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The temperature setting in a grow room is an important and complicated concept. Photosynthesis, build-up of sugars, respiration and growth all react differently to the temperature. It must therefore be carefully considered what’s most important. In addition, heating your grow room is very expensive. A good insight into the influence of the temperature on your marijuana plants is therefore important to achieve the best results possible.
Influence of temperature on marijuana plants

Influence of temperature on marijuana plants

The processes that take place inside the marijuana plant are mainly chemical ones. The rate of a chemical process is usually higher at higher temperatures, of course up to a certain maximum. Unlike humans and animals, plants can’t create warmth themselves, so the temperature in your grow room is very important. The temperature of a marijuana plant is therefore determined by the temperature of the environment, the amount of light and the evaporation of water from the plant.

How light is used Photosynthesis
Influence of temperature

A good example is the difference in sensitivity to temperature of the photosynthesis and the growth of organs the produced sugars go to. Within certain limits, photosynthesis is quite insensitive to temperature. For marijuana plants, this is between 59 and 86 degrees. For photosynthesis it therefore doesn’t matter if it’s 62 or 82 degrees. The plant can produce the same amount of sugars in both cases.

The distribution of the sugars of the leaves to the places they’re needed is however sensitive to temperature. The distribution of sugars is inhibited when it gets colder than 70 degrees. The sugars will accumulate, which can be harmful to your plants. If that only happens for a few days, it will solve itself as soon as it gets warmer and the stored sugars will still be distributed. This is only the case for mature plants. The growth of young plants is immediately inhibited when this accumulation takes place.

The respiration of a plant is also very sensitive to temperature. The maintenance respiration decreases when it gets colder. This is, in principle, a positive effect, because this is the energy the plant uses to stay alive. But if a heavier crop is created because of the lower temperature, the total crop respiration will increase. So when the temperature increases, it will use more energy in order to stay alive, leaving less energy to grow. Fortunately, there has been a lot of research on the ideal values for marijuana plants to grow optimally.

Temperature in your grow room

The temperature in your grow room is influenced by various factors. An important one is the location inside the building; a basement is a lot cooler than an attic with a flat roof. In addition, the size of the room, the airflow, the number of lamps and the air extraction play a major role. So take this into account when building your grow room.If the light is on, a temperature between 70 and 77 degrees is ideal for cuttings and seedlings. As the plants grow older, they can evaporate a bit more, and the temperature may increase to a maximum of 82 degrees. The temperature has to be between 62 and 72 degrees when the lights are off. Another important rule is that the temperature differences should not become too large, no more than about 20 degrees. So if it’s 82 degrees during the day, it should not be colder than 62 degrees at night. A temperature difference of about 10 degrees between daytime and night time is ideal.

Temperature in your grow room
Temperature in grow room

The different parts of the plant all react differently to the temperature. The most temperature sensitive part of the plant is the new growth. At a low temperature, the new growth can’t make new cells as quickly, so a new leaf or bud is formed more slowly. If young plants experience low temperatures, fewer leaves are split and the leaves will become thicker. This eventually harms photosynthesis, because there are fewer leaves, so less sugars will be produced.

The temperature of the new growth is therefore the most important part for the development of the plant. Therefore, it’s not desirable that the new growth is colder than the rest of the plant. Make sure you keep this in mind when applying cooling or ventilation from above.

Measuring the temperature in your grow room

You can measure the temperature in your grow room with a thermometer. You can choose between a digital or analogue thermometer. The cheapest ones only cost a couple of bucks. I always use a digital thermometer/hygrometer with a memory setting, so I can see what the highest and lowest temperature has been. It’s also smart to get one with a temperature sensor on a string, so you can place the display outside of the room, so you can also check the temperature at night. Check this one on Amazon.com

Marijuana temperature
Measuring the temperature

Always measure the temperature in the shade and at different spots in your grow room. Make sure you have a good airflow by placing several fans in the room. The lamps produce radiant heat that doesn’t affect the air temperature. Underneath the lamp it will therefore always be a couple of degrees warmer than in a shaded area. This is no problem, but just make sure there’s enough distance between the lamp and the plant, so the buds don’t literally burn.

Lowering the temperature in your grow room

Heat is often a problem in grow rooms. This is mainly because of the heat that’s generated from the lamps we use. Fluorescent lamps aren’t so bad, but HPS lamps can easily heat up a room to 122 degrees, which is fatal for your marijuana plants. First of all, you should consider the size of the room. Make sure you have at least 3’3” x 3’3” x 6’6” of space for a single 600 watt HPS lamp. Use the following formula for air extraction; number of watts / two = air extraction per 35 cubic feet. So 2 x 600 watt HPS lamps require 21000 cubic feet air extraction. The air extractor blows warm air out and sucks fresh air in. You can attach a carbon filter to this, so you won’t blow out a marijuana smell. Use ballast, preferably outside the room, because these generate a lot of heat.

Lowering the temperature in your growing room Carbon filter
Lowering the temperature

Possibly the easiest way to keep the temperature down is to have the light cycle at night. A couple of hours after sundown the lights turn on and then turn off a couple of hours after sunrise. This way, you have your lamps on during the coolest period of the day. You can imagine that when it’s 86 degrees outside, you’ll also bring this indoors through your extractor, so it will never be cooler than 86 degrees. Professional growers buy an AC to be able to have the light cycle during the day or during the night.

Increasing the temperature in your grow room

If the lights are on, you won’t have to increase the temperature. The lamps will do this for you. It is however important to distribute the air through the room, so the temperature is the same in the entire room. Use rotating fans to do this and aim them between the lamp and the plant. The fresh air you suck in from outside also has to be distributed well to prevent cold spots. Especially in winter, temperatures can be below freezing.

If the lights are on, you won’t have to increase the temperature. The lamps will do this for you. It is however important to distribute the air through the room, so the temperature is the same in the entire room. Use rotating fans to do this and aim them between the lamp and the plant. The fresh air you suck in from outside also has to be distributed well to prevent cold spots. Especially in winter, temperatures can be below freezing.

Increasing the temperature in your growing room
Increasing the temperature

It can get quite cold in your grow room when the lights are off. Fortunately, there are plenty of things you can do to increase the temperature. A basic electric heater with thermostat is usually enough to sufficiently heat your room. These however use a lot of electricity. A radiator with thermostat will also do fine. You could also turn off the fan that attracts fresh air once the lights go off. This can be done with a fan speed controller with thermostat or with a time switch.

Heat stress

A plant can experience heat stress when it gets too much warmth. Heat stress doesn’t usually occur outside, because it won’t get warm enough for that. We’re talking about temperatures above 105 degrees. Photosynthesis is greatly inhibited during heat stress. Furthermore, the activity of enzymes decreases and the production of proteins slows down. Certain proteins could break. If heat stress goes on too long, the plant will die.

Increasing the temperature in your growing room
Heat stress

You can identify heat stress by looking at the top leaves of the plants (see picture).

These turn yellow and the leaf curls inwards over the entire length. The plant tries to protect itself against the heat this way. If the lamp is placed too close to the plant, this will happen quite quickly. You can hang the lamp a bit higher or place a fan between the plants and the lamp to blow away some of the heat.

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