In order for your cannabis plants to thrive, they need optimum temperature and humidity conditions. Cannabis grows best when kept at a temperature between 68 and 77 degrees Fahrenheit (20 and 25 degrees Celsius), and humidity levels are between 40% and 70%. The temperature also shouldn’t vary by more than 18 degrees during the day (lights on) and night (lights off) cycle.
It is a good idea to keep your plants at the higher end of the temperature spectrum for the vegetative stage, and at the lower end for flowering. As for humidity, seedlings like 65% to 70%, with this gradually lowering during the vegetative stage (by about 5% per week) until flowering, when humidity should be around 40% to 50%. Finally, a week or two before harvest the humidity should be between 30% and 40%.
Altering the humidity in your grow tent helps prevent mould and keeps your plants thirsty so that they keep a high intake of nutrients and water. However, if humidity drops below 30% the plants will react to prevent dehydration by becoming dormant, so be careful.
When growing indoors, you are in control of your plants’ environment. It is up to you to monitor and maintain optimum growing conditions for your plants, and thankfully there are lots of tools out there to help you do this.
Basic Tools to Measure Humidity and Temperature in a Grow Room
The two most important tools to measure temperature and humidity are thermometers and hygrometers. They often come in an all in one tool called a hygro-thermometer, which can save you money and time by avoiding having to buy two separate tools and constantly check both.
Here are some models that we would recommend:
Extech 445713 Big Digit Indoor/Outdoor Hygro-Thermometer
This measuring tool is a favourite for its accuracy, being able to measure temperature down to +/-1.8 degrees and humidity down to +/-5%. It also has a min/max function, which allows you to view temperature and humidity fluctuations over the period of time since you last actively used it.
To record a measurement, simply stick the probe into the shady soil under your plants and wait. The measurements will be displayed on the large screen, which is easy to read even in low lighting. When not actively in use, hang or place the hygro-thermometer inside or outside your grow tent.
8inch Copper Finish Decorative Indoor/Outdoor Thermometer by Bjerg Instruments
This analogue wall thermometer may look elegant and classy, but it only measures temperature, not humidity. If you only have a couple of plants growing, then this thermometer will do, but you will need to get something else to measure humidity. It is however much cheaper that the Extech (above).
Ambient Weather WS-07-C Big Digit 8-Channel Wireless Thermo-Hygrometer Display Console
The price point for this tool is in between the two above, and it measures both the temperature and humidity in your plants’ grow space. It is slightly less expensive that the Extech because it doesn’t have a soil probe, but it still measures the grow environment of your plants.
How to Control the Conditions in your Grow Space
First off, you need to identify how climate-controlled your grow space is, and how much ventilation you have. If you are growing in a garage, your plants will be more exposed to the elements. In the winter for example, you might need to buy a space heater to keep your plants warm.
If you are growing inside in a climate-controlled area such as a bedroom, then you will need to think about ventilation. If you are using a grow tent or grow box or cabinet, then you will have much more control over your plants’ environment.
You will also need to be careful of what kind of grow lights you will use. HPS (or high-pressure sodium) lamps can be a great choice for growing cannabis, but it is important to note that they run hot and use a lot of electricity. If you have a small, closed off setup, then avoid HPS grow lights.
LED grow lights are a great choice for growing weed, as they run cool and don’t use up huge amounts of electricity. LEDs are a good idea of you have a small setup, or live in a country with higher temperatures and/or naturally higher humidity.
Grow Room Heaters
Here are some options for space heaters if your grow area tends to be on the cold side:
Holmes Digital Bathroom Heater Fan with Pre-Heat Timer and Max Heat Output, HFH436WGL-UM
This heater has a built-in thermostat and timer, and tolerates high levels of humidity well. It only has one setting though, so be careful not to point it directly at your plants. For the low price point, this little heater does the job.
Apollo Horticulture 10inch x 20inch Seedling Heat Mat for Propagation and Cloning
Another way to keep your plants warm is to use a heat mat, like this one from Apollo Horticulture. This one is best suited to seedling and clones, as it raises the temperature by 10 to 20 degrees F to encourage root growth. This heat mat is also waterproof, which is extremely useful for obvious reasons!
Grow Room Fans and Cooling
If you live in a place where the temperatures are high even at night and there are high levels of humidity naturally, you will need ways of cooling down your grow space so your plants can continue to thrive.
