Temperature Just Right

Although we’ve had a relatively mild autumn this year, ambient temperatures are now starting to drop, especially at night, as we head towards the grip of winter and every indoor gardener will be looking for ways to keep grow room temperatures at the optimum level of 25-28c during lights on and 18-21c while lights are off.

Effects Of Cold Temperatures

Low grow room temperatures can have a detrimental effect on your plants, one of the early signs is stems turning purple in colour, followed by slow/stunted growth as nutrient uptake reduces and photosynthesis levels decrease. The result will not only be a poor final yield, but also the overall quality of the harvest will be lower than expected, as lets face it, nobody wants a below par harvest after all the hard work you’ve put in over the previous weeks.

What Can Be Done To Maintain Optimum Temperatures?

One of the simplest ways to help raise grow room temperatures is to switch the time of day lights are on or off. Rather than setting your light timers to match that of mother nature, switch them around so lights are on during the night, when ambient temperatures are at their lowest, and lights are off during the day, when ambient temperatures are at their highest.

Another quick way to help raise temperatures is to use a Fan Speed Controller on your extraction and intake fans. This may not be feasible in the height of flower, but during vegetative growth and early bloom, by knocking the fan down by a few percents, warmer air stays in the room longer but still refreshes often enough.


For some set ups, this might be sufficient and if it is, you’re one of the lucky ones! For the rest of us, additional heating and/or insulation will probably be required. The cheapest of which are Tube Heaters, originally designed for greenhouses, the hydroponic indoor growing community quickly saw the potential as a cheap and efficient way to help raise grow room temperatures. Comprising of just a heating element, they are the perfect way to help get temperatures higher and as they can be placed at pot level, the roots will be as snug as a bug in a rug during the cold days and chilly nights.

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Another way is to make sure your grow room is well insulated, if using a grow tent, do not place it directly on a cold floor, place the tent on something like a carpet, or even building insulation, such as Kingspan, in fact some of our customers during winter wrap their entire tent with insulation. Another good tip is don’t place your pots, and trays, directly onto the floor, use two trays or saucers, place one upside down, then the other, directly on top and the right way up, as this will raise the pots, and more importantly the roots, a few inches away from the cold floor.

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If you run an intake fan, in spring/summer, cool fresh air is usually drawn from outside, during the winter months, however, the air will simply be too cold, a better set up would be to draw in air from another room, as the temperature will be warmer. This also reduces the chance of introducing pests, although, whether drawing air from outside or inside, during the summer or winter, protection like Bug Barriers should always be used to limit any kind of pest entering the grow room.

Nutrient Storage During Winter

Although not directly related to grow room temperatures, care, especially during winter, needs to be taken when storing nutrients, which should be kept in a dark room, away from sunlight with a temperature of between 18-21c. In its concentrated form, if the room temperature regularly fluctuates and frequently falls below 5c, crystallisation can, and probably will occur making the entire bottle unusable. When mixed, especially in large volumes and stored in a water tank, or similar, temperature needs to be maintained, if it becomes too cold, the solution can separate and again becomes unusable. One way to combat this is to use a Water Heater, often used in aquariums when keeping tropical fish, these handy submergible heaters are ideal as once placed in the water tank, they will keep the nutrient solution at the desired temperature.

And although this seems obvious, one of the most important items you should have is a Thermometer. Ideally, it should be digital that logs both max and min temperatures, as you can’t always be around to take readings. We also recommend using a thermometer with a probe, this means you can take separate readings from the canopy and root level, which can differ by a few degrees depending on set up.

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To recap, here’s a quick list of tips and tricks that can help keep your grow room at the optimum temperature during the winter months….

  • Lights on should be during the night and lights off during the day
  • Use fan speed controllers to reduce the amount of air intake and extraction
  • Insulate your grow room walls, floor and ceiling
  • Use secondary heating, for example, tube heaters
  • Intake air from a different room, not from outside
  • Nutrients are stored at the correct temperature
  • Use a water heater for large volumes of nutrient solution
  • Take regular maximum and minimum temperature readings

So there you go, a few simple steps to help you, the grower, maintain perfect grow room conditions during the cold winter months.