How the Kratky Method Works

Lesson 1 of 14

Before you buy anything, it helps to understand what is going on inside a Kratky jar. Once you see why it works, every later step makes sense.

Our indoor Kratky garden with a variety of plants

The Short Answer

The Kratky method grows a plant in a closed container of nutrient solution, with no pump, no air stone and no electricity. The plant sits in a net cup in the lid. Its roots grow down into the solution and drink it. As the level drops, the space left between the lid and the solution fills with air, and the upper roots breathe from that space.

The method was developed by Dr. Bernard Kratky at the University of Hawaii as a low-cost way to grow vegetables without soil or power.

What Happens in the Jar, Week by Week

  1. Start: a seedling goes into a net cup filled with a growing medium such as rockwool or clay pebbles. The solution is filled so that it just touches the bottom of the net cup. The young roots reach it straight away.
  2. Growth: the plant drinks. The solution level falls, and the roots grow down after it.
  3. Air gap: the space between the lid and the falling solution is the air gap. The roots in that space take in oxygen, while the roots below keep drinking. This split is what replaces the pump in other hydroponic systems.
  4. Harvest: for fast crops like lettuce, one fill can last from transplant to harvest. Longer crops need topping up along the way, which Lesson 10 covers.

Why Beginners Like It

  • Nothing to plug in. No pumps to fail and no noise.
  • Cheap to start. A jar, a net cup, nutrients and seeds are enough for your first plant.
  • Little daily work. You check the level and the plant, not a machine.
  • Easy to learn from. One container, one plant, so you can see exactly what each change does.

Where It Struggles

  • Big, thirsty plants like full-size tomatoes drink faster than a small container can supply, so they need large containers and topping up.
  • Light leaks into a clear container grow algae in the solution. Containers need to be dark or covered.
  • Refilling to the top drowns the upper roots that have adapted to air. Topping up is done carefully, never back to the lid.

The Three Rules to Remember

  1. Keep light out of the solution.
  2. Let the level drop and protect the air gap.
  3. Start with an easy plant and a simple setup.

Those three rules come up in every lesson that follows. Next, you will choose your first plant.

Related Articles

Sources

  1. Kratky, B.A. (2009). Three non-circulating hydroponic methods for growing lettuce. Acta Horticulturae 843: 65–72.

All lessons  |  Lesson 2: Choose Your First Kratky Plant →