Whether large or small, with indoor growing, you are basically creating a climate for your plants. The right temperature and humidity levels are important for establishing and maintaining healthier crops. Avoid extremes when controlling levels inside your grow room or grow tent, and fine-tune them to fit your plants' specific needs.
Use these links to quickly navigate to more specific information.
- Why Controlling Temperature and Humidity Is Important
- Signs of Extremes in Temperature
- Signs of Extremes in Humidity
- Air Circulation and Ventilation
- Temperature and Humidity Monitoring
- How to Control the Temperature Level
- How to Control the Humidity Level
- Other Tips
Why Controlling Temperature and Humidity Is Important
From germination through growing stages and flowering or fruiting, you can create an optimal environment for your plants by controlling the temperature and humidity levels. Plant metabolism needs cooler nights in order to rest--with a drop of about 10 degrees being desired.
Plants are constantly taking in water to absorb nutrients and grow to their full potential. Both temperature and humidity can affect how much water your plants take in, so that controlling these levels also gives you control over the plants’ nutrient intake.
Signs of Extremes in Temperature
When temperatures rise too high or fall too low, your plants are adversely affected.
When plants are too hot, you may notice:
- Leaves that are yellow or have brown/yellow spots
- Leaves that are curled up or wilted
- Blooms or fruit dropping off
- Blossom end rot for scalded plants
When plants are too cold, you may notice:
- Wilting or drooping leaves or stems
- Splits in woody stems
- Very loose root balls
- Spots on flowers and leaves that look burned
Signs of Extremes in Humidity
Humidity levels are important in indoor growing since they affect transpiration (plant breathing) and the ability of plants to absorb nutrients. Poor levels can also create an environment ripe for disease and pests.
When humidity levels are too high you may notice:
- Mold and bud rot that can make plants completely unusable
- White powdery mildew on leaves
- Excessively damp leaves
- Underdevelopment of young plants and slow growth
When humidity levels are too low, you may notice:
- Slowing or stopping plant growth
- Burning on the tips of leaves (too much nutrient intake due to lack of moisture)
Early growing stages require higher humidity levels than the rest of the cycle. It’s common to add a humidifier to grow spaces during the early germination or clone growing stage, while a dehumidifier may be required later in the growing process.
Air Circulation and Ventilation
Proper airflow in your grow area provides fresh air for temperature and humidity modulation. This can help fight off problems like white powdery mildew brought on by high humidity levels and other issues that can affect your plants.
Increasing the ventilation in your space with a powerful exhaust fan is a good solution for increased indoor humidity levels for people who live in areas with low relative humidity (the air coming in is lower than the humidity level inside).
Ventilation fans can also help increase or decrease room temperature if desired. In addition, CO2 settles at the bottom of your room, so airflow is important for the life of your plants as well.
Temperature and Humidity Monitoring
Adding a monitor is the best way to know the exact temperature and relative humidity in your grow room. Various options are available, many of which allow for remote monitoring from other rooms.
If you’re looking for precise growing conditions, these are fairly inexpensive and the best way to monitor temperature and humidity levels.
Control Temperature Level
Temperature and humidity are closely linked, so being able to maintain the right temperature in your grow room can also help control humidity levels. In addition, plants need cooler nights which can occur naturally, but may need help at times.
Even for well-insulated grow rooms, high temperatures can be problematic if you live in a hotter climate. And, if your grow room isn't accessible from your ducted HVAC system, cooling can be challenging.
One of the best ways to do this is with a Ductless Mini-Split AC. These systems have a condenser unit that operates outside the structure (building) and an air handler that is usually wall-mounted inside the grow room.
If temperatures tend to drop too much during cold months, many of these units also have heat pumps.
For smaller growing spaces, using a portable AC or a Through-the-Wall unit can be helpful in bringing down and maintaining the right temperature.
For grow rooms in colder climates, the logical answer is to add a sturdy heater that can produce more significant heat than most small room heaters. Both King Electric and QMark offer a variety of wall-mountable and portable heaters with adjustable thermostats and heat settings.
Control Your Humidity Level
Do you live in an area with high humidity, or does your grow room suffer from high relative humidity? Do you have difficulty keeping enough moisture in the air of your growing space?
The best way to lower humidity in grow rooms that tend to retain high levels is with a dehumidifier. Dehumidifiers pull moisture from the air in grow rooms or grow tents. They come in a variety of sizes, from portable units for small rooms to large, industrial dehumidifiers for large grow spaces. 
Keep in mind that dehumidifiers collect moisture, so they need to be emptied or drained.
Quest dehumidifiers are the most popular grow room dehumidifiers for commercial settings. If you're a hobbyist with a small grow room, a durable crawl space dehumidifier may work for your needs. Review all dehumidifiers here.
For grow rooms that tend to lose moisture easily, the most effective way to increase or maintain the correct RH is to add a humidifier. Look for large humidifiers with several-gallon tanks for less frequent refills. If you're using your HVAC system to control temperature, adding flow-through or drum humidifier to your system could be a good plan.
Common indoor-growing ducted humidifier brands include Honeywell and Aprilaire. Aircare is a trusted brand for standalone humidifiers. Review all humidifiers here.
Other Tips
- Take care of standing water - this can add more humidity to your space than you may think.
- Consider lights - when the lights are turned off, the temperature will drop. This also leads to an increase in relative humidity.
- Contact us at 770-224-7069 or email us if you have any temperature or humidity questions related to your grow room.
