Kratky Containers: Sizes, Types and Light Blocking
In a Kratky system, the container is the plant's whole supply of water and food. Choosing Kratky containers of the right size and keeping light out of them solves many problems before they start.
Why container size matters
A Kratky plant drinks from one batch of solution, ideally for the entire crop. If the container is too small for the plant, the solution runs out before harvest, or the plant is short of nutrients by the end. If it is generously sized, the plant finishes with solution to spare. Bigger plants that stay in the container longer need much more solution than small, quick crops.
Rough sizes by plant
These are rough guides, because growth depends on light, temperature and variety:
| Plant | Container | Notes |
|---|---|---|
| One herb or loose-leaf lettuce | Quart to half-gallon jar | Larger is safer for longer harvests |
| Head lettuce, bok choy | About 1 gallon per plant | More margin before the solution runs low |
| Several greens | Storage tote with a lid | Space net pots so plants can spread |
| Peppers, tomatoes | 5-gallon bucket or larger tote | Plus support for the stems |
The guide to the best plants for Kratky explains which crops are thirstiest.
Whatever the size, light on the solution causes algae; see algae in a Kratky kit.
Container types
Mason jars
Wide-mouth jars fit a 3-inch net pot neatly, look good on a counter and are easy to clean. They are clear, so they must be covered to block light. Ideal for single herbs or a lettuce. See the article on building a DIY Kratky kit from a mason jar.
Storage totes
Opaque plastic totes with lids hold several plants. Cut holes in the lid for net pots. A tote is a cheap way to grow a tray of lettuce.
Buckets
Five-gallon buckets with lids suit larger single plants. Choose an opaque, food-grade bucket.
Kit reservoirs
Purpose-made kit reservoirs are sized by the manufacturer. Check the volume per planting site when choosing a Kratky kit.
Materials: keep it food-safe
Use containers made for food or drink, such as glass jars and food-grade plastics. Avoid containers that held chemicals, and avoid metal containers that can corrode in nutrient solution. Wash new containers before use.
Blocking light
Light reaching the nutrient solution allows algae to grow. Algae competes with the plant for nutrients, can coat roots, and changes the solution as it grows and dies. Blocking light is the main way to prevent it:
- Use an opaque container where possible.
- Cover clear jars with a fabric sleeve, paper bag, foil, tape or paint. Leave a narrow vertical strip that can be uncovered to check the water level, then covered again.
- Make sure the lid fits without gaps, and cover unused net pot holes.
- Avoid light-colored grow media sitting right at the surface of exposed solution.
Temperature
Dark containers in direct sun can heat up, and warm water holds less dissolved oxygen, which raises the risk of root rot. Light-colored or covered containers stay cooler. Keep containers out of hot afternoon sun where possible.
Seeing the water level
Because the air gap is so important in the Kratky method, it helps to see the level without lifting the lid. Options include a covered viewing strip, a float indicator, or simply weighing a small container. The article on the Kratky air gap explains what level to expect as the plant grows.
Go deeper: mason jars, buckets and other reservoirs are covered in detail at Kratky Containers, our sister site on growing in containers with the Kratky method.
Frequently asked questions
Can I use a clear container for Kratky?
Yes, if you block the light. Wrap it in a sleeve, paper, foil, tape or paint, leaving a covered strip you can open to check the water level. Light reaching the solution leads to algae.
How big should a Kratky container be for lettuce?
A single loose-leaf lettuce can grow in a quart or half-gallon jar, but around a gallon per plant gives more margin, especially for head lettuce or under strong light.
Is a 5-gallon bucket good for Kratky?
A food-grade, opaque 5-gallon bucket with a lid suits one larger plant, such as a pepper, or several small greens if the lid is cut for more net pots.
Does the container need a lid?
Yes. The lid holds the net pots, keeps light out of the solution and helps keep the air gap humid for the roots.
Can I use a metal container for Kratky?
It is best avoided. Nutrient solution is slightly acidic and can corrode some metals, which can release unwanted minerals into the water. Glass and food-grade plastic are safer choices.
Key points
- Size the container to the plant; bigger and longer crops need more solution.
- Use food-safe materials.
- Block all light from the solution to prevent algae.
- Keep containers out of strong heat.
- Make it easy to check the water level.