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Heat stress in marijuana plants and what to do about it

heat stress with cannabis

In summer, it often gets really hot here in Spain, so that the cannabis plants get into quite a bit of stress, both indoors and outdoors. But also in other regions, very high temperatures can occur in summer, which do not exactly make life easier for the marijuana plants. The problem is, cannabis plants can only withstand a certain amount of heat. At a certain point, when the temperature gets too high, the plants start to show signs of stress: the leaves get brown spots, turn yellow or brown, light burn occurs in places with too much light.

Heat Stress Cannabis

Hydroponic cultivation at high outdoor temperatures

Hydroponic cultivation becomes a bit of a challenge especially when the outside temperatures are very high and needs some attention. Too high temperatures in the nutrient tank or deep water pot can favour and promote root rot. A really serious problem that can destroy the entire plants and thus the harvest.

A frequent check of the temperature in the nutrient tank is required. This should not be higher than 18-23° C. To achieve this, nutrient tank cooling or a climate device in the grow room is sometimes necessary. Basically, in wise foresight, hydroponic systems should not be set up in the attic or other particularly hot places in the house. Unfortunately, cooling devices for the nutrient tank are much more expensive than heating rods, which are of little importance in terms of costs, at least in the case of usually large systems.

Heat stress in outdoor plants

Even with outdoor growing, signs of heat stress can appear in the cannabis plants when temperatures have exceeded a certain point. Especially if they do not have enough water available.

If it is too hot, the leaf edges begin to curl and the leaves point upwards. Yellow or brown spots may appear on the leaves or they may even fall off. This can be especially problematic during the flowering phase, because the cannabis plant will not produce new leaves.

Sativas can handle higher temperatures more easily, while indicas are more prone to heat damage, especially during flowering. Heat damage that occurs during flowering can destroy important leaves, reduce crop yields and also results in fox-tailed, substance-minimised and airy flower growth.

Foxtail Cannabis

foxtail-like flower growth is typical for some varieties, but can also be a sign of heat stress

Solving the heat problem in grow

There are ways to get too high temperatures under control when growing cannabis. Sometimes several measures are necessary.

  • Water sufficiently, check the humidity in the pot and water several times a day if necessary.
  • The temperature in the grow room should be lowered somehow and if possible, and above all the air circulation should be increased with additional fans. A swivelling fan should blow directly over the tips of the plants so that no hot spots can develop.
  • Cover the topsoil or the soil around the plant with vermicult or perlite so that water cannot evaporate so quickly.
  • Use Hidro-Gel, which is granulate that increases in volume many times over when soaked in water and completely absorbs and stores water. It is mixed under the soil and then releases water to the plants as needed without overwatering. The cannabis plants are thus protected from drying out and heat damage.

hidrogel

  • You may have to decide to increase the distance to the lamp more than is normal.
  • When temperatures are very high, it is often impossible to do without air conditioning, otherwise there will be considerable losses in the harvest.
  • If possible, the supply air for the grow room should ideally be drawn from a cold room in the house, e.g. the cellar. It is even better if the grow box is located in the cellar.
  • Seaweed extracts can help cannabis plants cope better with heat stress and help protect them from potential damage. Many growers therefore use algae/seeweed extracts, e.g. Seeweed von, when they notice heat damage or signs on the plants during high outdoor temperatures in summer.
  • The use of a Cool-Tube-Reflector for indoor cultivation. The Cool-Tube-Reflector is directly connected to a fan and sucks the hot air away from the lamp.

cool-tube-

  • For indoor growing, the fan should have sufficient capacity and run at full power. Check often to see when watering is needed again.
  • Outdoors, of course, it is more difficult to reduce the heat. The options are limited. For example, you can put up a sun sail or move the plant pots into the shade.
    Watering plants outdoors is best done in the evening and early morning hours to prevent damage from scorching and soil salinisation. Before a plant dies from drying out during the day, water it instead! Never let a cannabis plant die of your own dogma 😀 If watering is also necessary during the day, the soil should be mulched, do not let the leaves come into contact with water, no drops on the leaves.
  • Use white plant pots outdoors, which do not heat up as much as black pots.

It is important to remember that if the cannabis plants are protected from too much sunlight for too many days with the help of a sun sail or sunshade, they can still become more sensitive to high sunlight.

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