# Kratky (passive, non-circulating hydroponics)

> Kratky is hydroponics with no pump: the plant sits above a container of nutrient solution, drinks it down, and the growing air gap gives the upper roots oxygen. It was developed and published for lettuce and tomatoes at the University of Hawaii. Cannabis can be grown this way, but its large water use means big containers, careful refills that keep the air gap, and close attention to water temperature and EC drift.

- Canonical: https://genetixbay.com/knowledge/systems/kratky.html
- Category: Systems · Level: Beginner
- Evidence: Supported · Status: Draft: awaiting expert review (some figures are being checked)
- Stage: Seedling, Veg, Flower · Systems: DWC
- Updated: 2026-10-31 · Licence: CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0/), attribute "GenetixBay Knowledge Base (genetixbay.com/knowledge)"

## Questions and answers

**Can you grow cannabis with the Kratky method?** Cannabis can be grown with the Kratky method, though that is grower practice; the published research covers lettuce and tomatoes. Kratky uses no pump or [air stone](https://genetixbay.com/knowledge/glossary/air-stone.md): the plant drinks the solution down and the growing air gap gives the upper roots oxygen. It suits [autoflowers](https://genetixbay.com/knowledge/glossary/autoflower.md), small [photoperiod plants](https://genetixbay.com/knowledge/glossary/photoperiod.md), [mothers](https://genetixbay.com/knowledge/glossary/mother-plant.md) and clones, kept in large containers with solution at 18–22 °C.

**How big a Kratky container for cannabis?** A full-size cannabis plant in Kratky needs a large container, roughly 40–100 litres per plant, a grower estimate worth verifying. Cannabis drinks far more than lettuce, and a big photoperiod plant can empty a 20-litre tote within days in flower. Use an opaque, light-tight container, and choose smaller plants such as autoflowers if space is limited.

**Should I refill a Kratky reservoir to the top?** Do not refill a Kratky reservoir to the top. As the level drops, the upper roots adapt to the humid air gap, and refilling to the brim submerges those air roots, inviting rot. Refill only partway, to about half to two-thirds of the original level, or with weak solution, a grower approach extrapolated from Kratky's tomato research.

**Why are my Kratky roots rotting?** Kratky roots usually rot because the air roots were submerged by topping up too high, or because the solution got too warm. With no aeration, Kratky has less margin than [DWC](https://genetixbay.com/knowledge/glossary/dwc.md), so keep the solution at 18–22 °C (64–72 °F) in a light-tight container. If roots turn brown and slimy, add an air stone and convert the system to DWC.

**Short answer.** Kratky needs **no pump, no air stone and no electricity**: a [net pot](https://genetixbay.com/knowledge/glossary/net-pot.md) sits in a lid over nutrient solution; the plant drinks the level down, and the expanding **air gap** gives the upper roots oxygen while the lower roots keep drinking [1][2]. It's proven for lettuce and was extended to tomatoes with refills [3]. Cannabis drinks far more than lettuce, so you need **large containers**, refills that **don't drown the air-root zone**, and **cool solution**.

## How it works

At the start the solution touches the bottom of the net pot. As the plant drinks, roots follow the water down; the upper root mass becomes "air roots" in a humid gap. Refilling to the top would submerge those air roots, which are not adapted to anaerobic water, inviting rot [1][3].

## Numbers that matter

| Parameter | Target | Notes |
|---|---|---|
| Starting level | Just touching net pot base | [1] |
| Refill | Only partway (e.g. to ~1/2–2/3 of the original level), or with weak solution | |
| Solution temperature | 18–22 °C (64–72 °F) | No aeration = less margin than DWC |
| [pH](https://genetixbay.com/knowledge/glossary/ph.md) | 5.6–6.2 | Drifts up as plants feed |
| [EC](https://genetixbay.com/knowledge/glossary/ec.md) | Start 0.8–1.4 [mS/cm](https://genetixbay.com/knowledge/glossary/ms-cm.md) | EC often rises as water use outpaces uptake |
| Container | Opaque, light-tight | Algae otherwise |

## How to run it

1. Best for **autoflowers, small photoperiod plants, mothers and clones**.
2. Start with a [seedling](https://genetixbay.com/knowledge/glossary/seedling.md) or rooted clone in a net pot with [clay pebbles](https://genetixbay.com/knowledge/glossary/clay-pebbles.md).
3. Check level, pH and EC every few days; add plain water if EC climbs fast, weak feed if it falls.
4. Keep the container cool and dark.
5. If roots go brown and slimy, convert to DWC with an air stone.

## Jardin notes

Jardin method "Kratky (monitor only)": a Root node in monitor mode reads pH/EC/water temperature and level or mass; dosing is disabled in software and channels unmapped, so no actuator can misbehave. Alerts like "EC rising fast, add water" and "water temp 26 °C". Automation level 0. It's the cheapest entry to the data side and the natural path to DWC [4].

## Common mistakes

- Topping up to the brim.
- Small containers for big photoperiod plants (they drink a 20 L tote in days in flower).
- Clear containers.

## Evidence

The method is *Established* for lettuce and *Supported* for tomatoes with refills [1][2][3]. Kratky for cannabis is *Grower practice*.

## Sources

1. Kratky BA, 2004. A suspended pot, non-circulating hydroponic method. Acta Horticulturae 648:83–89. https://www.ctahr.hawaii.edu/hawaii/downloads/A_Suspended_Pot_Non-circulating_Hydroponic_Method.pdf
2. Kratky BA, 2009. Three non-circulating hydroponic methods for growing lettuce. Acta Horticulturae 843:65–72. https://www.ctahr.hawaii.edu/Hawaii/downloads/Three_Non-circulating_Hydroponic_Methods_for_Growing_lettuce.pdf
3. Kratky BA, Maehira GT, Cupples RJ, Bernabe CC, 2005. Non-circulating hydroponic methods for growing tomatoes. Proc. National Agricultural Plastics Congress 32:31–36. https://www.ctahr.hawaii.edu/Hawaii/downloads/Non-Circulating_Hydroponic_methods_for_growing_tomatoes.pdf
4. GenetixBay Jardin engineering notes, 2026 (system profiles and failure margins).