Kratky Lettuce Kit: Growing Your First Crop

Lettuce is the crop the Kratky method was largely developed around, and a Kratky lettuce kit is the easiest way to see passive hydroponics work. This walkthrough covers a first crop from seed to salad.

Why lettuce suits Kratky so well

Lettuce grows quickly, stays compact and drinks modestly, so one fill of solution can carry it to harvest in a reasonably sized container. Dr. Bernard Kratky's own research included comparisons of non-circulating methods for lettuce (Acta Horticulturae 843, 2009). The guide to the best plants for Kratky explains what makes a crop suitable.

Choosing a variety

• Loose-leaf lettuces (green and red leaf types) are the most forgiving and can be harvested a few leaves at a time.
• Butterhead types form soft heads and do well indoors.
• Romaine grows taller and needs a little more room.
• Iceberg is slower and less suited to small kits.

Varieties described as slow to bolt are a good choice if your growing area is warm.

Starting the seeds

Sow two or three seeds into a moist rockwool cube or plug, and keep it warm and lightly covered until they sprout. Thin to the strongest seedling. Lettuce seed needs light to germinate well, so do not bury it deeply. Move the seedling into the kit when it has a couple of true leaves and roots showing at the base of the cube. See starting seeds for a Kratky kit.

Setting up the kit

1. Mix a complete hydroponic nutrient at the rate the label gives for leafy greens.
2. Adjust pH into the range commonly used for lettuce, about 5.5 to 6.5.
3. Fill until the solution just touches the bottom of the net pots.
4. Plant the seedlings and cover any unused holes.

The full process is in Kratky kit setup.

Light and temperature

Lettuce likes plenty of light but prefers mild temperatures. Under a grow light, a schedule of roughly 12 to 16 hours a day is common. Heat and very long days encourage bolting, when the plant sends up a flower stalk and the leaves turn bitter. A cool room and moderate day length help. See Kratky grow lights.

While it grows

• Let the water level fall. The air gap is where the upper roots breathe; see the Kratky air gap.
• Check that light is not reaching the solution.
• Look at roots and leaves once a week. White roots and steady new leaves mean things are on track.

Harvesting

Loose-leaf lettuce can be harvested by picking outer leaves once the plant has enough of them, leaving the center to keep growing. Head types are usually cut whole once they reach a usable size. Harvest before the plant starts to stretch upward, which is the first sign of bolting. Exact timing depends on variety, light and temperature, so seed packet guidance is a useful starting point.

Common first-crop problems

• Brown inner leaf tips (tipburn): often linked to heat and fast growth; keep the area cooler.
• Bitter, tall plants: bolting from heat or long days; harvest earlier next time.
• Pale leaves: too little light or a weak solution.

The Kratky problems and fixes guide covers these and more.

The Kratky harvest guide compares whole-plant and cut-and-come-again harvesting.

After harvest

Empty the container, clean it, and start again with fresh solution. See cleaning and reusing a Kratky kit. For more on what a good Kratky hydroponics kit includes, see the home page.

Go deeper: lettuce varieties, spacing and growing lettuce beyond a first kit are covered at Kratky Lettuce, our sister site on growing lettuce with the Kratky method.

Frequently asked questions