (larvae), pour the solution out onto soil or a dry area, rinse the container, and start a fresh batch with a better seal.

Light and heat

Leafy greens need bright light: about 4–6 hours of direct sun, or very bright shade. Indoors, use a grow light close above the plants. But the solution itself must stay as cool as possible. In strong afternoon sun, water in a dark box can become very warm; warm water holds less oxygen and encourages root rot. Keep the container in shade, use light-coloured or wrapped boxes, and choose spots with morning sun where possible.

Healthy roots are white or cream. Brown, slimy, bad-smelling roots usually mean the solution is too warm, the air gap was flooded, or light is getting in.

Which crops to grow

• Pak choy and choy sum - the most reliable for beginners in lowland heat.
• Lettuce - grows well with cool solution and morning sun; heat can make it bitter or bolt.
• Kangkung - very fast and heat-loving.
• Avoid large fruiting crops (tomato, cucumber) for your first Kratky try: they drink too much and need support.

GuidesHow to Grow Pak Choy at Home in Malaysia (Soil or Hydroponic)Grow pak choy from seed in pots or a simple Kratky hydroponic setup. Sowing, spacing, watering, feeding, pests and harvesting for Malaysian balconies.

GuidesHow to Grow Lettuce from Seed in Malaysia: The Complete GuideGrow lettuce in Malaysia's heat: choose loose-leaf types, sow for cool germination, give afternoon shade or go hydroponic, and avoid bolting and tip burn.

GuidesHow to Grow Kangkung (Water Spinach) at Home in MalaysiaGrow kangkung from seed in pots, polybags or water. Sowing, watering, feeding, pests and cut-and-come-again harvesting for Malaysian homes.

FAQ

Can I use tap water? Generally yes. If you have a meter, measure the EC of your tap water first and count it as part of the target. Rainwater or filtered water gives a cleaner starting point.

My lettuce seeds won't sprout. Why? Many lettuce varieties germinate poorly when it is too hot, and some germinate better with a little light. Sow in the coolest shaded spot you have, press the seeds onto the surface or cover them only very thinly.

Guides25 Vegetable Seeds That Do Not Need Light to GerminateMost vegetable seeds sprout in the dark when sown at the right depth. 25 common crops with sowing depth and germination days, plus seeds that need light.

Why are the leaves turning pale yellow? Often the solution is too weak or used up, or the pH is out of range so nutrients can't be taken up. Check EC and pH, and top up correctly.

Do I need to change the solution? For a single short crop, usually not. For a second crop in the same box, empty it, clean it, and start with fresh solution.