Any indoor gardener can tell you that temperatures affect the indoor growroom environment, yet humidity often gets overlooked. Inattentive growers may not realize until after the fact that humidity also plays a role in the outcome of their produce.

Whether you are starting seedlings, rooting cuttings or drying herbs or vegetables, the ambient outdoor humidity can drastically change conditions indoors. Therefore, in addition to temperature, humidity must also be addressed during all growth stages.

Maintaining proper humidity is especially important in the early stages of plant growth. Plants are usually able to withstand a broader humidity range after they have formed a healthy root system and hardened into the environment.

However, different plant varieties are affected in different ways, so you have to control the humidity according to the specific varieties you’re growing.

How to Control Humidity in the Growroom

To answer the question, “Do I need a humidifier?” growers first need to monitor humidity readings in the growroom by using a hygrometer, a.k.a. a humidistat. The necessary humidity range for most indoor crops lies between 3-55%, with an ideal target range of 40-45%.

Humidity rising above 60% can lead to molds, mildews, and other unwanted biological growth. Furthermore, high humidity prevents plants from properly taking in CO2, preventing efficient photosynthesis. In this case, a dehumidifier can be used to lower the humidity to an appropriate level.

Humidity falling below 30% can lead to reduced transpiration capabilities, which slows growth and prevents efficient photosynthesis. In this case, a humidifier or ultrasonic fogger can be used to raise the humidity to an appropriate level.

Growers can achieve these results economically by placing the device on a timer and monitoring resulting humidity levels, or if your budget allows, a hygrometer controller can be used to trigger the device only when needed.

As mentioned earlier, seasonal changes in outdoor humidity also affect the overall humidity inside the growroom. Climates with lower-than-average ambient humidity, such as the dry, mountainous regions of Colorado, can sometimes see humidity levels as low as 10% in winter months.

In this situation, a humidifier can be a useful tool to quickly and effectively bring humidity levels back up. There are other means of raising humidity that can happen by default. For example, a growroom has its own ambient humidity range, depending on its ventilation configuration, plant density and plant transpiration rates.

Ventilation and Humidity in the Growrom

The most practical method of controlling humidity is often through ventilation. Humidity can easily be lowered by increasing, or raised by decreasing, the amount of air being exchanged throughout the room.

This is usually done by adding a can-style exhaust fan. To increase the effectiveness of the exhaust fan, an intake hole can be added or widened, resulting in an increased amount of air being moved through the room.

A light baffle may need to be constructed around the intake hole to prevent light leakage and still allow plenty of airflow.

For more reliable automation and increased efficiency, a hygrometer controller can be used to power the fan rather than a timer or thermostat. Because plants technically need fresh CO2 rather than constant fresh air, many rooms configured with CO2 injection systems have little or no ventilation.

Rooms configured in this fashion use dehumidifiers to control humidity and extra air conditioning to control temperature. The enclosed area and lack of ventilation causes a rise in humidity.

Furthermore, the increased metabolic rates the plants achieve from the added CO2 lead to increased transpiration and an additional increase in humidity. In this situation, a humidifier would not be needed.

Transpiration of Plants and Its Effect on the Growroom Environment

Plant density also impacts humidity levels. The air in the room is displaced by increasing the number of plants, which also act as windbreaks and prevent other areas of the room from receiving fresh air and CO2.

In addition to air displacement, additional plants cause a rise in humidity due to increased transpiration. Without proper ventilation or dehumidification, the water vapor transpired by the plants has nowhere to go. For this reason, as plant density increases in the garden, so too will ventilation needs.

In this scenario, if the exhaust fans cannot keep up with the high humidity created from increased transpiration, a dehumidifier may also be necessary. If the high level of humidity is not corrected, transpiration is hindered, and overall plant metabolic rates decrease.

To prevent lowered metabolism and the slowing of plant growth, keep in mind the impact of plant density and transpiration and be proactive by controlling humidity levels before these issues arise.

Humidity During Germination & Propagation

While germinating seeds or rooting cuttings, it is extremely important to keep humidity levels much higher than normal (80-90%) to promote initial root growth.

If humidity is too low, the vapor pressure deficit will encourage transpiration to occur too rapidly to allow energy for root development. A humidifier or ultrasonic fogger may be added to ensure relative humidity remains at an appropriate level.

Another useful tool to keep humidity levels high enough for seedlings or cuttings is a humidity dome, which can be placed over the young plants to create a micro-climate where humidity levels remain higher than ambient levels.

