What are the best indoor ambient temperatures and relative humidity levels for maximum photosynthesis?

By Eric Hopper | Last updated: February 26, 2019

Finding the optimal temperature for a CO2-enriched growroom is essential for maximizing your return on CO2 equipment. Regardless of the type of artificial lighting system being used, a growroom with enriched CO2 levels will have a higher optimal operating temperature than a growroom without CO2 enrichment.

The reason you have heard about the importance of leaf surface temperature is because plant metabolism is affected by temperature and most plant metabolism occurs within a plant’s leaves. The main type of plant metabolism indoor growers are concerned with is photosynthesis.

Photosynthesis is a chemical reaction that has multiple variables which affect each other. In other words, if you alter the temperature or CO2 level (assuming everything else stays the same), you will affect the rate at which photosynthesis occurs. The main contributor to the temperature of the leaf surface is the grow room’s ambient air temperature. In other words, if you control the air temperature, you can mostly control the leaf surface temperature.

That said, changes in relative humidity can affect the leaf surface temperature as well. The stomata on the leaves open during daylight hours to transpire moisture, which cools the leaf surface through evaporation. This means, for most indoor gardens, the leaf surface temperature is already slightly lower than the ambient air temperature. A higher relative humidity will slow this evaporative cooling process and can actually raise the surface temperature of the leaf.

All plants and growing spaces are a little different, which means determining the optimal temperature, relative humidity, and concentration of CO2 will take experimentation. If you plan to enrich the garden with 1,200 to 1,500 ppm of CO2, a good starting point for your ambient temperature would be 80°F.

If you are unable to achieve a temperature this high with the LED lighting system, you may need to add a heater to your grow room. A good starting point for relative humidity would be 55-60 per cent. From there, you can make slight adjustments to find the optimal combination of temperature, humidity, and CO2 levels.

Personally, I would not worry too much about the leaf surface temperature as the ambient temperature will affect this variable the most. If you do wish to raise or lower the leaf surface temperature without affecting the ambient temperature, you could experiment with raising or lowering the relative humidity.

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Written by Eric Hopper | Writer, Consultant, Product Tester

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Eric Hopper’s past experiences within the indoor gardening industry include being a hydroponic retail store manager and owner. Currently, he works as a writer, consultant and product tester for various indoor horticulture companies. His inquisitive nature keeps him busy seeking new technologies and methods that could help maximize a garden’s performance.

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