best temperature to grow cannabis

Indoor Cannabis Growing: Relative Humidity and Temperatures

The most refined techniques to grow cannabis become irrelevant when relative humidity and temperatures are not being controlled – learn more about these two major factors.


Final results of an indoor grow are greatly influenced by the way growers keep in control of parameters that influence their plants growth. There are two basic factors that can easily be forgotten when we’re busy thinking of other ways to increase yields, size, and overall health of our plants – temperature & relative humidity. This blog summarizes ways to keep both of these factors within an optimum range, and provides specific information what conditions should be maintained to achieve best results.


It’s important to know that humidity levels and temperatures are closely related to one another. When we talk about humidity, we usually mean relative humidity (RH), which is the ratio of partial pressure of water vapor to the maximum vapor pressure of water at the same temperature. You get the whole idea when knowing the basic principle that warm air holds more water vapor than cold air. This is one of the reasons why it’s necessary to extract a lot of warm air from our grow room, and ideally allow cool air to enter – warm air simply holds too much water vapor in it.


We need to define what humidity and temperature control actually means when growing cannabis. It makes sense to divide the life of cannabis plants into 4 different stages in which humidity levels, and temperatures, should be adjusted to ensure healthy growth. Don’t think that humidity and temperature control is complicated and not worth it! It’s generally very easy, and more about keeping parameters within a certain range, and as constant as possible.

The first thing you need to do is to buy a hygrometer and thermometer, preferably a digital one with memory function, also showing maximum and minimum values of the past. Some hygrometers aren’t the most accurate, so don’t bother having several devices in your grow room to compare values. Now that we’re able to closely monitor our conditions, we can get to the essence of humidity and temperature control – the actual humidity levels and temperatures we aim for.

1. Seedling Stage

  • Seedlings and clones like high humidity levels of 65-70%
  • Reason: The root system is not established
  • High humidity levels allow water intake through leaves
  • Temperatures with lights on: 20-25 C° (lights off: 4-5 C° lower)

2. Vegetation Period

  • Humidity levels can be lowered by 5% each week (acceptable range: 40-70%)
  • Temperatures can be increased a little bit (no obligation)
  • Reason: Roots absorb more water; evaporation through leaves cools plant(s)
  • Temperatures with lights on: 22-28 C° (lights off: 4-5 C° lower)

3. Flowering Period

  • Humidity levels need to be lowered to 40-50% (extremely important)
  • You can get away with 55% (anything over 60% is real bad)
  • It’s best to slightly lower temperatures in flowering
  • Temperatures with lights on: 20-26 C° (avoid high temperatures)

4. Late flowering (1-2 weeks before harvest)

  • The following steps are no necessity, but can improve yield, flavour and appearance
  • Bring down humidity levels as much as you can: 30-40%
  • Lower daytime temperatures, and also increase the temperature difference (day/night)
  • Temperatures with lights on: 18-24 °C (lights off: minus 5-10 C°)


We’ve got a pretty good idea on humidity levels and temperatures we aim for. Now it’s time to get to the practical part, and to find ways to bring things back in balance when they’re not. Most growers will struggle to keep both relative humidity and temperatures down, which is of primary importance in the flowering period – we got that. In some colder regions, and depending on the lighting solution, the opposite scenario might be the case, and temperatures or humidity levels must be raised.

Remember the basic principle that warm air holds more water than cold air? Keep this in mind, and be aware of the fact that relative humidity and temperatures interact with one another.

Finding ways to control humidity and temperatures is crucial when growing cannabis indoors. This blog shows practical steps for best results.

Keeping Temperatures Down In Summer For Your Indoor Grow

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.


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.

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.

When addressing heat concerns, humidity will also need to be considered. Heat and humidity go hand-in-hand. Warmer air is more capable of carrying moisture, raising the relative humidity (RH) of the grow space. Even well-vented spaces will not drop in humidity if the inlet air is warm and already moisture-laden. Humidity can be a marijuana flower’s worst enemy, causing ideal conditions for pests and pathogens.


Cannabis grows best when provided with ideal temperatures and humidity levels for each phase of growth. Staying within these limits is the challenge facing the summertime grower.

Late Flowering


Cost can be the primary factor in deciding whether to grow in summer or not. When yield-to-cost is considered, a summer crop is always cost-effective. Mechanical intervention is more expensive to run, but the relative amount of cannabis produced outweighs the costs ten fold.

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.

  1. 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.
  2. 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.
  3. 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.
  4. 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.
  1. Grow at night. Make your day cycle during the night. Expect a 10–15°C difference in ambient air temperature at night.
  2. 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.
  3. 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.


Having complete control over the grow room environment will mean installing mechanical solutions. Initial outlay or price of upgrading will be offset by fat buds before summer ends.

  1. 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.
  2. Use exhausted light covers to control radiated heat from globes. This is a minimal investment, and should be considered when setting up a grow.
  3. 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.
  4. 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.
  5. Opt for larger intake and exhaust fans. Larger fans running on a lower setting are more effective than small fans running on overdrive.
  1. Make sure all ducting is unobstructed and kink-free, and avoid sharp angles when changing direction. Interrupted airflow makes the fans less efficient.
  2. 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.
  3. 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.
  4. 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.
  5. 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.

Increasing overall metabolism helps plants resist the effects of heat. Bigger, healthier, and more robust plants will tolerate environmental extremes better. When CO₂ density is increased to 1,200–1,500ppm, ideal gas exchange happens at 29°C.

The extra few degrees of heat is a good thing when using carbon dioxide. However, enhanced plant metabolism means extra demands all around. The plants will take up more water and nutrients, and will require extra ventilation. Check the nutrient reservoir more often.

Marijuana grows best when it's kept within a particular temperature and humidity range. Here are the top tips for growing summer weed indoors.