The Kratky method is the simplest form of hydroponics: no pump, no air stone, no electricity. A plant sits in a net pot above a container of nutrient solution, and as it drinks the solution down, an air gap opens up where its roots can breathe. It was developed by Bernard Kratky at the University of Hawaii, and his own extension publications are the best guide to doing it right. This guide follows those CTAHR publications.
How the Kratky Method Works
In CTAHR publication VC-1 (2010), Kratky describes the system: a tank is filled almost to the top with nutrient solution and covered with a lid that has holes for net pots. At planting, the lower half inch or so of each net pot sits in the solution, and the growing medium in the pot is wetted by capillary action. Within a few weeks the solution level drops below the net pots, but by then the roots have grown down into it.
From that point there are two kinds of roots. Roots in the solution keep taking up water and nutrients, while roots in the space between the lid and the solution become “oxygen roots” that take up air from the humid air layer. The whole crop is grown with the single initial fill of water and nutrients; no pumps, wicks or aerators are needed.
| Feature | Kratky method | Most other hydroponic systems |
|---|---|---|
| Pumps or air stones | None | Usually required |
| Electricity | None | Usually required |
| Water and nutrients | All added at the start | Topped up and recirculated |
| Best for | Short leafy crops (lettuce, greens, herbs) | Short and long-term crops |
Source: Kratky, B.A. (2010). CTAHR VC-1. For other systems, see our hydroponic systems guide.
For the bigger picture of hydroponic types, see our hydroponic systems guide.
What You Can Grow
Kratky says the method is intended for crops that need less than 2 gallons of water per plant for the whole season, and lists leafy, romaine and semi-head lettuces, cilantro, green onions, kai choy, pak choi and watercress. His home-garden kit (CTAHR HG-42) uses a 1-gallon bottle for a single lettuce or kai choy plant. Long-season crops such as tomatoes and cucumbers use much more water and need larger, different non-circulating setups (VC-1, HG-42).
Kratky Method Mason Jar Setup (Step by Step)
Kratky’s HG-42 kit uses a 1-gallon bottle; a large wide-mouth jar works the same way for one small plant. Based on HG-42:
- Clean the container. HG-42 says to rinse it twice with water, without bleach, and rinse well if you use dish soap.
- Add hydroponic fertilizer and water. HG-42 uses about 1 teaspoon of a hydroponic fertilizer per gallon, then fills to about 1.5 inches from the top.
- Fill a net pot with growing medium (HG-42 lists at least two of peat, perlite, vermiculite or coir), tapping it to settle without packing it tight.
- Set the net pot in the opening so it fits snugly; HG-42 notes that gaps let mosquitoes in. The bottom half of the net pot should sit in the solution.
- Plant one or two seeds a quarter inch deep, or transplant a 1- to 2-week-old seedling (HG-42).
- Block the light by painting the container or putting a bag around it (see below).
- Leave it alone: HG-42 says no more water or fertilizer, and don’t pull the net pot out, which damages the roots.
For a jar, use one large enough for the crop: a full head of lettuce drinks most of a gallon. Small jars suit herbs and baby greens harvested young.
Kratky Method Nutrient Solution
Use a complete hydroponic fertilizer, not a general garden fertilizer. Kratky notes in VC-1 that field fertilizers usually have too much ammonium nitrogen for hydroponics. Key numbers from VC-1:
| Measure | Kratky’s guidance (VC-1) |
|---|---|
| Solution strength (EC) | About 1.5 mS; lower (1.2 to 1.5) in hot weather, higher (1.6 to 2.0) in cool weather; be wary above 2.0 and avoid above 3.0 |
| pH | Recommended 5.5 to 6.5 |
| Water quality | Avoid salty water: salts concentrate as the plant uses the solution |
| Volume | About 1.5 to 2 gallons of solution per lettuce plant in tanks |
Source: Kratky, B.A. (2010). CTAHR VC-1.
If you don’t have an EC meter, follow the fertilizer label’s hydroponic rate. Kratky also explains that as plants use water, salts already in the water concentrate: water starting at 0.5 mS reaches 2.0 mS when three-quarters of the solution has been used, so start with good-quality water or rainwater.
