Kratky hydroponics is a passive way to grow plants in nutrient-rich water with no pump, no air stone, and no electricity for the system itself. You suspend a plant in a net pot over a covered container, let the lower roots drink, and let the falling water level create the air space the upper roots need for oxygen. That one rule explains almost every success and failure with the method. After reading this, you will be able to size a container for a crop, mix a starting solution, set the first water level correctly, and know when to leave a plant alone instead of topping it off.
What Kratky Hydroponics Is and Why the Air Gap Does the Work
Kratky hydroponics is a non-circulating hydroponic method developed by Dr. Bernard Kratky at the University of Hawaii. A plant sits in a net pot supported by a lid, its roots reach into a static reservoir, and the crop is meant to finish on the solution it starts with, or with only careful refills for longer crops.
The system works because roots specialize as the level drops. Lower roots stay in the solution and take up water and nutrients. Upper roots stay in the moist air between the solution and the lid and take up oxygen. According to UF/IFAS Extension St. Lucie County's overview of the Kratky method, that expanding air gap is what gives roots access to oxygen without aeration.
Fill a Kratky container to the net pot at the start, then protect the air gap that forms later.
That is the difference from a jar of water on a windowsill. A plant kept with its whole root system submerged has no reliable oxygen zone. A plant started correctly, then left to lower the level itself, builds both zones on its own.
What to Grow First, and What to Save for Later
Short, fast crops fit Kratky hydroponics best because they can finish before the solution runs low or its chemistry drifts too far. Lettuce, other leafy greens, and many herbs are the practical starting point. UF/IFAS lists lettuce, spinach, kale, basil, and mint as crops that work well, and notes that tomatoes, peppers, and cucumbers need larger containers and more careful nutrient management.
Choose your first crop by harvest time and thirst, not by what looks most impressive. A head of lettuce that finishes in one fill teaches you the method. A tomato in a quart jar teaches you only that the container was too small.
| Crop | Starting container per plant | Why it fits or strains the method | EC range | pH range |
|---|---|---|---|---|
| Lettuce | 1 gallon (3.8 L) | Fast, shallow-rooted, harvests on one fill | 1.2–1.8 mS/cm | 6.0–7.0 |
| Basil | 1 quart to 1 gallon (0.9–3.8 L) | Fast herb, easy to size up if it gets bushy | 1.0–1.6 mS/cm | 5.5–6.0 |
| Spinach | 1 gallon (3.8 L) | Leafy and quick, but prefers cool conditions | 1.8–2.3 mS/cm | 6.0–7.0 |
| Kale | 1–2 gallons (3.8–7.6 L) | Leafy but larger and thirstier than lettuce | — | — |
| Tomato | 5 gallons (19 L) or larger, with refills | Long season, heavy feeder, outgrows a passive fill | 2.0–4.0 mS/cm | 6.0–6.5 |
The EC and pH ranges in the table are from Oklahoma State University Extension's EC and pH guide for hydroponics. EC is electrical conductivity, a measure of dissolved salts in the solution. pH affects whether those nutrients stay available to the plant, which is why both numbers matter even in a hands-off system.
For lettuce specifically, UF/IFAS guidance on growing lettuce in small hydroponic systems gives a narrower target of 1.4 to 1.8 mS/cm and a pH of 6.0 to 7.0. If you grow only lettuce, use that range. If you grow mixed herbs, follow the range for the crop in that container and do not try to share one reservoir between plants with different needs.
What You Need for a First Kratky Setup
You need a container, a way to hold the plant, a complete hydroponic nutrient, and light. Everything else is a tool that makes checking easier, not a part that makes the system run.
- Opaque container with a lid: a 1-gallon jug, a 5-gallon (19 L) bucket, or a tote. Clear containers must be covered, because light reaching the solution encourages algae.
- Net pots: 2- to 3-inch (5–7.6 cm) pots are a practical size for lettuce and herbs in a lid.
- Growing medium: rockwool cubes, expanded clay pebbles, or coco coir to anchor the seedling in the net pot. UF/IFAS lists rockwool, perlite, coconut fiber, and light expanded clay aggregate among the media used in small hydroponic systems.
- Hydroponic nutrient: a product labeled for hydroponics and for edible crops, mixed exactly to its label. Soil fertilizer alone is often incomplete for soilless growing because it may lack calcium, magnesium, or trace elements in usable amounts.
- Seeds or seedlings: loose-leaf lettuce is more forgiving than head lettuce for a first run.
- Light: a very bright window or an LED grow light. The plant still needs electricity for light indoors, even though the water system does not.
- Optional meters: a pH test kit and an EC meter help you mix correctly the first time and diagnose a struggling plant later.
