Keeping Humidity Under Control

The trick to growing healthy plants is controlling their environments. Most plants can only thrive under fairly specific conditions, and growing them outside of those conditions can lead to reduced productivity and faltering growth. The majority of growers understand that they need to control the water supply, nutrients, and even the temperature, but they often overlook humidity. That is a shame because it can have just as much of an impact as any other environmental factor.

Why Humidity Matters

Almost every plant has structures called¬†stomata. They are little holes in the leaves that let carbon dioxide to enter the plant and oxygen to leave. The plants can’t photosynthesize without that gas exchange, so the stomata are vital to healthy plants. Unfortunately, they also allow some water to escape when they open to release oxygen.

The rate at which the leaves lose water depends on the humidity. If the area has low humidity, the plants lose water quickly. If the area is very humid, they lose it slowly. On the other hand, high humidity can encourage mold and bacteria to grow, which can easily kill a plant. This means that every plant has an optimal humidity level where it can retain enough water to thrive, but it will not succumb to mold.

Which Plants Care?

All plants have their own growing conditions. In general, plants that evolved in arid environments need low humidity, since they evolved ways to conserve water in spite of losing some while exchanging gas. Plants from humid environments need the opposite since they are usually more resistant to mold than they are to water loss. It’s important to look up each plant’s requirements, but most vegetables that are commonly grown need a relative humidity of 50% or so.

Humidifiers and Dehumidifiers

The easiest way to adjust the humidity in a room is to install a dedicated humidifier or dehumidifier. Some areas will need both because humidity can vary significantly from one season to the next, but others will only need one or the other. A relatively small one will be sufficient in most cases.

It is surprisingly easy to overuse these devices, so it is also necessary to use a hygrometer to make sure that the room is at an appropriate humidity level. Some models will have an integral hygrometer, but a separate one will work just as well and make it easier to test the room’s humidity prior to installation.


A simple fan can also help to compensate for high humidity. Circulating air helps to prevent mold from growing on the plants, which is one of the biggest problems that high humidity can cause.

Small, rotating fans are ideal. High power is not necessary, and extreme winds can even damage plants, so it is safer and more convenient to keep the unit small. The leaves on the plants should move slightly in the breeze, but the bulk of the plant should stay stationary instead of blowing around.

Ventilation fans can also be used to exhaust excess heat and humidity from the grow space. They can be used continuously, or periodically with the use of a timer or used with an atmospheric control unit that measures your temperature and relative humidity and turn the exhaust fan on when values climb above your settings.