Indoor Cannabis Temperature by Grow Stage
The short answer: keep your grow room warm while the lights are on, a little cooler when they are off, and above all avoid heat peaks. A well-cited study on cannabis leaves found photosynthesis works best between 25 and 30 degrees Celsius (77 to 86 degrees Fahrenheit), but only under strong light. Many of the exact ranges per stage you see online are grower rules of thumb, not research results.
What the research says about temperature
Researchers at the University of Mississippi measured how cannabis leaves respond to light, temperature and CO2 (Chandra et al., 2008). They tested 20, 25, 30, 35 and 40°C. The highest rate of photosynthesis was at 30°C with a light level of about 1500 µmol m⁻² s⁻¹, and they concluded the optimum lies at 25 to 30°C.
Two things are easy to miss in that result.
- It needs strong light. At 20 to 25°C, photosynthesis kept rising with more light. At 30°C it only rose up to about 1500 µmol and dropped above that. Unless you know your lamp delivers that much at the canopy, assume the 30°C peak does not apply to you. With weaker light, this study gives no reason to turn the room up to 30°C.
- It measured leaves, not harvests. The study looked at gas exchange in leaves. It did not measure yield, bud quality, smell or THC. A temperature that suits a leaf on a measuring device is not automatically the best temperature for the buds.
Heat means thirst
The same study found that transpiration, the water a plant loses through its leaves, kept rising with temperature all the way to 40°C. Stomatal conductance, which is how far the leaf pores open, increased up to 30°C and dropped above that. The authors also saw that high temperature combined with strong light reduced photosynthesis and water use efficiency. In practice a hot room can make your plants drink more without growing more. If you have to water much more often than usual, check the temperature before you change your feed.
Vegetative stage
During the vegetative stage the plant builds leaves, stems and roots, and this is where the photosynthesis result above is most useful. A warm, stable room with good light should support fast growth. If your light is modest, aim for the cooler end of the 25 to 30°C range rather than the top of it.
Measure at canopy height, not at the floor or next to the door. The top of a tent, under the lamp, is often warmer than pot level, and the top is where most leaves are. Check both lights-on and lights-off, because the swing between the two tells you more than one reading at noon.
Seedlings and clones are a separate case. They have small roots and lose water easily, so they are more sensitive to dry heat than an established plant. We cover that in temperature and humidity for seedlings.
Flowering stage
In flower the question changes from "how fast does it grow" to "what happens to the buds". Recent work from Wageningen University on medicinal cannabis found that higher air temperatures during flowering lowered cannabinoid concentrations, through abnormal formation of the flowers, and that the effect on yield differed between genotypes (Holweg, PhD thesis, Wageningen University, 2025). The summary does not give the exact temperatures tested, so we will not turn it into a number. The direction looks clear: heat in flower can cost quality, and how much depends on the strain.
So in flower, the goal is to stop the room from getting hot rather than to make it warmer. Late in flowering the buds are dense and humidity becomes the bigger risk. Warm air that cools sharply when the lights go off can leave moisture in the buds, which can set bud rot off. Read preventing bud rot indoors and humidity for indoor cannabis together with this article, because temperature and humidity affect each other. VPD is the way to look at both at once.
Night temperature
With the lights off, the room cools down. A moderate drop is normal and fine. Two things cause trouble: a big, fast drop that makes humidity spike, and a room that gets really cold, such as a tent in an unheated attic or garage in winter. We found no study that sets a hard minimum night temperature for cannabis, so we do not give one. See growing in winter for that situation.
Keeping the temperature under control
- A thermometer and hygrometer at canopy height. Ideally one that records the minimum and maximum, so you see what happens when you are not there.
- Ventilation and air movement. An exhaust fan removes the heat the lamp produces, and small fans inside the tent break up hot spots. More on that in grow room ventilation and air circulation.
- Insulation. A grow room against an outside wall follows the weather. Insulating walls and sealing doors makes it far easier to hold a steady temperature.
- Heating or cooling where needed. A small heater for cold nights or an air conditioner for summer. Running the lights at night, when the house is coolest, is a cheap trick in summer.
If your plants already show heat damage, such as curled or bleached top leaves, see managing excessive temperatures and light burn and heat stress. For how temperature fits with humidity and light stress as a whole, read environmental stress factors.
Our view
We grew and keep all our own base genetics, and every strain in our range has been grown here at least once. Some of us have close to 40 years of growing behind us. What that teaches is mostly modesty about fixed numbers: strains differ, rooms differ, and the Wageningen thesis points the same way for heat in flower. Use the research as a frame, measure your own room, and watch the plant. A steady room without heat peaks beats chasing a perfect figure.
Temperature is only one part of the picture. Light intensity, feeding and pH matter just as much, and a temperature that works under a strong lamp will not suit a weak one.
When your room is under control, choosing the right genetics is the next step. Our premium-quality marijuana seeds from Amsterdam Marijuana Seeds include strains for every kind of setup, and our advice is the same for all of them: grow with passion, not for the money.
