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Hemp and autumn cold snap. How to keep crops from cold

With the onset of autumn, the daylight hours become shorter, the weather is colder, and the cones are more magnificent and weightier. In this article, we will tell you how plants react to low temperatures and how to reduce the impact of autumn cold on future crops. Choose the stealth growing box.

Air temperature is very important for plant growth and flowering, and cannabis is no exception. The rate of development and health of your plants directly depends on this parameter.

Depending on the variety, the optimal temperature regime may differ. For varieties with a predominance of indica and auto-flowering, the optimum will be 20-28 degrees during the day, 16-18 degrees at night. Sativa is more thermophilic: in the daytime 22-30 degrees, at night not lower than 18-20 degrees.

Influence of low temperatures

A temperature of 15 degrees is considered critical for the growth process of cannabis.

If the drop in temperature is short-lived, it will not harm the plant. But when exposed to temperatures no higher than 15 degrees for several days, all vital processes slow down and the transport of sugars inside the plant stops, growth and flowering cease. The rate of evaporation of moisture from the surface of the leaves decreases. As a result, the suction force that forces the roots to absorb nutrients is weakened. Mineral salts that the plant cannot absorb remain in the ground. The soil becomes salty, which causes the plant to absorb less water and nutrients, and growth may stop altogether.

Such conditions, which persist for a long time, negatively affect the health of plants – their immunity becomes weaker. Another major threat to cold-weather blooming cannabis is mold. Humidity and low temperatures are ideal conditions for fungus to spread. The most resistant auto and indica varieties can grow at 10-12 degrees. But in order to obtain a decent harvest, the plants should be provided with an optimal temperature regime.

The problem of cold weather can arise both when growing in an open field and in a grow box, if the box is in an unheated room. And, accordingly, it will be solved in different ways.


When the lamps are on, especially if they are on lamps of the type DNaT or DRI (MGL), the heating problem does not arise. The heat generated by the lamps is sufficient. But heated air can accumulate in the upper part of the box, and cold air can stagnate at the bottom. This is solved by additional fans for blowing lamps and plants. If cold LED lamps are used, the heat generated by them may not be enough, then an additional electric heater must be installed in the box or LED lighting must be combined with HPS.

When the lamps are off, the temperature should be maintained between 15 degrees and 22 degrees. An important rule: the difference between day and night temperatures should not exceed 10 degrees. For example, if the day is 27 degrees at night, you need to maintain 17 degrees.

A couple of nights in cool temperatures won’t do any noticeable damage to your crop, but if it lasts for the full flowering period, something needs to be done about it.
The first thing to do to raise the night temperature is to turn off the exhaust fan along with turning off the lighting. To do this, the fan just needs to be connected to the same timer as the lamps. If this is not enough, a simple 1.5-2 kW electric heater with a thermostat is usually installed.

Open ground

Most outdoor varieties tolerate autumn temperatures down to 10 degrees Celsius quite easily. A drop in temperature to 4 degrees can damage plant tissues. At negative temperatures, the water inside the plant begins to freeze, and this will certainly lead to the destruction of plant cells and the death of tissues, cause stress in the plant and can lead to its death. Frostbite parts of the plant are easily vulnerable to mold, therefore, if the hemp has undergone severe frosts, it is necessary to urgently remove the frostbite parts of the plant, if the damage is extensive, so as not to lose the entire crop, it will have to be harvested as soon as possible.

Outdoor plants will feel comfortable under the film. Heating the greenhouse at night will further increase protection against cold nights and unexpected frosts, gas or electric heaters can help.

As a rule, cold weather is accompanied not only by a decrease in temperature but also by heavy rainfall. With high humidity and prolonged rains, there is a great danger in the form of mold, which can easily ruin the bushes. Therefore, it is recommended to inspect the buds as often as possible for the presence of mold and, if it appears, remove the affected inflorescences.

It should also be remembered that healthy plants are less susceptible to cold temperatures and mold, so you should pay attention to plant nutrition. Micronutrient feeding and the use of biostimulants such as Biobizz® Alg · A · Mic ™ can improve the overall health of the plant and increase its resistance to cold and other risk factors.

Cold hardening before harvesting

On the other hand, the short-term stress experienced by a healthy plant in the final stages of plant life may even be beneficial. At least for the grower. Indeed, in such extreme situations, the plant is forced to intensively produce resins through trichomes, which can significantly increase the THC content in the buds.