Blocking Light and Stopping Algae
Algae need light to grow in nutrient solution. Kratky’s HG-42 says plainly: paint the bottle or put a bag around it to exclude light from a clear container, “otherwise green algae may form in the bottle and slow the growth of the crop.” In his commercial system (VC-1), tanks are lined with black polyethylene and covered with lids, and he recommends painting covers white so they don’t overheat in the sun.
- Use an opaque container, or wrap a clear jar in foil, a sock, a paper bag or paint.
- Cover the top with a lid that holds the net pot snugly, so light (and mosquitoes) can’t get in.
- Leave a small window (a strip you can uncover) if you want to check the level, but keep it covered the rest of the time.
- Light-colored outer wrapping keeps containers cooler in sun than black ones.
The Golden Rule: Don’t Raise the Water Level
Kratky stresses that the solution level may stay the same or fall, but it should not be raised, because submerging the oxygen roots makes the plant “drown.” That is why his outdoor tanks need rain shelters: rain raises the level and the plants wilt or die. He also warns not to lift the lid while the crop is growing, because the roots can tear.
The one exception: if the solution is about to run out before harvest, VC-1 suggests adding about half an inch to an inch of water or half-strength solution, not a full refill. If that happens every time, use a bigger container.
Kratky Method Lettuce: Timing and Harvest
VC-1 says leafy or semi-head lettuce is ready about 4 to 6 weeks after transplanting, depending on seedling age, season and variety, and HG-42 says a home kit needs little attention for 5 to 6 weeks before harvest. Lettuce grows best in cool temperatures (Kratky gives 60 to 70°F); high temperatures can cause tip burn and bitterness. Some midday wilting is normal and plants usually recover by late afternoon (VC-1).
After harvest, remove the roots and medium, wash the container and start again (HG-42). Kratky suggests using leftover solution to water plants in soil rather than pouring it down one spot (VC-1).
Problems and Fixes
| Problem | Likely cause | Fix |
|---|---|---|
| Green water, slimy roots, slow growth | Light reaching the solution (HG-42) | Make the container opaque; cover the top |
| Plant wilts and collapses mid-crop | Solution level raised over the oxygen roots (VC-1) | Don’t refill; keep containers out of the rain |
| Mosquito larvae in the solution | Gaps around the net pot or lid (HG-42, VC-1) | Fit the net pot snugly; cover gaps |
| Poor growth, burned leaf edges | Solution too strong or salty water (VC-1) | Use the label rate and good-quality water |
| Seeds don’t sprout | Old or heat-damaged seed (HG-42) | Store seed in the fridge; use fresh seed |
| Solution runs out before harvest | Container too small (VC-1) | Add a little water or half-strength solution; use a bigger container next time |
Sources: Kratky, B.A. CTAHR VC-1 (2010) and HG-42 (2002).
Because there is no soil, the Kratky method also avoids soil-dwelling pests such as fungus gnat larvae; see our fungus gnat guide if you have them in your potted plants.
For herbs you can start in plain water before moving them to a nutrient solution, see our guide to herbs that grow in water.
Frequently Asked Questions
Do you need an air pump for the Kratky method?
No. Kratky’s method uses no pumps, wicks or aerators: roots in the air gap above the solution take up oxygen from the moist air (VC-1).
Should I top up the water in a Kratky jar?
Generally no. Kratky says the level can stay the same or fall but should not be raised, because submerging the oxygen roots can drown the plant. If it is about to run dry before harvest, add only half an inch to an inch (VC-1).
Why is my Kratky water turning green?
Algae, because light is reaching the solution. Kratky’s HG-42 says to paint or bag a clear container to exclude light.
How long does Kratky method lettuce take?
About 4 to 6 weeks after transplanting, depending on seedling age, season and variety (VC-1); HG-42 gives 5 to 6 weeks for the home kit.
What is the best nutrient solution for the Kratky method?
A complete hydroponic fertilizer at about 1.5 mS EC and pH 5.5 to 6.5, per Kratky’s VC-1. Avoid ordinary garden fertilizers, which usually have too much ammonium nitrogen.
Can you grow tomatoes with the Kratky method?
Not in a small jar. Kratky’s suspended-pot method is meant for crops that need less than 2 gallons of water per plant; tomatoes and cucumbers need much larger non-circulating setups (VC-1).