Does Grow Light Heat Warm Kratky Water?
It can. Grow light heat warms Kratky water when the light is close, the space is enclosed or the containers are dark and small. LED lights put out less heat than older lamps, but they still warm the air around them, and a small container of still water picks that warmth up.
Most of the time, a well-placed LED doesn’t warm the solution enough to matter. It becomes a problem in a closed closet or tent, on the top shelf of a lit rack, or in a warm room in summer.
Why Warm Solution Is a Problem
Warm water holds less dissolved oxygen than cool water. In Kratky, nothing adds oxygen to the solution, so the roots under the surface depend on what is already there. As the solution warms, it holds less, and roots under stress are more open to rot.
As a guide, solution between about 65 and 75°F (18 to 24°C) suits lettuce and most herbs. Above about 80°F (27°C), the risk of root problems climbs.
How to Check
- Put a thermometer in one container, or stick one to the outside below the water line.
- Check it after the light has been on for several hours, which is when it is warmest.
- Compare a container on the top shelf with one on the bottom.
A quicker check: touch the container. If it feels warm in your hand, the solution is warmer than it should be.
Signs of Heat Trouble
- Roots turning brown or slimy instead of white or tan.
- A sour or rotten smell from the container.
- Plants wilting while there is plenty of solution.
- Leaves curling or scorching at the top of the plant, nearest the light.
How to Keep It Cool
- Raise the light. Move it up a few inches. The light distance guide covers the right height.
- Move air. A small fan carries warm air away from the light and the plants.
- Use light-colored containers. White or light opaque containers absorb less heat than black ones. Keep them opaque so light can’t reach the solution.
- Use bigger containers. More water warms more slowly.
- Run the light in the cool part of the day. On a timer, set the light period overnight in summer if the room heats up in the afternoon.
- Keep the light’s driver away from containers. On many LEDs the driver box gets warm; hang it outside the tent or away from the solution.
Is Some Warmth Ever Good?
In a cold basement or garage in winter, a little warmth from the light can help bring a cold solution up to a better temperature. The goal is steady, cool-to-mild solution, not as cold as possible.
Common Mistakes
- Lowering the light to speed growth and cooking the top of the plant.
- Using black containers on a lit shelf in a warm room.
- Never checking the solution temperature until roots have already rotted.
Common Questions
Do LED grow lights get the water hot?
Not usually, if the light is at the right height and air can move. In a closed space, a close light or a warm room, they can warm the solution enough to matter.
What temperature should Kratky water be?
About 65 to 75°F (18 to 24°C) suits lettuce and most herbs. Above about 80°F, root problems become more likely.
Can I put ice in the container to cool it?
It cools the solution only briefly and dilutes it. Raising the light, adding airflow and using a larger, light-colored container work better.
Related guide: growing Kratky under grow lights.
Related article: How Do You Grow Kratky in a Closet or Grow Tent?