Humidity, simply put, is the amount of water in the air. Even more, warmer air can hold more water than cooler air. So for garden purposes let’s talk Relative Humidity, or the amount of moisture in the air as a percentage of the maximum for a given temperature. Humidity affects plants much like it does people. For example, 80°F feels much hotter in Florida than it does in California. Why? The Relative Humidity of course! While the temperature may be the same in both locations, the Relative Humidity in Florida will be close to 80% but only 35% in California. The more moisture that is being held in the air, the less the air can continue to take on. At a Relative Humidity of 80% the air is unable to evaporate your perspiration like it would at 35%, and this is how the human body cools itself. When there is less Relative Humidity, the air is able to “absorb” the moisture coming off of your body, and in turn, cools you off.

Similarly, plants also “sweat” or transpire. Only about 5% of the water plants absorb through their roots is retained, the other 95% is transpired back into the air through the plants “pores” or stomata. Almost all the water is just passing through the plant. This constantly cools the plant, keeping it from overheating and drying, so that the plant can continue on with growth and flower production. Also note that plants only photosynthesize when the stomata are open. If the stomata close, photosynthesis completely stops.

Back to relative humidity. We know that the more humidity that is in the air, the less moisture the air can “absorb”. We also know that plants must transpire 95% of the water they absorb so they can keep cool and keep photosynthesizing. So, if humidity is too high in the room the plants will not be able to transpire and will not be photosynthesizing or growing properly. Not transpiring also means they are holding water, which means they are not drinking more water, so they are not taking in nutrients. High humidity is also ideal for mold and mildew. On the other end, if humidity is too low, the air will practically pull the moisture right out of your plants and they will close their stomata in defense again shutting down photosynthesis.

The lesson here: If humidity is too low or too high, your plants are not growing.