Kratky in Full Sun: Stopping the Solution Overheating

Basil growing outdoors in a container in full sun, where Kratky solution can overheat

Shade the container, not the plant. Kratky in full sun works when the leaves get the sun and the container doesn’t. Use light-colored, opaque containers, keep them large, and shade their sides with a box, a board or other plants so the solution stays cool.

Full sun is a gift for basil, peppers and tomatoes, but a container of still water sitting in it all afternoon can get warm enough to harm the roots.

Why Full Sun Heats the Solution

A dark container in direct sun absorbs heat all day, and a small one has little water to soak it up. Warm solution holds less oxygen, and in Kratky the roots below the surface depend on the oxygen already in the water. Hot solution also encourages root rot.

As a guide, try to keep the solution below about 80°F (27°C). On a hot day a black container in the sun can go well past that.

How to Keep It Cool

Choose light colors. White or light-colored opaque containers reflect sun. If yours are dark, wrap them in white fabric, reflective insulation or light-colored tape.
Go bigger. More water warms more slowly. A large tote stays cooler through an afternoon than a small bucket.
Shade the sides. Set containers inside a slightly larger box or crate, behind a low board, or among other pots so the sides stay out of the sun.
Double up. Stand the container inside a second, larger one with an air gap between them. The outer one takes the sun.
Get them off hot surfaces. Concrete and dark decking get very hot. Put containers on a wooden pallet, a rack or a mat.
Use afternoon shade. If you can move the containers, give them morning sun and afternoon shade during heat waves.

How to Check

On a hot afternoon, put your hand flat on the side of the container. If it feels warm, the solution is warm.
For a number, put a thermometer in the solution in the late afternoon, the hottest point of the day.
Check the roots once a week. White or tan is healthy; brown and slimy means heat trouble.

Crops That Handle Summer Sun

Basil, peppers, tomatoes and cucumbers like sun and warmth, in large containers with plenty of solution.
Lettuce and leafy greens bolt quickly in summer heat. Grow them in spring and fall, or in a shaded spot in summer.

Big summer plants drink a lot in the heat, so expect to add solution part-way through. Add it only up to the lower roots, so the upper roots stay in the air gap.

Common Mistakes

Using black buckets in full afternoon sun.
Standing containers straight on hot concrete.
Growing lettuce on a south-facing balcony in July.

Common Questions

What color container is best for Kratky in the sun?

A light-colored container that is still opaque. White or light gray reflects sun; it must still block light to the solution, so wrap thin white plastic if light shines through.

How hot is too hot for Kratky solution?

Try to keep it below about 80°F (27°C). Above that, roots are more likely to rot.

Can I put Kratky containers in full sun all day?

The plants can have sun all day, but shade the containers themselves, especially in the afternoon.

Related guide: growing Kratky on a balcony or patio.

How Do You Stop Kratky Solution Overheating in Full Sun?