Buy the nutrient and the opaque container before you buy a bigger light or a second system.
Most first failures come from solution and sizing mistakes, not from having too few gadgets.
How to Set Up Kratky Hydroponics, Step by Step
A correct setup takes about 20 minutes, most of it spent mixing and checking the solution. Work in this order, because changing the level after the plant is in place is what causes trouble later.
- Wash the container and lid. Rinse away soap residue, dust, and anything left from a previous crop. Old roots and algae left in a container carry problems into the next plant.
- Cut one hole per plant in the lid. Space lettuce holes at least 6 inches (15 cm) apart center to center in a tote, so leaves are not crowded as they widen. The net pot should sit snugly and not fall through.
- Start the seed in the medium. Sow in a moist rockwool cube or fill the net pot with clay pebbles or coir and sow into it. Keep the medium damp, not flooded, until roots show at the bottom of the pot.
- Mix the nutrient solution in a separate jug. Add nutrient to water at the label rate for your crop and volume, stir, then check EC and pH if you have a way to measure them. Adjust pH only after the nutrient is fully mixed, because the nutrient itself changes pH.
- Fill the container so the solution touches the bottom of the net pot. UF/IFAS gives this contact as the starting point, because it lets moisture wick into the medium and reach young roots. Do not submerge most of the net pot.
- Set the net pot in the lid and put the container in light. A sunny spot outdoors or a light hung close enough to prevent stretched, pale growth works. Keep the lid on to block light and slow evaporation.
- Leave the level alone. Check the plant, not the water, for the first week. Roots should grow down as the level begins to drop, and a visible air gap should open under the lid.
That last step is the hardest for beginners. A falling water level is the method working, not a problem to fix.
If you started a seedling with very short roots, the solution must touch the medium at first. If you transplant a seedling with roots already hanging below the pot, you can start with a small gap of about 0.5 inch (1.3 cm) below the pot, as long as root tips reach the solution.
Light, Temperature, and Solution Checks Without Overcomplicating It
Plants in a Kratky container still fail from weak light and warm, stale solution, even when the fill level is right. Keep your checks simple and tied to a visible symptom.
Light that is too far away produces spindly, stretched plants. Light that is too close can burn leaves. UF/IFAS notes both problems for lettuce grown under supplemental lights, and also notes that lettuce cultivars differ in how many hours of light they are adapted to. Start with bright light for most of the day, then move the light closer or farther based on whether leaves look compact and green or stretched and pale.
Solution temperature matters because warm water holds less dissolved oxygen and stresses roots. Oklahoma State Extension lists a water temperature of 72 to 75°F (22–24°C) as optimal in its hydroponics guide. You do not need a chiller for one bucket, but move a container out of hot afternoon sun, off hot concrete, and out of a closed, overheated room if roots start to look brown and the solution smells stale.
Check pH and EC at mixing, not every day, for a true one-fill crop. In Kratky, repeated opening, stirring, and adjusting can do more harm than a solution that started in range.
If a plant stalls, yellows, or develops burned leaf edges mid-cycle, then a check is useful. A reading far outside the crop range tells you the mix was off or the solution has concentrated as the plant drank mostly water. That is diagnostic information for the next crop, and for long crops it may justify a careful partial refill with a correctly mixed solution.
Kratky Hydroponics vs. Deep Water Culture: Which One Fits Your Space?
Deep water culture, often shortened to DWC, also suspends plants over a reservoir, but it uses an air pump and air stone to oxygenate the solution. That one part changes the cost, the noise, the maintenance, and the kinds of crops you can push.
| Factor | Kratky | Deep water culture |
|---|---|---|
| Pump or air stone | None | Required |
| Power for the system | None | Continuous for the pump |
| Starting cost for one plant | Container, net pot, medium, nutrient | Same basics, plus pump, tubing, and air stone |
| Refilling | Avoid for short crops; careful partial refills for long crops | Top off and adjust as a normal routine |
| Best first use | One-fill lettuce and herbs | Larger plants, longer crops, growers who want control |
| Main risk | Running dry, drowning air roots by overfilling | Pump failure, more parts to clean |
Choose Kratky if you want one or a few short crops, quiet operation, and the lowest possible setup. Choose DWC if you want to grow a larger plant for longer, adjust the solution during the crop, or reuse the same reservoir system across seasons with active control.
Neither choice is better in the abstract. Kratky trades control for simplicity, and that trade works only when the crop is short enough to accept it.
The Mistakes That Kill Most First Kratky Plants
Most dead Kratky plants were not killed by the method. They were killed by a container that was too small, a refill that drowned the air roots, or a solution mixed for the wrong crop.
