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Grow Room Temperatures


During the hot summer months, it can be difficult to control the temperature in your grow room. This article advises you on how to maintain a temperature that promotes healthy plant growth.

How important is temperature?

Your plants are constantly processing energy 24 hours a day. In the light cycle they make and store sugars, during the dark cycle these sugars are transported throughout your plants. Your plants need a continuous energy supply in order to grow and mature. The speed at which your plants process energy is directly related to temperature.

What is the ideal temperature for plant healthy growth?

If you want your plants to grow quickly and stay healthy, it’s important that you keep them within their ‘comfort zone’. This is between 72-82°F. Most plants are very comfortable up to 82°F, but are sensitive to higher temperatures. 

You can use a thermometer to measure the temperature of your grow room. Ideally, choose a thermometer that records the maximum and minimum temperature. This’ll allow you to monitor the hottest and coolest temperatures in your grow room over a twenty four hour period. Where you place your thermometer is often overlooked, but very important. The idea is to capture ambient air temperature. So the ideal place is half way up your grow room and out of direct light. If the air in your grow room is well mixed with the use of oscillating fans, your thermometer will be reading an accurate temperature.

Consequences of high temperature

Small pores on the leaves (stomata) allow water vapour and gasses to leave the leaf and keep the plant cool (transpiration). If your plants are within their temperature ‘comfort zone’ (72-82°F) and are transpiring healthily, they’ll keep themselves up to 4°F cooler than the ambient air temperature.

As a defence mechanism to conserve water, your plants’ stomata will start to close at around 85°F (at ambient CO2 levels ~390ppm). With the stomata shut, your plants will not be able to transpire to keep themselves cool. If the temperature remains too high, the water trapped in your plants’ leaves can overheat causing internal cell and enzyme damage. The result is poor growth and unhealthy plants.

If the temperature in your grow room is too high, your plants will respond in a few ways. They’ll appear stretched as the inter-node distance increases with a high temperature. This results in very ‘airy’ flowers or poor fruit quality. During high temperatures, your plants will have difficultly laying down dense material because their sugar energy is being used so fast that they cut back on the energy used to make flowers and fruit. 

High heat is often associated with summer and low humidity in grow rooms. As a result, the roots of plants take more water than nutrient salts to compensate for the loss of water from the leaf due to high transpiration. This increases the nutrient strength in your reservoir or media leading to nutrient lockout, which can cause over-fertilization.

Controlling Temperature

To keep the growing environment within your plants ‘comfort zone’ you must remove the hot air from your grow room. Using an inline extractor fan you can take hot air and duct it out of the room. Due to heat rising, you should place your extractor fan at the top of your room. 

The lights in your room are the main heat generators, so the more lights you have, the larger your fan will have to be. 

As well as removing the hot air, you also need to supply the room with fresh cooler air. This can be done by using another inline fan to pump fresh, C0² rich air into your room. Relying on one extractor to take the wasted air out and draw the air in through vents is not as effective in bringing temperatures down.

For effective temperature control, we recommend that you use ventilation system with a thermostatic fan speed controller. With these devices you set the desired maximum temp (around 82°F). If the room temperature exceeds this, the fans adjust their speed to move more air, as the temperature decreases, so do the fans speed. Thermostatic fan speed controllers are excellent for summers where the outside temperature varies greatly from day to day.

Heat can often build up in your room under the lights. It’s important that you have good air exchange and keep the air well mixed to avoid heat build-up. This can be done using a pedestal fan to move the air around from under the lights. Heat shields are also a good way to avoid a high temperature under lights and are especially good for rooms with low ceilings as they disperse heat very well.

An efficient way of removing heat from your lights is to use an enclosed air cooled reflector, or with glass cool shade reflectors. Air and heat is pushed through these reflectors using an extractor fan. Air cooled reflectors are very efficient and are commonly used to remove heat from the grow lights in summer running grow rooms. If heat becomes an issue for you, the use of air cooled reflectors can bring room temperature down significantly.

The time of day your lights come on can also have a big effect on your grow room temperature. In the hotter months, it’s advisable that you start the light cycle in the evening, running the lights through the night. This allows you to bring in cooler night time air into your room during the light cycle and avoids extreme mid-day temperatures. (Running your lights through the night is great in winter also, counteracting extreme low temperatures outside at night).

If your grow room temperature is constantly high and growth is affected, you may want to invest in an air conditioner. Air conditioners are the most effective way to reduce high temperatures in the hotter months. They also remove water from the air, thus lowering humidity, so a humidifier may also be needed when using air conditioners.

If your temperature is high and humidity is low, the use of a humidifier may be enough to increase humidity and lower temperatures. Increasing the humidity has a cooling effect and can bring the temperature down by as much as 5°F.

The last resort for a high temperature is to simply cut back on the number of lights in your room, or if you have dimmable ballast you can reduce the wattage. If you have two 600W lights and are finding that your day time temperature is constantly above 85°F and your plants are struggling, you’ll get a poor yield and poor quality produce. By cutting back to one 600W light, or by turning both down to the 400W setting, the temperature will drop to a manageable level and your plants will remain healthy. Obviously, this’ll drop yield, but it will allow you to grow better quality produce.

Roots and temperature

If roots are to function efficiently and take up the nutrients, they need to be kept at a good temperature 24 hours a day. For established plants with a good root mass, the temperature around the root zone (rhizosphere) should be 65-75°F. High root temperatures can increase the risk of root disease and lower the dissolved oxygen content of the nutrient solution. Roots with temperature below 68°F will have a weaker ability to take up nutrients. When growing in pots/containers, temperature around the roots can be increased using warming mats/pads. For people using re-circulating NFT systems, it’s better to keep the solution nearer to 70°F because a higher temperature will increase the risk of pathogens. This can be done with the use of a water heater. If your water temperature is too high, you can reduce the temperature with a nutrient chillier, capable of lowering the nutrient solution temperature by 10°F.

Day Time and Night Time Temperatures

Plants are very sensitive to differences in temperature between the light cycle and dark cycle. As mentioned earlier, to measure the night and day temperature you need a max-min thermometer. If you want fast vegetative growth, you need to try and keep your night and day temperatures as close as possible. However, to promote flowering and fruiting, you need to increase the difference between night and day by approx. 10°F (day). If your temperature differential is bigger than 20°F, you’ll put your plant under stress.

The sugars your plants have made during the light cycle will be poorly transported in the dark cycle, thus lowering growth rates. To keep night  time temperature up, use an electric fan heater or an oil filled radiator during the night cycle.

us lowering growth rates. To keep night time temperature up, use an electric fan heater or an oil filled radiator during the night cycle.

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