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Temperature in the Grow Room: Man’s not hot!

Us humans deal with hot temperatures differently to that of our plant friends

Temperature is THE most important thing for your indoor grow room. Underestimate temperature issues or do not adequately control your temperature throughout your grow, and you will never achieve the record-breaking yield of your dreams. One or two extra degrees of heat during warm days, too cold a night time temperature, or even too great a swing in your day/night temperature difference all play a direct part in adding to your yield. Getting them right will see more of an addition to your yields than any bottle of nutrient can give you; quick maths.

Understanding how temperature impacts your plants and understanding how you can steer your growth by controlling grow room temperatures will quickly set you on the path to becoming a hydroponic god. This blog is a quick insight into temperature and a few pointers about the sort of equipment you can use to control it all. So, without further ado, let’s get cracking!

What temperature does

There are two key things that temperature impacts a plant with; its metabolic rate and its cooling capacity, which are essentially at constant logger-heads.

Your plant needs heat to maintain a healthy metabolism. Every bio-chemical reaction happens quicker in hotter temperatures and a plant is no different, so a plants metabolic rate, is pretty much directly linked to temperature. This means the warmer a plant is, the more energy it can produce, as it can photosynthesise more from having a higher metabolism. The opposite is true for cold temperatures.

At the same time, the plant can only cope with a temperature if it can transpire an adequate amount to keep itself cooling correctly. Ninety-nine percent of all the water that is taken up by a plant is used for keeping it cool! Only one percent is used for photosynthesis and other functions! The hotter the air temperature (or IR level of the light source) is, the more water the plant needs to transpire to cool itself.

So, there is a limit to the rate at which a plant can metabolise, which is based on its overall capacity to cool itself. Finding the correct balance of temperature versus plant age/size will ensure you are produce as much energy as physically possible. So, exactly what else do we need to consider to make this a reality? 

Daytime Temperatures

The first and most obvious aspect of temperature over a 24-hour period is that of the daytime, or lights-on time. This is when your plant is doing all the work to produce energy by photosynthesising energy from the light source, it is also the hottest time because of all the extra heat energy from the lights. If your temperatures are too high for your plant during this time, they will soon show you, as they will start to wilt or droop.

If it is too hot, your plant will begin to wilt, and your final yield will suffer!!


The maximum daytime temperature depends on a few parameters. Plant size and mass being most important in this context. The smaller your plant is, the less capacity it will have to cool itself, so you will want to aim for a (relatively) lower temperature. The larger your plant, the more it can cool, so the warmer the temperature it can take. So, for example, the daytime temperature for a young plant in its vegetative cycle would be roughly 24/25°C.  A larger plant that can adequately cool itself will be able to tolerate temperatures up to roughly 28°C during the daytime.

RVK Extraction Fan

Making sure that your extract fans can cope with keeping temperatures down to these levels is key! If you are unsure of which size fan you need, we can help make sure you get the right one for your room. Give us a ring or pop down to the store and we can talk you through what’s available.


Night-time temperatures

Do not simply think that you need to pay attention to your daytime temperatures only! Although your plant does not photosynthesise and produce energy at night, it is quietly performing many functions that are all effected by the temperature of the grow room as a whole.

The most common problem with temperature during the night-time period is that it is too low. This slows the metabolic rate of the plant and limits how well it performs its night-time functions. You will know in your grow room if it is getting too cold at night as you will notice the purpling of stems, as the plant is struggling transport phosphor. It will also not be storing an adequate amount of water overnight for its daytime photosynthesis, so it is a double whammy.

Plants like cold nights, just like you and me! Make sure you keep them from getting cold feet

Ideally, you don’t want to see your night temperature fall much short of 20°C. Preventing cold nights effecting your yields can be easy with the use of heaters. Whether you choose a fan heater or oil heater is mainly dependant on personal preference, but ultimately whatever heater you choose for your hydroponic grow room, make sure it has the correct capacity to deal with any seasonal cold snaps mother nature can throw at you.

Temperature Differences

In particular, the difference between your day and night time temperatures. The temperature difference between your day and night time hours, has a direct impact on the growth habits of your plant. The reason it impacts plant growth is all down to your plant’s own ability to produce hormones. A smaller temperature difference causes the production of more vegetative hormones, and a larger temperature difference causes a plant to naturally produce flowering hormones.

An unnatural or too great a temperature difference can have detrimental effects on plant growth

So, a young plant in its vegetative cycle will be happier with a daytime and night-time temperature of 24°C (0°C temp diff). A flowering plant will be more productive with a daytime temperature of 27/28°C and a night-time temperature of 20°C (so a 7/8°C temp diff).

Total Awareness!

Having fan controllers that regulate the extract fans speed based on temperatures are the first step to correctly controlling your temperatures. We have all sorts of fan speed controllers, with an option to suit you whatever your budget may be. Don’t let them make you become complacent though! You need to know what your temperatures are at all times! Adequately measuring them with thermo-hygrometers is extremely important. Ideally, having a few placed around your room will give you a good average of the day-to-day conditions and how best to react.

Ideally, having a few placed around your room will give you a good average of the day-to-day conditions and how best to react.

Hopefully you have gained something from this, or even better, have had an idea of how to improve your hydroponic grow room off the back of it! We will be going into how to make the most out of equipment to control your temperatures in a later blog, so keep your eyes peeled for more tips to come!!

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