Plants and Air Temperature
Plants like their air to be kept reasonably temperate. As the colder weather begins to draw in, it can be difficult to keep the air around your plants in the sweet-spot of 23C – 28C (28C being the absolute maximum temp you ever want at the tips of the plant without CO2 supplementation).
23C-28C is the ideal temperature only during the lights-on period. During the dark-period a few degrees of temperature drop is a desirable thing (see below), although the temperature should ideally not go below 18C, and should never go below 15C (see below).
Obviously, a grow lamp creates quite a bit of heat. While this can cause problems of high temperature during the summer months, it is actually a very useful side-effect during the winter.
For most growers, the simple addition of a fan speed controller to reduce the amount of hot air being exchanged for cold air within the grow environment, is enough to keep the air within the right limits. As long as enough air is being exchanged to refresh CO2 levels and to keep humidity moderate then this approach can be a great solution to keeping temp in the 23C-28C range during the lights-on period.
When the lights go out
Temperature during the dark-period is something that many growers do not consider. Maintaining the right temperature during the lights-off period is just as important as getting the temps right when the lights are on. As stated before, while it is a good idea to have a temperature drop of a few degrees during the lights-off period, it is important to not let the temperature go below 18C and it is absolutely vital the temps NEVER go below 15C.
Plants sense the temperature even during the dark-period. A temperature drop during the lights-off period actually helps plants to respond better to the photoperiod. A drop of a few degrees when the lights go out will help kick plants into flowering/fruiting more quickly when they are switched to 12/12, and promotes stronger flowering/fruiting thereafter.
Brrrr It's cold, winter must be coming, I think I'll stop growing now...
Plants use temperature to sense when winter is coming. Plants also need certain temperatures for the biological act of living and growing. Plant growth goes down markedly below 18C and plants can “stall” (stop growing completely) below 15C. It may only take one or two cold (sub 15C) lights-off periods for the plant to decide that winter is coming. This sets off a process whereby the plant ends flower/fruit bud production, finishes-up the flowering/fruiting maturation process in a hurry, and then gets ready to die or recede until the springtime.
If plants “stall” in this way, it can take a long time (sometimes up to 2-3 weeks) for them to get back to normal flower/fruit production. Usually, they will never recover what has been lost during the “stall” period. As I’m sure you can imagine, 2 wasted weeks out of an 8 week flowering or fruiting period will have a huge detrimental effect on the size of the final crop.
Keep them snug with a grow-room heater
So that your plants do not “stall” or slow down during this critical period, you need to ensure that the temperatures in your grow-room never go below 18C. Therefore, it is a very good idea to have a method of keeping the air warm within the grow-space, even during the dark-period.
If there is no method of continuous heating in the grow-space then it will usually be necessary to add one. Fortunately there's a couple of easy ways to do this, and it doesn’t have to break the bank.
First of all there are the tubular "Greenhouse Heaters". These bazooka like objects are designed to take up very little space in a grow-tent or other grow space. They are available in a range of wattages to suit the size of your tent/grow-space and how much heating will be required. These devices will certainly keep the chill off without you having to re-arrange your whole tent:
24 inch 80W
36 Inch 135 Watt
48 Inch 180 Watt
The above bazooka-type heater are great for most situations. However, they are not thermostatically controlled (they do not switch off when a certain temperature is reached).
For complete control, a small oil-filled radiator featuring a thermostat is the way to go. This little one will fit into virtually any tent and will keep temperatures just right:
With solutions this simple, why not go ahead and treat your plants to the warmth they love. They'll repay you handsomely for it!