Another thing to consider while germinating seeds or rooting clones is that although the relative humidity needs to be kept much higher than ambient levels, the moisture level at the roots should be treated normally by giving them plenty of oxygen in between waterings.

Dehydration, Drying & Curing

Humidity plays a vital role in proper dehydration, drying, and curing methods. Dehydrating foods requires a high amount of ventilation and an initial low level of humidity. Some produce must be dried and cured slowly instead.

In a well-constructed area, the environment is controlled in such a way that the process occurs slowly over time, allowing chlorophyll to break down and other important chemical processes to occur.

If this process is rushed, chlorophyll can be trapped inside, negatively affecting the taste, quality and overall outcome. To manipulate the amount of humidity and air exchange, small exhaust fans are used. In a sophisticated processing area, fans are placed on either a thermostat or hygrometer controller, depending on which variable is more important to control.

If the ambient humidity is too high or too low to be controlled initially by the use of ventilation fans, a humidifier or dehumidifier can be used to bring the relative humidity to an appropriate ambient level, where it is then controlled by fans. Finally, further controlling the temperature sometimes requires heaters or air conditioners.

Vapor Pressure Deficit in a Sealed Growroom

It is often said that the environment is the main factor affecting the quality of your harvests. Humidity is undervalued and growers focus mostly on temperature, which is also important.

The role temperature and humidity play together, as well as the amount of air exchange, or more specifically, the amount of CO2 available to the stomata throughout the day, make up key components of a controlled environment that can greatly determine the quality and yield of an indoor garden.

Consulting on the NASA-funded X-Hab project at the University of Colorado in Boulder, I recently heard a term that was new to me after more than 11 years of experience and research in indoor gardening.

The PhD students used the term vapor pressure deficit (VPD) to describe the environment, or more specifically, the relationship plants have to temperature and humidity and their ability to transpire in a given environment.

Understanding VPD helps growers aggregate important environmental factors into one parameter. (See: Why Vapor Pressure Deficit Should Matter to Greenhouse Growers)

Temperature and humidity are arguably the two most important environmental factors in the garden. It is the plant’s ability to transpire and metabolize nutrients that is effected by these two important factors, and their relationship with each other can either assist or prevent a plant from performing at its full potential.

VPD measures the difference between the relative humidity and the amount of humidity the air can hold until saturation. Temperature greatly affects humidity and air pressure, so it is also a part of the VPD equation. As temperature rises, air expands and can hold more water vapor. As temperature falls, air contracts and loses humidity through condensation.

VPD is a measurement of the drying power of the air, or how much the plant is encouraged by the environment to transpire. When humidity is too high, the plant cannot transpire as quickly as it could before.

The amount of humidity and pressure in the air relative to the amount of water in the leaf is too high to encourage transpiration. If relative humidity (RH) is brought down to the proper level, the water in the leaf is encouraged to transpire by the lower humidity and air pressure.

As mentioned before, temperature also plays a vital role in this process. When temperatures are too high, a plant will need to transpire more rapidly than normal to cool itself off.

So, in a high-temperature, low-humidity room, the VPD is great enough to encourage transpiration to cool plants off, but with a lower VPD, such as in a high-temperature, high-humidity room, plants are not able to cool themselves off as easily via transpiration.

Most plants generally grow well with a VPD between 0.8 and 0.95 kPa.


Gardeners need to pay close attention to temperature and humidity levels, as well as how they affect each other, to fully understand how environmental factors can impact plants. If both temperature and humidity levels are high, plants are not encouraged to transpire and growth slows.

To encourage transpiration and maximize growth potential, a greater VPD must be reached by either decreasing relative humidity or increasing temperatures. Because increasing temperature levels can further raise humidity levels and stress plants, the former is the most optimal choice.

If both temperature and humidity levels are low, plants are not encouraged to transpire and growth slows. To encourage transpiration and maximize growth potential, a greater VPD must be reached by increasing temperatures to a safe level while maintaining lower humidity levels.

Understanding VPD will not only enable a grower to make necessary corrections to the environment, but also anticipate environmental problems before their effect has negatively impacted their crop.

For maximum yields and quality, humidity levels need to be controlled with the use of a hygrometer controller, fans, and/or a dehumidifier or humidifier.

It is important to understand how temperature affects relative humidity and their combined effect on transpiration rates. Your plants will thank you for learning the roles these environmental factors play in the garden, and how and when to control them.

For more tips on growroom humidity, check out Maximum Yield's humidity article archives.