Thermal stress and your plants

Thermal stress can have a dramatic effect on your plants, but it is also easy to repair. What should you do when the thermometer climbs to 28ºC in your growing room or greenhouse and continues to climb? Here are some methods to fight the heat and to be able to harvest dense and tasty buds.

Your plant can only support a certain amount of heat and light. If your cannabis begins to show signs of stress on the leaves near the light sources and / or heat. Your leaves will have yellow or brown brown spots and may appear burned in places where there is too much light. It is also common for the leaves to bend up or down, bend inward like cones, and the serrated edges of the leaves begin to rise. Even open-grown outdoor plants can suffer from the intense heat of summer. Besides the need for large amounts of water, here are the symptoms that you might encounter if you grow in the red zone.

  • Damage from some insects can quickly kill your plants in hot, dry weather: they breed up to twice as fast during these times.
  • Excess heat conditions cause burns and leaf diseases, whether wet or dry, they invite burns, molds, fungi and bacteria that cause disease in your crop.
  • Anaerobic bacteria and fungi that cause decay in the lower parts of cannabis plants often prefer warm, moist conditions around the roots because, at warmer temperatures, the water retains less oxygen, weakening the roots. .
  • Low quality, low yielding buds are a telltale sign of the stress of a crop in excessively hot growing conditions.

Solution: Find a way to monitor the temperature. Control the heat by any means necessary by following the steps described below.

5 tips for controlling heat in the growing room

Inside, find a way to lower the temperature by increasing the air circulation in the growing area. Having a small fan blowing on top of your plants will help prevent the formation of hot spots directly under your growth lights.

You can consider positioning crop lamps higher from the top of plants if heat is a problem.

When growing, it is best to try to keep a comfortable room temperature at all times for optimal growth. If it's too hot for you, it's probably too hot for your plants.

Keep the roots cool : If you can keep your roots cool, it will help your plant cope with the heat affecting the top of the plant.

When cannabis plants recover from heat shock, some producers recommend the use ofkelp seaweed extract (often available as liquid fertilizer) to help plants recover from stress and possibly protect plants from heat stress in the future.

Many indoor installations will require you to ventilate the hot air with a fan or exhaust system. By creating a good suction with an efficient exhaust system and adding a carbon scrubber, you can also eliminate all odor of the culture room.

An oscillating fan will circulate the air in the room and provide a light breeze to your plants.

Outside You have fewer options for reducing heat during a heat wave, but you can monitor your local weather through weather forecasts.

It is possible to partially protect your plants when you know that the temperature will heat up. You can also adjust your watering schedule to make sure the plants have at least a lot of water.

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Some tricks to try when you know that the weather outside will be hot or dry:

  • water the plants in the evening or early in the morning to help prevent the evaporation of water during the hottest hours
  • Keep the roots cool for example by placing your potted plant in a ceramic pot to insulate the roots of the sun. Some growers dig a hole in the ground to place their potted plant because the soil is generally cooler than air when the temperature is high.
  • Thekelp extract for roots can help protect against heat stress
  • increase the shadow to reduce the heat felt by plants, you can use an old sheet or other cloth as a short-term solution, or get a professionally made "shade sail", specially designed to create shade outdoors . It is important to remember that shading the plants for more than a few days will make them softer than in the sun, and you may need to slowly reintroduce full sun to avoid being shocked by the intensity. light.
  • move potted plants : fortunately with potted plants, it's usually easier to move them out of direct sunlight during a heat wave

When growing cannabis outdoors, it often takes a few weeks for the plant to regenerate after a hot or dry period. Prevention is therefore the best medicine for outdoor plants.

Hightimes also offers some techniques as a bonus :


If you already have an industrial air conditioner, chances are you'll stay quiet (even if you're not yet safe from all the problems). However, for most growers, there are some things you will want or need to do to ensure healthy plants and high quality crops. Naturally, the first step is to try to lower the temperature by applying this method of air conditioning. This makes it a serious investment for those who can afford it.

Namely: the aeration systems usually exchange the indoor air with the outside air to ensure cooling. This means that you are likely to put some odors outside. In-line charcoal filters or odor neutralizers can be added to this process to absorb as much odors as possible.

Water-cooled heat exchangers

If you have access to a free stream of water that stays cold and plentiful, a water-cooled heat exchanger can be a great way to keep a greenhouse cool during the hottest months of summer or even the summer. year in a closed place.

The water circulating in a heat exchanger remains clean and very useful afterwards. Liquid cooling can be very efficient and the heat can be directed as far as possible from the growing room, depending on the conditions of your circulation lines. Water coolers, tanks and geothermal principles can also be used to cool the water in a heat exchange system.

Tip: The fact that your water is cold and plentiful in winter and spring does not mean it will be the same in summer. A water-cooled heat exchanger depends greatly on the temperature of the water used to function properly. Make sure the intake air is cool enough to handle the temperatures needed for your valuable crop.

Fan cooling

Extractor fans for growing rooms, tents and indoor greenhouses can be effective for cooling even during the summer months but this largely depends on where you live and how lucky you are with weather. Even the best fan-cooled installations (ie in / out exhaust) generally require a difference of approximately 10 ° C between the outside air temperature and the desired room temperature to operate within the optimal limits of Culture.

Cooling of hydroponic crops

Hydroponic methods of water culture, such as the nutrient film technique (NFT), deep-water culture (DWC), recirculated deep water culture (RDWC) and aeroponics, require little or no culture medium. Naked cannabis roots drink freely in a circular, well-aerated mineral-rich solution. Temperatures in the hydroponic reservoir above 25 ° C can create problems such as poor nutrient uptake or even root disease in some cases.

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The good thing about hydroponics in hot weather is that savvy growers cool the nutrient solution instead of the air. It is much more efficient from the point of view of equipment costs and energy consumption. Remember that these are the plants that must be kept cool, not the air ...

What about less expensive solutions?

If you have a budget and you can not spend a lot on greenhouse technology, here are some suggestions:

Although they are not really cheap, air-cooled HID light reflectors are a great way to dissipate heat from grow lamps even before they enter the growth environment.

Alternative light sources such as LEDs (light emitting diodes) or LECs (such as ceramic metal halides) may cost more for a surface of the same size as HID lamps, but they often produce much less heat. for the level of bud production they offer. Another added benefit of offsetting the initial cost is that you use less electricity to power them, or any other equipment you could use to cool your growing room, which means your cooling needs and power consumption are going down. proportionally.

A simple and economical way to reduce 30% heat in the growing room is to operate your HID lighting fixtures with remote ballasts. These are lighting systems that allow the driver to be placed (which makes the lamp turn on) in an area outside the growing room, at the other end of the cord, thus maintaining the heat generated by the lights. electronic components away from your plants.

(Photo by Erik Biksa)

The fans cool the lights, maintain the room temperature and allow fresh air to circulate in the canopy.

Plant Genetics

Some varieties of cannabis handle heat better than others, some even want it. The sativa exotic varieties (eg, Cambodian varieties) may actually prefer higher heat with higher moisture levels than the more common domestic hybrids. Desert stumps, like Afghans, can also withstand heat, but do not resist moisture.

Strains with susceptible roots or prone to insect infestations are also discouraged when grown under less controlled conditions during the warmer months. Because frequent watering is required in hot climates, the chances of root rot are much better in warm, moist soils.

The summer season is approaching, take advantage of these tips for prevention, a good holiday.

Tags : Agriculture / Growself-cultivationpermaculturePlantingPrevention