Problems at low temperatures and why it is better to avoid them at all costs

Winter is slowly knocking on the doors, which means just one thing for indoor growers – the heating season is beginning. Maintaining sufficiently high temperatures is essential for fast and healthy plant growth. Cold, on the other hand, brings a number of problems, from slower growth to difficulties with nutrition and moulds. Why is it better to avoid low temperatures, and which heaters should you get for your growbox or growing room?

It is generally stated that the optimum temperature in the growing area is between 22–28 °C when the lights are on and a few degrees lower in darkness. Of course, this is not a universal optimum. For example, plants can grow very well at higher temperatures under intense lighting, with high transpiration or CO₂ supplementation. Excessive cold, on the other hand, always has a negative effect on plants grown indoors. At low temperatures, the metabolic processes in plants that are responsible for rapid growth and healthy development slow down. Photosynthesis is also slower, and plants are unable to use the energy from grow lights efficiently. A cold environment over a long period leads to difficulties with the uptake of nutrients and growth, and increases the risk of mould development.

You may also be interested in: What are the ideal conditions for indoor growing?

What happens to plants when temperatures fall too low?

Plants stop growing: The main advantage of indoor growing is rapid plant growth. However, if plants do not have enough warmth, enzymatic reactions and other processes required for the formation of new tissues slow down. Photosynthesis is also slower, and plants are unable to use the energy from grow lights efficiently.

Cold roots absorb water and nutrients less effectively: In a cold environment, plants typically show nutrient deficiencies even though there are sufficient nutrients in the substrate or nutrient solution. This is because roots work more slowly in cold conditions. The optimum root-zone temperature is approximately 20–24 °C in soil and 18–22 °C in hydroponic nutrient solution.

Substrate dries out slowly: The first problem you will usually notice when temperatures fall is that the substrate dries out slowly. This is because plants transpire less and less water evaporates from the substrate at the same time. A persistently damp environment prevents roots from breathing and increases the risk of rot. Less water absorbed also means fewer nutrients, and plants will grow more slowly.

Higher humidity and condensation increase the risk of moulds: A cooler environment often brings humidity problems. When the temperature drops quickly, for example when the lights are switched off, air humidity condenses on the plants in much the same way as morning dew outdoors. Wet leaves are the perfect environment for powdery mildew, botrytis and other moulds that attack plants.

How to keep temperatures in the growing area sufficiently high

It probably needs no explanation that no indoor growing room can do without a quality thermometer with a humidity metre. Devices with a memory function that records the highest and lowest measured values are excellent. Read these values every day, write them down and reset the memory. After a few days, you will gain a perfect overview not only of the highest and lowest temperatures, but also of the fluctuations between them, which is equally important. In general, it is recommended that the difference between daytime and night-time temperatures should not exceed 5 °C.

When choosing heaters for your growing room, you have several options:

Tubular heaters can be placed directly inside the tent next to the plants. This type of heater is best placed on the floor so that it warms the root area and the warm air rises upwards. Tubular heaters are best suited to small and medium-sized growboxes.

Fan heaters are intended for larger growing rooms. They should not be placed in the immediate vicinity of plants, and when growing in growboxes, the most suitable option is to place them in the room containing the growbox, not inside the tent. If the heater you choose does not have its own thermostat, we recommend purchasing a plug-in thermostat.

Heating mats are intended for seed germination and cloning. Some seeds germinate best at temperatures slightly higher than room temperature. Chilli is a typical example, but it is far from the only one.

In addition to heaters, you can also increase temperatures in the growing area using economical methods such as the intake of warm air from another part of the house, insulation or replacing LED lighting with HPS HID lamps which emit more heat. We covered winterising the growbox in detail in the article Winter is coming: Prepare your growbox for winter. You can find more advice and guides for growers on the Higarden blog, where we bring you freshly gathered information about indoor and outdoor growing every week.