Water Vapor in Grow Rooms: Moisture, and How to Measure It

Part I: Relative Humidity

Plants need airborne moisture. In addition, the water vapor (or humidity) in your grow room can determine how healthy your plants are. Healthy plants require proper humidity levels for each stage of growth.

What is humidity and how is it measured?

Humidity is the amount of water vapor present in the air. There are three common ways to measure it in the grow room. This includes:

• Relative humidity (RH)
• Grains per pound
• Dew point temperature

In this article, we’ll talk about relative humidity, which is expressed as a percentage: %RH.

What is relative humidity?

Relative humidity denotes what the “air/water mixture” is at a particular temperature. The higher the percentage of water to air, the more humid the air. The higher the air temperature, the more water the air can hold. If the air temperature drops sufficiently, water will condense out of the air and form droplets on surfaces.

A dehumidifier does its work in a grow room by removing moisture from the air as the temperature drops (as it does when the lights shut off). Moisture returns to the air when the temperature rises again, after the lights come back on. This means that in a sealed grow room, moisture is continually pulled out of the air by the dehumidifier when temperatures are cooler and then returned to the air when temperatures are warmer.

Why is it so important to be able to measure %RH in a grow room?

Different stages of plant growth need different levels of relative humidity. If %RH is too high or too low, plants can become stressed. This can reduce harvests and/or result in problems like powdery mildew.

What are optimal levels of %RH in a grow room?

In general, seedlings need 70 to 80% RH, 50 to 60% RH through the start of the flowering phase, and 40 to 50% RH at the flowering phase. Additionally, some growers find improved flower production below 40% RH – even as low as 20% during the last two to three weeks of flowering.

In the next article: Learn about grains per pound (gpp) as a measure of moisture in your grow room.