For a lot of indoor growers, that is all you need to worry about. If it feels too hot or too cold for you in your grow area, it’s probably too hot or too cold for your cannabis plants as well.
The Vegetative Stage: Best Grow Room Temperature
If your grow room feels warm or cold, humid or dry, that is a sign that you may want to look into changing the temperature or humidity of your grow area.
The Flowering Stage: Best Grow Room Temperature
Cannabis plants like a temperature similar to humans, or a little warmer — not too dry, not too humid.
The best grow room temperature during the flowering stage of growth is 68–75 degrees during the day and no more than 10–15 degrees cooler at night. If you’re supplementing with CO2, daytime temps can be as high as 75–82 or so. During flowering, you should lower your relative humidity to 35–45% and even lower (30%) for the last couple of weeks before harvest. This will help you avoid issues with mold, bud rot and PM (Powdery Mildew) that can arise in higher humidity.
Room temperature with lights on should be 18–24°C. Increase the difference between day/night temps by up to 10°C for even better results.
Basic Hygrometers and Thermometers
During late flowering, improvements to overall quality can be achieved by keeping temps and humidity as low as possible. Humidity under 40% RH is ideal.
Cannabis plants like a temperature similar to humans, or a little warmer — not too dry, not too humid. For a lot of indoor growers, that is all you need to worry about. If it feels too hot or too…
- Grow at night. Make your day cycle during the night. Expect a 10–15°C difference in ambient air temperature at night.
- Bottles of frozen water, rather than ice cubes are a resource-friendly way to add cool to the room. A 2-litre bottle in front of a fan cools the airstream considerably, without adding moisture to the air. Ice cubes evaporate and increase humidity.
- Use insulation where practical. If building a grow room, insulate the complete structure well. Foam core panelling is quick and easy, is very soundproof, and also easy to clean. Flexible rolls of insulation can be used to drape over grow tents or pack around smaller spaces. Insulation also reduces the heat signature of clandestine grows in cannabis-unfriendly countries.
- First, you will need a hygrometer and a thermometer, or a combo unit. This way, you can maintain environmental conditions with complete accuracy. Advanced kits act as switches that run the complete environmental system automatically.
- Use exhausted light covers to control radiated heat from globes. This is a minimal investment, and should be considered when setting up a grow.
- Dimmable ballasts give you some control over the radiated heat from lights. Turning down the wattage during the hottest parts of the artificial day, but not interrupting the grow cycle, can cut heat output in half from the lights, and keep plants growing steadily.
- Make sure air circulation is thorough. Still air is the enemy of marijuana. Put pedestal fans and mount wall fans where they provide the best air movement around plants. Always have them set on oscillate.
- Opt for larger intake and exhaust fans. Larger fans running on a lower setting are more effective than small fans running on overdrive.
Growing indoor marijuana in summer can present some unique challenges. Cannabis grows best within a particular temperature and humidity range. These top tips will help you keep your indoor cannabis happy this summer.
Growing in summer can be a challenge. Especially so for those grows closer to the equator. A common sense, passive setup with mechanical intervention when necessary makes summertime growing drama-free.
- Make sure all ducting is unobstructed and kink-free, and avoid sharp angles when changing direction. Interrupted airflow makes the fans less efficient.
- Air conditioning units are relatively inexpensive and will ensure correct temperatures in summer. When connected to a temperature switch, temps will automatically stay in the desired range. Where possible, connect the AC unit to a fresh air intake to aid in air exchange. Reverse cycle units will act as heaters when winter arrives.
- Optimum humidity can mean adding or subtracting moisture from the air. Combination humidifier/dehumidifier units ensure stable humidity. Set the unit to the ideal humidity for each phase of growth. Check reservoirs regularly and fill or empty as needed. Lower humidity means lower temperatures are more easily maintained.
- LED lighting can reduce temperatures significantly. They use energy differently than HID lights and produce far less heat. Growing with LEDs is a bit different, so do some research first. The lack of heat is what will need to be considered when the cooler months arrive.
- Reservoir chillers are electrical units that keep water in the reservoir at an ideal temperature. Protecting roots from heat prevents plants from slowing their growth due to heat stress. The cool water flowing through the system also acts like a passive air conditioner, helping to keep ambient temps down.
- By nature, electronics generate heat. Keep as many motors, ballasts, pumps, and timers that run on electricity out of the grow room. It ends up being far more convenient to have a single spot where everything is accessible at a glance.
- Have your grow space on the northern side of the house in the Northern Hemisphere. These rooms never receive direct sunlight on their roofs or walls.
- Source fresh intake air from a cool place. Sub-floor is ideal, but fresh air is the absolute minimum requirement. Put exhaust outlets as far away as practical for your situation. Try and vent to the outside, or at least another room or roof space. A 400W lamp will heat a standard room very quickly.
- If the grow room is on a sunward side, especially the west side of a building, make efforts to shade the room. Plant quick-growing vines to protect the walls. Install awnings or sunshades.
Cannabis grows best indoors under high-intensity discharge (HID) lights, which do their best to imitate the sun. The side effect of generating lots of lumens is heat. Most heat is created by light ballasts, but globes still emit enough heat to burn plants. Add to this some high summer temperatures, and environments can quickly become unsuitable for proper growth.
Summer is coming, and the lucky outdoor grower is stoked. Plenty of sun, wind, and even rain makes for superb cannabis. For the indoor grower, summer is a different story with its own unique set of challenges.
Before spending money on equipment, make sure your grow space optimises passive heat control measures. Providing the coolest ambient air temperatures is the first step in combating summer heat.
Marijuana grows best when it's kept within a particular temperature and humidity range. Here are the top tips for growing summer weed indoors.