Refilling to the top
Topping a half-empty container back to the brim submerges the air roots the plant built in the gap. If a long crop needs more solution, refill only partway, leaving at least 1 to 2 inches (2.5–5 cm) of air space below the lid. For a short lettuce crop, do not refill at all unless the container is nearly dry well before harvest.
Using a clear container in bright light
Light in the reservoir grows algae, which competes for oxygen and makes roots and containers slimy. Paint, wrap, or cover a clear jar or jug, or start with an opaque food-grade container. A lid alone does not fix clear sides.
Sizing for the seedling instead of the mature plant
A basil seedling fits in a mason jar. A mature basil plant may drink that jar dry and crowd its own roots. Size for the plant at harvest, using the table above, and size up when a crop is still vigorous but the level is falling fast.
Mixing nutrient by guesswork
Too weak a mix starves the plant. Too strong a mix can burn leaf edges as the solution concentrates over time. Follow the product label for the volume you actually mixed, write the date and rate on tape on the lid, and check EC and pH when you can.
Keeping a plant with foul, slimy roots
White or light tan roots with no bad smell are normal. Dark, slimy roots that smell sour or rotten point to root disease and warm, low-oxygen conditions. Remove that plant, discard the solution, wash the container before reuse, and do not eat a crop that has rotted at the root. Start the next crop cooler, with a correctly set starting level.
When to Refill, When to Restart, and When to Harvest
A one-fill crop is finished when the plant is ready and the solution is low. That timing is the goal, not a coincidence to worry about.
Harvest loose-leaf lettuce by cutting outer leaves while the center keeps growing, or cut the whole plant at the base when it reaches usable size and before it bolts and turns bitter in heat. Herbs such as basil are harvested by pinching above a pair of leaves, which encourages branching, rather than stripping one stem bare.
Restart instead of rescue when a short crop has nearly emptied a small container and looks tired, pale, or bitter. A fresh mix for a new seedling costs less time than weeks spent correcting a spent solution.
For a longer crop in a large container, inspect the level every few days once the plant is big. Refill with plain water for a small top-up if EC was in range, or with a correctly mixed nutrient solution if the plant is still feeding heavily, and always preserve the air gap. Record the date and amount, because a plant needing constant large refills is telling you the next container should be bigger.
Use only nutrients labeled for edible hydroponic crops, follow label handling directions, and wash harvested greens in clean drinking water before eating. Keep electrical lights and cords dry and away from splashing solution, even though the Kratky reservoir itself uses no power.
Kratky Lettuce and Basil Questions, Answered Shortly
Do you have to change the water in a Kratky system?
No, not for a short crop sized correctly. The method is designed for the plant to grow to maturity on the starting solution, as UF/IFAS describes. Change the solution completely between crops, after washing the container.
Can you grow Kratky hydroponics indoors without a grow light?
Only in a genuinely bright window that keeps leaves compact and green. Most indoor windows give too little light for fast, tight lettuce growth, and stretched plants are the usual result. A small LED light run on a timer is the practical fix.
Why are some roots white and some roots in the air gap fuzzy?
Roots in the air gap often look thicker and fuzzier because they are adapted to taking up oxygen from moist air. Roots in solution are usually smoother and whiter. Color alone is not a problem; slime and a rotten smell are.
Will mosquitoes breed in a Kratky container?
They can if openings leave standing solution exposed. Keep the lid fitted, cover any unused holes, and avoid open gaps around net pots outdoors. A tight lid removes most of the access mosquitoes need.
Can you reuse clay pebbles and net pots?
Yes, after you remove old roots, wash the pieces, and let them dry fully before the next crop. Replace rockwool and coco coir, which break down and hold old root material. Reusing hard parts is part of why a simple Kratky setup stays cheap.
What is the first crop to try if you have never grown anything?
Loose-leaf lettuce in a 1-gallon opaque container is the clearest first test. It grows fast, its EC and pH targets are well documented by extension guides, and it can finish on one fill. Basil in a quart jar is a good second test if counter space is very tight.
How This Guide Was Put Together
This guide is based on the suspended-pot, non-circulating method described by UF/IFAS Extension, UF/IFAS guidance for lettuce in small hydroponic systems, and Oklahoma State University Extension guidance on EC and pH for hydroponic crops. It does not claim first-hand testing, expert review, or results from a specific home setup.
Sources
- UF/IFAS Extension St. Lucie County, Kratky Hydroponic Method: A Simple Way to Grow Plants Without Soil, 2025
- Sandoya, Bosques, Rivera, and Campoverde, UF/IFAS Extension, Growing Lettuce in Small Hydroponic Systems, HS1422, 2021, reviewed 2024
- Singh and Dunn, Oklahoma State University Extension, Electrical Conductivity and pH Guide for Hydroponics, 2017
