The Best Heat and Humidity for Your Grow Room

Whether using a compact indoor grow cabinet or a high-end, complete indoor grow tent package, it’s crucial to use the ideal heat and humidity. Just like any plant on earth, cannabis plants thrive when their conditions allow them to grow to their fullest potential. Finding out the optimal temperatures and humidity for your cannabis crops is the key to having healthy, happy plants.

While light sources and nutrition are commonly discussed topics, many often overlook the best heat and humidity for growing cannabis plants.

Choosing the best heat for your plants.

At different stages of growth, your plants prefer different heat and humidity settings. While LED setups typically don’t generate as much heat, fluorescent and HID bulbs discharge a large amount of radiant energy.

Using an effective cooling and ventilation system is a must when using any of these setups. A cannabis plant being roasted alive by your lights is a recipe for disaster, leading to death or in worst case scenarios – a fire.

A digital thermometer coupled with a humidity controller is instrumental for any successful grow. Ensuring you have the appropriate temperature for your particular plants will help ensure healthier plants, larger yields, and possibly higher cannabinoid concentrations.

Effective methods of heating/cooling your plants include the use of electronic heating pads, the lights you use, fans, and ventilation. Leave your plants at a constant temperature and monitor the ambient heat from other sources, including sunlight if your plants are near windows.

Sprouting and seedling stages

While just sprouting and in seedling form, a temperature between 65-80°F/18-26°C is optimal. Typically, sitting right in the middle is safe, though your independent setting may vary, depending on which strains you’re growing. While the lights are off, drop your ambient temperature by ~40°F/5°C.

This temperature is ideal since your plant hasn’t developed large enough leaves yet, hindering the rate at which it can transpire and breathe.

Vegetative stage

The best temperature to grow cannabis increases towards 75-85°F/23-29°C during this stage. The plants naturally become larger, and thus are able to withstand slightly higher maximum heat through evaporation.

Carefully watch your plants if you do increase the temperatures, as many strains are sensitive to even a slight increase in heat, and will develop problems as a result. Floating between 75-80°F/23-25°C is a safe choice.

You’ll find at these higher temperatures your cannabis plants are able to grow quickly and develop stronger, healthier stalks too.

Flowering stage

During the final stages of growth in the flowering stage, drop the temperatures back down towards 65-80°F/18-26°C, as it was during the seedling stage. The taste and smell of your buds will vary drastically depending on the heat used at this point. Potency can take a hit, growth may slow down and diseases like powdery mildew and root rot occur with higher frequency at higher temperatures.

Choosing the best humidity for your plants.

While the temperature at which your plants grow at is an important factor, picking the right humidity for growing your plants at is another crucial step. Likewise, with picking the ideal temperature, the best setting may vary depending on the strain itself.

With improper humidity control, the likelihood of crops developing bacterial diseases or pests multiplies. Root rot, powdery mildew, mold, and mites can occur to your precious cannabis plants very easily if left unattended.

Some effective ways of controlling your humidity include the use of a humidifier, an ultrasonic diffuser, damp towels or even a spray bottle. Monitor your humidity controller so you know when to adjust the climate accordingly. When trying to lower humidity, simply introduce more ventilation to the grow area.

When using a large, complete indoor grow package, since there’s more area to be covered, it can be difficult at times to maintain a high humidity, especially throughout the early stages of growth. On the flip side, a small solution such as an indoor grow cabinet or compact grow box will likely have higher ambient humidity, requiring an effective ventilation system.

Seedling & sprout stage

Humidity during the seedling stage should remain high since the seed hasn’t had to opportunity to fully develop roots yet. A healthy humidity at this point is ~70%, +- 5%. The majority of the hydration your plant will get at this point is through its leaves, so a high humidity is a must. Especially with a complete indoor grow tent, it can be difficult to maintain a constant 70% humidity.

Once the plant starts developing lush leaves, it’s safe to begin slowly dropping the humidity. At this point, you’re likely approaching the next stage of growth.

Vegetative stage

The root system of the plants has started to develop and will begin delivering water to the plant this way. It’s recommended to lower your humidity by around 5% weekly, with a minimum threshold of 40%.

Budding & flowering stage

Once the plant shows signs of budding and flowering, it’s time to drop the humidity to at most 50%. Bacterial growth and mold become a serious factor, and at this point, can be a major setback. Towards the later points of flowering, dropping the humidity to between 30-40% is highly recommended. When using a compact solution like an indoor grow cabinet, it’s easy to accidentally let the humidity rise to dangerous levels.

The humidity at which you leave your grow area during this stage greatly influences the flavor, scent, terpene and cannabinoid content too. Though all the stages are important, having the correct humidity at this point is certainly the biggest contributor to end product!

Heat & Humidity Cheat Sheet

In short, the best heat and humidity settings for each stage of growth are as follows:

• Seedling & sprout stage: 70% +- 5% humidity, 65-80°F/18-26°C degrees.
• Vegetative stage: 40-60% humidity, optimally around 50%, 75-85°F/23-29°C degrees.
• Budding & flowering stage: 30-40% humidity, 65-80°F/18-26°C degrees.

During each of these stages, lower your ambient temperature by around ~40°F/5°C if you’ve turned your lights off. Carefully monitor the plants daily and ensure they’re growing properly.

The settings at which your temperature and humidity are set to can vary based on the strain you’re growing, individual genetics of your particular strain, and even your grow space. Ensure you have appropriate climate control solutions to adjust your temperature and humidity easily. Few things are more heartbreaking than checking in on your plants and discovering they’re wilting or hosting spider mites!