Project: grow lettuce in a jar
How can a plant grow in a jar with no pump?
There are no bubbles in this jar. So how do the roots breathe?
Build the simplest system there is.
Think of a glass of juice with a straw. As you sip, the juice drops and air fills the top.
No pump and no plug. A jar does the whole job.
A plant in a jar works the same way. It drinks the water down, a little each day.
Growers call this the Kratky methodA way to grow where you fill a tub once. As the water drops, the top roots get air.
See it in the glossary method.
On day one, the bottom of the pot sits in the water.
The roots can drink right away. Plant food in the water feeds them.
- A · lettuce seedling
- B · net pot
- C · roots
- D · plant water
As the plant drinks, the water drops. A space opens between the pot and the water.
That space is the air gapThe space of air between the bottom of the pot and the top of the water.
See it in the glossary.
- A · lettuce seedling
- B · net pot
- C · roots
- D · plant water
- E · air gap
Top roots breathe in the air gap. Low roots drink the water.
So you never fill the jar back to the top.
- A · lettuce seedling
- B · net pot
- C · roots
- D · plant water
- E · air gap
Build the jar, step by step.
Put the build steps in order, from first to last.
Move the steps up or down. Then check.
- Put the jar in a bright spot, out of the rain. Right spot Not quite
- Wrap the jar in dark paper so light stays out. Right spot Not quite
- Fill the jar with the plant water, almost to the top. Right spot Not quite
- Tuck the seedling into the net pot, with growing mix around it. Right spot Not quite
- Set the net pot in the lid so its bottom sits in the water. Right spot Not quite
- An adult mixes plant food into clean water, following the label. Right spot Not quite
Put the build steps in order, from first to last.
The right order:
- Wrap the jar in dark paper so light stays out.
- An adult mixes plant food into clean water, following the label.
- Fill the jar with the plant water, almost to the top.
- Tuck the seedling into the net pot, with growing mix around it.
- Set the net pot in the lid so its bottom sits in the water.
- Put the jar in a bright spot, out of the rain.
Light out first, then water, then the plant.
Set the jar in a bright spot, out of the rain. In about a month, your lettuce is ready to eat.
Ask an adult to print the project sheet and build one together.
Check 1 of 4
This jar has no pump. Where do the top roots get air?
Show the answer
In the air gap above the water. Right. As the water drops, the gap gives the roots air.
Check 2 of 4
After two weeks, the water is lower. What do you do?
Show the answer
Leave it. The gap is part of the plan.. Right. The roots grow down to follow the water.
Check 3 of 4
Why do you wrap the jar in dark paper?
Show the answer
To keep light out of the water. Right. Light lets green slime grow in the water.
Check 4 of 4
Who mixes the plant food into the water?
Show the answer
An adult. Right. Plant food is strong stuff. An adult measures and mixes it.
You just learned how a jar with no pump can grow lettuce.
That's the end of this track for now. More lessons are on the way.
You finished Track 2. You can print a certificate for it.
Go deeper: why the jar worksOpenClose
This method is named after B. A. Kratky, a researcher at the University of Hawaii. He wrote about it from the 1990s on.
It is a passive, non-circulating method. You add all the water and plant food once, at the start. There is no pump, timer, or air stone.
As the level falls, the roots in the moist air take in oxygen. The roots still in the solution take in water and nutrients.
One lettuce plant drinks about 3 to 6 liters over its life, depending on the guide. Kratky grew one lettuce in a 4-liter bottle, about 1 gallon. A small jar can run dry before the lettuce is grown.
Guides give different timings. Kratky’s bottle lettuce was ready about 5 to 7 weeks after seeding. Transplanted seedlings are ready about 3 to 6 weeks after they go in, depending on the guide, the season, and the variety.
It suits small, fast leafy crops such as lettuce and bok choy. Tomatoes and cucumbers drink far too much for one small jar.
Rain can raise the water and drown the air roots, so the jar stays under cover. If the jar may run dry before harvest, add only a little water. Never cover the air roots.
Sources
- Kratky, B. A. A Simple Hydroponic Growing Kit for Short-Term Vegetables. Home Garden HG-42. College of Tropical Agriculture and Human Resources, University of Hawaii, 2002. Link
- Kratky, B. A. Three non-circulating hydroponic methods for growing lettuce. Acta Horticulturae 843: 65-72, 2009. Copy hosted by the University of Hawaii CTAHR. Link
- Kratky, B. A. A Suspended Net-Pot, Non-Circulating Hydroponic Method for Commercial Production of Leafy, Romaine, and Semi-Head Lettuce. Vegetable Crops VC-1. College of Tropical Agriculture and Human Resources, University of Hawaii at Manoa, 2010. Link
- Eason, H. 'Set It and Forget It' Hydroponic Lettuce. HS1488. UF/IFAS Extension. Link
- Hoidal, N., Reardon, A., Worth, L., and Rogers, M. Small-scale hydroponics. University of Minnesota Extension. Link
- Sharma, A., K. Leonberger, N. Gauthier, and Q. Ying. Hydroponic Lettuce Production in Controlled Environments. CCD-CP-63. Center for Crop Diversification, University of Kentucky, 2025. Link