You can do this by using air conditioning and/or ventilation fans, such as the ones below:
Frigidaire 5000 BTU 115V Window-Mounted Mini-Compact Air Conditioner with Full-Function Remote Control
This small window unit can cool and dehumidify air in a room up to 150 square feet large. It runs on household current, and comes with a removable, washable air filter to keep the air smelling fresh. You can also direct the air flow to keep it away from blowing directly onto your plants.
Honeywell HT-900 TurboForce Air Circulator Fan
Circulatory fans are a great way to regulate temperatures in your grow space. This one from Honeywell runs quietly and has three different blade speeds to suit your requirements. You can place it in your grow space or hang it on a wall.
Grow Room Humidifiers
If the natural humidity levels where you live are lower than 30%, you will need to invest in a humidifier to grow your weed. In some places, you might be able to get away with spraying your plants or leaving open containers of water in your grow space, but if you live in a dry area, humidifiers are the way to go.
Here are some of our favourites:
Holmes Cool Mist Comfort Humidifier with Digital Control Panel, HM1865-NU
This humidifier can run up to 48 hours without a refill, and has four settings for even more control. It has a timer, and will also shut off once it reaches the desired humidity level, so you don’t have to worry about accidentally drowning your plants. It does emit cool mist though, so keep an eye on your temperature to make sure it doesn’t drop too much.
Holmes Warm Mist Humidifier HWM6008-NUM
This humidifier is also from Holmes, but is not as state-of-the-art as their cool mist humidifier, or as long lasting – it needs a refill every 24 hours. Again, be careful with the temperature of your grow space, as this emits warm mist.
Grow Room Dehumidifiers
If you live in an area where high humidity is a problem, then you may be looking for a dehumidifier instead. Air conditioners will work too, but plain circulation fans will not.
Here are some dehumidifiers for you to browse:
Ivation GDM20 Thermo-Electric Dehumidifier 1100 Cubic Feet
This silent dehumidifier condenses the water in the air and fills the transparent cup at the bottom, shutting down when this becomes full until you empty it. It works well in small grow spaces, just make sure you monitor humidity levels in case it works too well.
Frigidaire FAD504DWD Energy Star 50-pint Dehumidifier
Instead of filling a cup that needs to be emptied, this dehumidifier can be linked up to a hose that runs into a drain, so it can work around the clock. This is especially handy if you have a large grow space with lots of plants on the go.
DampRid FG50T Hi-Capacity Moisture Absorber 4-Pound Tub
This small, affordable tool is great for budget growers with a small grow space. Make sure you regularly check your humidity levels though in case they drop too much.
Grow Room Controllers
Grow room controllers can monitor temperature, humidity, CO2 levels, and even grow lights and watering too! Having everything in one place makes growing so much easier, both for traditional soil and hydroponic grow systems.
Here are some of the best available:
Titan Controls Hyperion 1 Wireless Environmental Controller
This Titan control panel can be mounted on a wall up to 150 feet away from your grow space and keeps track of temperature, humidity, and CO2 levels. You can also add a lighting controller, so it can manage your grow lights too. This control system is great if you have lots of growing to do but not a huge amount of time to be constantly checking thermometers.
AutoPilot MDT Master Digital Combination Recycling and Lighting Timer, APCTMDT
This control system is made by Hydrofarm, a company that specialises in hydroponics. It keeps optimum temperature and humidity conditions for your weed, as well as being able to manage your grow lights and control CO2 levels with its CO2 injection setup.
Titan Controls 702850 Saturn 4 Digital Day and Night Environmental Controller with Photocell
Another controller from Titan Controls, the Saturn 4 is an analogue control system that manages a dehumidifier, a fan and a CO2 device. It uses the photocell to determine when it is night time (or lights off) for your plants. You can plant its 15-foot cable in the soil, as it has a remote sensor at the end which it uses to measure your plants’ environment.
Other Grow Tools
Finally, timers are very important, as they remind us when it is time to switch the grow lights on/off, among other things. The Intermatic DT122K 15 Amp Two-Outlet Heavy Duty Digital Indoor Timer is one of many that will do the job. It has two outlets for two different devices such as a grow light, a dehumidifier, a fan or a heater.
Hopefully, you have learned something from this guide to temperature and humidity control, and you have some idea of the products you need to suit your personal grow space. Happy growing!