How to build your first Kratky jar

Gather your jar and cup

Almost any wide-mouth jar between 500ml and 1 litre will work. A pint mason jar is the classic. You want the jar dark or wrapped — light on the roots grows algae. Aluminium foil, a sock, a brown paper bag all work. The net cup should sit in the jar’s mouth without falling through. 2-inch cups fit pint mason jars perfectly.

Prepare the net cup

Soak your growing medium — rockwool, clay pebbles, or coco — for an hour in pH-balanced water (pH 5.5 to 6.0 is fine; lemon juice brings it down if your tap water is alkaline). Press one seed about 5mm deep into the medium. Two seeds is fine if you want to thin later; it’s actually a good idea for lettuce.

Mix the nutrient solution

Follow the directions on your bottle. Most hydroponic nutrients are sold as A+B (concentrates). For lettuce, you want an EC (electrical conductivity) of 1.2 to 1.6 mS/cm. If you don’t have a meter, follow the “lettuce” line on the bottle — the manufacturer has already thought about this.

Fill the jar so the water touches the bottom 1cm of the net cup. The roots will drink from there and then, crucially, will find air as the level drops.

Choose a sunny window

South-facing, if you’re in the northern hemisphere. Six hours of light minimum. If your window is shy, a small grow light 30cm above the jar will do the trick — you don’t need much. A warm white 15W LED is fine.

Watch, don’t touch

For the first ten days, do nothing. Really nothing. Don’t top up the water, don’t check the pH, don’t move the jar. The seed will either sprout or it won’t. If it sprouts, the roots will grow down and find water. As the water drops, the roots will split — some drinking, some breathing. This is the whole method. This is the quiet magic.

After three weeks, you can harvest the outer leaves. After five, you will have eaten something you grew, on a windowsill, in a jar of water. This is how it starts.