Kratky method: easy low-cost hydroponics on a windowsill with no pump

TUREFF• •0

The Kratky method is the simplest way to try hydroponics. There is no pump, no air stone, no electricity and no plumbing: just a container, a lid, a net pot and nutrient solution. It was developed by Bernard Kratky, a researcher at the University of Hawaii, and published in the late 2000s. It is perfect for a windowsill and for learning how plants behave in water before spending money on equipment.

How it works

A plant sits in a net pot held in a lid, with the bottom of the pot touching the nutrient solution. As the plant grows and drinks, the water level falls. That leaves a gap of humid air between the lid and the water. The upper roots take oxygen from that air, while the lower roots keep drinking from the solution. Because the plant creates its own air gap as it grows, no pump is needed. The key point: do not keep topping the container back up to the brim. Once roots have grown into the air gap and adapted to air, flooding them again can harm them. If the level drops too far before harvest, top up only partway, keeping the upper roots in air.

What you need

A container: a large jar, a food-grade tub or bucket. It must be opaque, or wrapped in something that blocks light, to stop algae growing in the water.
A lid with a hole cut to hold a net pot. A jar with a wide mouth can hold the pot in the neck.
A net pot and a little growing medium: clay pebbles, rockwool or coco coir.
Hydroponic nutrient, mixed at the strength on the label, starting on the lower side for young plants.
A pH test if possible. Most leafy crops do well around pH 5.5 to 6.5.
Seeds or seedlings.

Setting one up

Start seeds in a rockwool cube or plug until roots poke out of the bottom.
Mix the nutrient solution in clean water and check the pH.
Fill the container so the bottom of the net pot just sits in the solution.
Place the seedling in the net pot with a little medium around it.
Put it in your brightest window, or under a small grow light.
Leave it alone. Check the level occasionally and watch the roots grow down.

Choosing crops

Kratky suits plants that grow quickly and do not drink enormous amounts:

Lettuce and salad leaves: the classic Kratky crop. One plant per container, and a container big enough to hold enough solution for its whole life.
Herbs: basil, parsley, mint, chives.
Leafy greens: pak choi, spinach, rocket, chard.

Larger fruiting plants such as tomatoes drink far more, so they need big containers and usually some top-ups. They are possible but not where to start. Size the container to the plant. A small jar suits a single herb; a full-sized lettuce needs a few litres. Running out of solution before harvest is the most common Kratky problem.

Windowsill tips

Light: a bright south-facing window in summer can be enough for leafy greens. In winter, in a dim room or in a north-facing window, plants often stretch; a small LED grow light makes a big difference.
Heat: glass in full summer sun can warm the solution. Keep containers out of the hottest spot, as warm water holds less oxygen and encourages root problems.
Algae: block light from the solution. Wrap jars in foil, paper or a sock.
Pests: cover any openings so insects cannot get in or breed in the water.
Spills: stand containers on a tray, and keep them away from electrics.

Keeping costs down

Reuse jars and food-grade tubs instead of buying a kit.
Cut net pots from plastic cups with holes punched in the sides if you prefer.
Start seeds in small cubes rather than buying seedlings.
Buy one concentrated nutrient that lasts many batches.

Harvesting and starting again

Pick outer leaves of lettuce and herbs as needed, or harvest the whole plant. Afterwards, empty the old solution, wash the container and net pot, and start again with fresh solution. Share photos of your windowsill jars, or ask about a container or crop, in the comments below.