By Luke Sumpter

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.

  1. Seedlings

  • Seedlings and clones respond best to higher humidity levels of 65–75% RH as root systems are not yet well-established. Clones and young plants take in moisture through their leaves.
  • Room temperature with lights on should be 20–25°C. Lights off, 15–20°C.
  1. Vegetation

  • During vegetation, lower the humidity by 5% per week. Between 40–70% RH is acceptable. Roots are now established and the plants are beginning to transpire.
  • Room temperature with lights on should be 22–28°C. Lights off, 18–22°C.
  1.  Flowering

  • During flowering, low humidity is important. Decrease to 40–50% RH. Over 60% can mean slowed growth and pathogen risks.
  • Room temperature with lights on should be 20–26°C. Lights off, 16–22°C. The goal is to imitate autumn, so temps need to be kept low.
  1. Late Flowering

  • 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.
  • 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.
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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.

Cool Temperature For Growing Cannabis

  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.

Indoor Cultivation At Night In Summertime

  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.

Indoor Cannabis Ventilation

  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.

Light For Cannabis


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.


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.

You might feel it in the hip pocket to start with, but it will balance out over the life of the equipment. The investment is relative to multiple crops over a number of years. Smelly, fat, home-grown buds have a mystique all their own. No matter the season, they are always worth it. Happy summer growing.

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