Short answer: Oklahoma State University Extension gives 72 to 75 °F (about 22 to 24 °C) as the optimal nutrient solution temperature for hydroponics. For lettuce and other cool-season greens, cooler is safer: Cornell University's lettuce research found a quality crop could be grown in a warm greenhouse only if the root zone stayed below 20 °C (68 °F). In a Kratky container the main job is to stop the solution getting warm.
Why temperature matters more in Kratky
A Kratky container has no pump and no air stone. The roots get their oxygen from the air gap that forms as the level drops (UC Master Gardeners) and from what is dissolved in the water. The U.S. Geological Survey notes that cold water can hold more dissolved oxygen than warm water. So as a container warms up, the water holds less oxygen at the very time the roots are using more.
Cornell's research recommended dissolved oxygen of at least 4 mg/L for good lettuce growth and saw severe stress below 2 mg/L. That work used aerated systems; a still Kratky container has no way to top the oxygen back up, which is why warm weather is when Kratky growers most often see root problems.
Warm water and root rot
Low oxygen weakens roots, and weak roots are where water molds such as Pythium take hold. Penn State Extension describes the results: brown, dead root tips, stunted plants and wilting at mid-day. See root rot in Kratky for the signs and what to do.
Which number to aim for
The two sources are not in conflict. Oklahoma State University Extension gives a general figure for hydroponic solutions; Cornell's figure comes from lettuce specifically, grown under warm greenhouse air. My own rule of thumb follows both:
- Lettuce and leafy greens: keep the solution below about 68 °F where you can, especially in summer.
- Other crops: aim for about 72 to 75 °F, and treat anything well above that as a warning.
- Cold: very cold solution slows growth, so I avoid filling containers straight from a cold tap in winter.
How I keep containers cool
- No direct afternoon sun. A small container in full sun can warm up far faster than the air around it.
- Light-colored, lightproof containers. Lightproofing stops algae; a light color reflects heat. If I wrap a jar, I use a light wrap rather than black.
- Bigger volume. More water changes temperature more slowly, which is one more reason to size containers generously.
- Room-temperature fill. I fill containers with water at room temperature, not straight from a hot or very cold tap.
- Grow with the season. Lettuce in the cooler months, heat-tolerant crops in summer.
Measuring temperature
Many EC and pH meters show the solution temperature, which is an easy way to keep track. Temperature also affects the readings themselves. Meters with automatic temperature compensation correct for it; with a basic meter, measure at a similar temperature each time so readings stay comparable. Oklahoma State University Extension also recommends checking at the same time of day.
Signs a container is running too warm
- Plants wilting in the afternoon and recovering overnight
- Roots turning tan or brown instead of white
- A sour smell or cloudy solution (see why Kratky solution turns cloudy or smelly)
- Lettuce bolting or turning bitter in hot weather
Moving containers through the seasons
Room temperature changes through the year, and so does the risk. In summer I look for the coolest spot indoors, such as a basement or a room that gets no afternoon sun, rather than a sunny windowsill. In winter a windowsill can do the opposite and chill a jar overnight. A cheap thermometer next to the containers tells you more than the thermostat does. If the solution regularly reads warm in one spot, I move the containers rather than trying to cool the water: moving them deals with the cause, while adding cold water raises the level and floods the air roots.
Quick questions
Can I add ice to cool a container?
I would not. Ice raises the level and can flood the air roots as it melts. Moving the container somewhere cooler is safer.
Does solution temperature change the EC?
The amount of nutrient does not change, but the reading does. That is what temperature compensation on a meter is for.
Related guide: water for Kratky nutrient solution.
Sources
- Dunn, B. and Singh, H. Oklahoma State University Extension. Electrical Conductivity and pH Guide for Hydroponics (HLA-6722).
- Both, A.J. Ten Years of Hydroponic Lettuce Research (Cornell University Controlled Environment Agriculture program).
- U.S. Geological Survey Water Science School. Dissolved Oxygen and Water.
- UC Master Gardeners of Orange County. Hydroponics for the Home Gardener.
- Moorman, G.W. Penn State Extension. Pythium.