Everything you need to know about growing cannabis at home

Relative Humidity and Cannabis Plants: Your Questions Answered

by | Apr 25, 2022 | Cannabis, Environmental Threats, Getting Started

Humidity can play an important factor in your grow space to help ensure you have healthy cannabis plants. Relative humidity, or RH, can be defined as the amount of water vapor in the air compared to the maximum amount that could exist at that temperature. If there is too much humidity for cannabis plants, it can cause mold growth, powdery mildew, and other problems. Learn more about humidity and how it affects your plants here!

What are humidity and relative humidity?

Humidity can be defined as how much water vapor exists in the air and is measured by grams or kilograms per cubic meter. Relative humidity, or RH, refers to how much of this water vapor actually IS present in relation to what it would need to be for the air to be completely saturated and is measured as a percentage up to 100%, which is full saturation. A relative humidity of 100% means that the air has as much water vapor as possible; 50% RH means half of what could be present before reaching saturation point.

Why is humidity good for cannabis plants?

While humidity can cause problems for your cannabis plants, it is important to ensure humidity levels are high enough. High humidity ensures the plant has access to water and prevents dryness in the grow space which reduces risks of damage or disease. It is very important to have both temperature control and humidity control in your indoor grow space and we recommend buying a thermometer and hygrometer to maintain ideal environmental conditions in your grow tent or indoor space. For outdoor conditions, there are not many things you can do to help your plants as you are at the mercy of mother nature.

What happens if there is too much humidity for cannabis plants?

If humidity levels are higher than optimal humidity for your stage of growth, humidity can cause issues such as mold or mildew. Too much humidity is not good for your cannabis plants and should be prevented to ensure healthy growth.

Common problems that occur with too much humidity are:

  • Bud Rot
  • Mold Growth
  • Powdery Mildew

What happens if there is not enough humidity for cannabis plants?

If humidity levels are too low for your plants, it causes problems such as dryness which leads to a risk of damage or disease. Problems that occur from a lack of humidity are a lack of hydration because your plant has trouble drawing water through the roots. This occurs due to the rate of transpiration, or the way plants breathe. If humidity levels are lower than the optimal range, the plant will try to combat this by transpiring, which means losing water vapor through leaves. When this occurs, it can lead to dryness in your cannabis plants also lead to other deficiencies.

What is the ideal relative humidity for the germination and seedling stage of cannabis plants?

The relative humidity during germination and seedling phases is the highest during the life of the plant. It is important to ensure humidity levels are high for germination because cannabis plants have access to little water at the seedling stage. In fact, a common problem for new growers is overwatering seedlings and their mediums. If humidity isn’t maintained, your plant may dry out or become damaged due to a lack of humidity which makes it hard for seeds and young plants to survive until they can be transplanted into larger pots with proper humidity levels. The ideal humidity levels are between 65%-70%. You can always use a dome or clear plastic cup over the seed or seedling to help raise RH levels.

What is the ideal humidity for vegetative growth of cannabis plants?

During the vegetative stage, humidity levels should be maintained in a wide range between 40-70% RH to ensure your plants have enough humidity without causing any problems such as mold or mildew from too much humidity. It’s important to maintain a high enough humidity during this stage because it helps prevent dryness and helps your plant get enough water to stay hydrated and grow.

What is the ideal RH for the flowering stage of cannabis plants?

During the flowering growth stages, humidity levels are best between 40%-60% RH. This is the ideal humidity range for cannabis plants to ensure they have enough humidity without mold or mildew occurring due to too much humidity in your grow space. Maintaining a RH of around 50% is ideal although it will vary with temperature. It is important for the humidity not to be too high during this stage as it can invite bud rot into the cannabis buds or colas.

What is the ideal humidity for drying and curing cannabis plants?

When drying your cannabis it is ideal to maintain a RH of around 60%. The rule of thumb for drying is that your temperature should be around 60°F and 60% RH although there are some slight variations on it. During curing, humidity should be maintained between 55-65%. The process of curing cannabis helps reduce risks for bud rot and prevent humidity from being too high. It also helps the humidity get down to around 60% RH for optimal curing conditions without committing any mistakes that could cause humidity levels to be higher than desired.

To achieve your optimal RH during curing, you can always throw humidity control packs into your storage container. There are several companies that make these packs to control humidity levels to around the 58%-62% range, which is ideal.

How do humidity and temperature affect one another?

Humidity and temperature are related to each other. Temperature affects humidity by influencing the amount of water vapor in your grow space, which will cause humidity levels to rise or fall depending on if there is more or less water vapor present. It also influences how easy it is for your plant to transpire, meaning moisture loss through leaves despite humidity being high. Also, at different temperatures, different humidities may be necessary to grow marijuana plants. To get a better idea of what humidities are ideal for which temperatures we suggest looking at a VPD chart.

What is VPD?

VPD stands for Vapor Pressure Deficit and it is a way of measuring humidity in the air. It doesn’t just measure humidity, but also temperature and how close they are to each other. VPD can help you determine what humidity levels should be like at different temperatures during specific stages of growth such as germination, vegetative growth, or flowering. Please take a look at our VPD chart and our article about VPD to learn more.


Humidity is important in all stages of cannabis plant growth. The way most cannabis growers measure humidity is by relative humidity, which measures the percentage of how saturated the air is on a scale from 0% to 100%, with 100% being the most saturated the air can be at that temperature. We listed the ideal RHs above for all stages of plant growth. If your humidity is too high, your plants become susceptible to various problems including bud rot, fungus, and powdery mildew. If your humidity is too low, your plants are susceptible to drying out by not being able to transpire at the proper rate to allow the roots to take water. To figure out what RH is ideal for the temperature you are growing in, please refer to our vapor pressure deficit (VPD) chart.

Thank you for reading this article, should you have any further questions please read our articles referencing the ideal temperature to grow cannabis plants and on VPD.

To learn more about growing cannabis at home consider checking out our guides on: Getting Started, Troubleshooting, and Optimizing your Grow or search our Knowledge Base.

We strive to help your plants thrive.