Top Tips For Growing In Cold Conditions

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 (check out our range) 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 temperatures 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 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

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:

Lighthouse Ecoheat 45W – 305mm Click Here

Lighthouse Ecoheat 80W – 608mm Click Here

Lighthouse Ecoheat 135W – 915mm Click Here

The above bazooka-type heaters 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:

Oil-filled Radiator Click Here

Lighthouse 2kw Greenhouse Heater (23cm x 19cm x 32cm) Click Here


It’s not just air temperature – it’s your nutrient solution too!

If you have a reservoir in your hydroponics system, it is best to keep your nutrient solution at about 18C (65F). It is not just cold air that can cause slowed growth in plants, the temperature of their feed water too can cause it too. Again, anything below 18C could begin to cause problems. Anything below 15C and there is a definite risk of stalling.

Fortunately, a simple aquarium tank heater can do the trick:

Adjustable aquarium tank heater

We stock several size/wattages to suit any reservoir.

50W – Up to 25 Litres
100W – Up to 75 Litres
150W – Up to 125 Litres
200W – Up to 180 Litres
300W – Up to 280 Litres

They’re really easy to use. Just set the temperature, fit it (submerged) to the side or bottom of your tank and plug it in.

Adding another light

One of the easiest things we always recommend is that you add another light into your setup. Adding another light will give you some added heat which can help to keep your temperature higher during cold periods. Winter is the perfect time to really push your lighting setup and yields to their max as you don’t have to worry about your temperature being unbearable due to external heat like you do in Summer.

Protect your Root Mass

Another way in which you could easily help to keep your plants happier during the cold is to keep your root mass warm. Your roots act as the engine for your plant, and you want them to be in the optimum conditions all the time. As we all know, heat rises, meaning the bottom of your tent could be the coldest area of your grow room, but there’s a few things you can do to make sure it doesn’t get too cold. The first thing would be to invest in a nutrient heater. This would keep your solution at an ideal temperature which in turn would maximise nutrient uptake throughout your plant and keep your yields to their max.

Another simple thing you could do is raise the pots off the floor ever so slightly to insulate the bottom of the pot. You could use a layer of polystyrene to raise it up an inch or two (if your system will allow you to), this slight adjustment would make a huge difference to the temperature of the roots, keeping them that little bit warmer.

Cut out Cold Spots

Another fantastic way to get the air circulating in your room to reduce the chance of cold spots would be with a DiffuseAir. Instead of directing air movement straight at plants, it diffuses it uniformly around the grow area, preventing issues with windburn and eliminating micro-climates. This also means it’s perfect for when it’s cold, as it’ll keep the hot air rotating through your grow room when paired alongside a heater to help push the temperatures up. A thermostatically controlled heater would allow you to add some heat to your crop when your lights are turned off (this is also the time in which your tent would be the coolest due to the lack of heat from the lamps).

Give your crop some Heat 

The last thing on our list would be using Vitalink Heat. Heat is a fantastic product which gives you protection from those unwanted temperature spikes and allows your crop to flourish with rich micro-nutrients. A little goes a long way when it comes to using heat in your grow room, so make sure you doing over do it when the amount as it can have a negative affect on your plants. All guides for Heat can be found on our website or on the bottle! If you’re unsure, go a little under the recommended amount to be safe! You want to be applying Heat into your schedule ideally before the weather gets too cold to allow your plants to become resistant to the cold! Did we also mention there’s 15% off at the moment?

The 0.4ml / litre dosage is important. It’s the sweet-spot for how much to use. Too much or too little of this product means that it will not work as well.  It can also be used as a foliar spray (use at 5ml / litre). Tests have shown that plants can keep growing normally at temperatures as low as 5C !!!

Hopefully these little tips and products which can save your winter crop have been useful! If you’re still struggling, don’t hesitate to get in touch with us and we can help you identify where you can improve your setup to maximise your yields in the cold!

Happy Growing