Winter is coming: Prepare your growbox for winter
One of the main draws of indoor growing is that you can cultivate year‑round, regardless of the whims of the weather. Still, it’s not entirely true that seasonal changes don’t affect indoor growers at all. Even a small drop in temperature in the grow space will cause VPD fluctuations and a slowdown of the plants’ metabolism. You will learn how to prevent this in the following article.
Most plants grown indoors perform best at temperatures between 21–29 °C, with the difference between day and night temperatures not exceeding 5 °C. Once temperatures drop, plants slow their metabolism and the rate of photosynthesis decreases. That means that even if we supply a large amount of energy in the form of light, they will not be able to use it to produce sugars and will grow more slowly than they would at higher temperatures. If the temperature falls below 15 °C, plants stop growing and remain stunted.
Plants also respond to low temperatures by slowing transpiration, i.e., the rate at which excess water evaporates from the leaves. Transpiration is key for the transport of nutrients and water. A smaller volume of water transpired by the leaves means that plants will consume less water but will also stop taking up nutrients from the substrate or nutrient solution. The consequence is again slower growth and an increased risk of substrate waterlogging and root rot.
Growing at lower temperatures can also bring some benefits, mainly in preventing pests and molds. Cold air holds less moisture and is drier than warm air. This can be useful in preventing molds such as powdery mildew or Botrytis cinerea, which causes flower rot. At low temperatures some insect pests, like spider mites or aphids, also reproduce more slowly. However, remember that if the temperature drops below 15 °C, plant metabolism stops completely and growth ceases.
Effect of low temperatures on VPD
Plants are affected by the temperature and humidity of the air surrounding them. Air temperature influences transpiration and relative humidity plays a significant role in CO2 uptake, which is important for photosynthesis. The effects of climate conditions on plants are best captured by the vapor pressure deficit (VPD). We covered this topic in detail in an earlier article on our blog. VPD values are derived from the temperature and relative humidity of the air in the grow space, and even small fluctuations in temperature (+-1 °C) or humidity (+-5 % RH) can represent significant changes in vapor pressure deficit. If you use VPD as a guide for indoor plant cultivation, it is extremely important to keep the temperature and humidity in the grow space consistently stable and optimal.
How to keep the grow box sufficiently warm
During winter you have several options for increasing the temperature in the grow box. The simplest and cheapest solutions are insulation, directing warm air into the space, or moving the grow tent to another location. A bit more expensive (but more reliable) is heating the grow space with a heater or changing the grow lighting.
Insulate the grow box:
Before you start heating the grow space, make sure the grow box seals well and check all zippers so warm air won’t escape. If pots or the irrigation system sit on a cold floor, place them on suitable insulating material, such as polystyrene.
Use warm air from another part of the house: During the winter months it can be advantageous to bring warm air into the grow space from heated rooms. It’s also a good idea to place the grow tent closer to heat sources, for example next to a heater.
Heating units for the grow box: If insulation or supplying warm air is not enough, consider getting an appropriate heater. Grow boxes are especially suited to tube heaters for greenhouses or hanging heating units. Most heaters are equipped with a thermostat that will keep a stable temperature in the grow space. Heating the room where the grow tent stands is also a solution. Oil-filled or electric radiators are suitable.
Switch to a different type of grow light: Modern LED grow lights are very efficient and therefore do not emit much heat to their surroundings. Older grow fixtures using sodium vapor lamps (HPS) emit a significant amount of heat, which can be advantageous in the winter months.
Whether you choose insulation, moving the grow box, or investing in heating and new lighting, always keep in mind that a stable temperature is the key to success. For more tips and tricks on how to grow during the winter months, visit our Higarden blog.