How to protect marijuana plants from thermal stress caused by heat?

The stress caused by high temperatures can hinder the development of your cannabis plants, as extreme heat is simply too much for the plant biochemistry to handle. This results in the deterioration of the plants’ health, and can even lead to their death. But there’s no need to panic: in this post we describe everything you need to know to protect your marijuana plants from this type of thermal stress.

Although marijuana is a highly resistant and adaptive plant, certain parameters need to fall within an optimal range for it to develop properly and produce delicious yields of aromatic buds.

Temperature is one of the most important factors to consider, especially when growing outdoors as the temperature is more difficult to control in this environment. Thermal stress occurs when marijuana plants are exposed to extreme temperatures or sharp temperature fluctuations, which may affect their metabolism, nutrient absorption, or their ability to perform photosynthesis.

How to identify thermal stress in marijuana plants?

One of the most important skills for cannabis growers to develop is to learn to ‘read’ the signs that plants show when something’s not quite right.

The ideal temperature for cannabis plants ranges between 19 and 26ºC. Many strains can safely withstand higher temperatures; but, in most cases, problems start to appear when the temperature rises above 30 degrees.

Therefore, measuring the temperature should always be the first step to prevent future issues. In the case of heat stress, there are certain symptoms which are a clear warning sign:

• The angle of the leaves is slightly down, and their edges start curling upwards in an attempt to block the light.
• The leaves look dry and withered. Their colour is not vibrant green but has become a lighter tone.
• Brown spots appear on the edges of the leaves. Normally, the largest fan leaves are the first ones to fall, sacrificing themselves when faced with the stress caused by excessive heat.
• During the flowering stage, new shoots appear within the flowers that have already formed. Most growers spot this quickly because these flowers take on a strange shape, with elongated shoots protruding from the typical rounded shape of the buds. This is known as ‘foxtailing’.
• Due to the stress caused by excessive heat, certain terpenes may become highly volatile and lose their essence.
• When dry roots, curly leaves, and stunted bud growth are combined, the result is a poor quality flower in terms of both yield and resin production.

How to protect your cannabis plants from heat?

Once the symptoms of heat stress have been identified, you need to act swiftly to alleviate this problem. Depending on the type of grow, you’ll need to take different measures. Below we explain what to do in each case.

How to avoid excess heat when growing marijuana indoors

One of the greatest challenges of indoor growing is doing it through the summer, when the outside temperatures are already high. Therefore, specific equipment is required, because otherwise the combination of the summer heat with the heat produced by the lamps will turn your grow tent into hell itself!

In order to control the temperature of your indoor cannabis grow, you’ll need to keep these points in mind:

• Times when the lights are on. It’s always a good idea to schedule this part of the light cycle during the coolest hours, i.e. at night and at the start or end of the day.
• Choose your lighting wisely: sodium lamps emit much more heat than other systems such as LED lights.
• Check the distance between the lamp and the tallest part of the plant.
• Use fans inside the tent.
• Use air conditioning units: if you want to grow indoors in summer in regions like southern Europe, it is most likely that you’ll need to get hold of an air conditioning system.
• Set up a good extraction system: if you live in a cold region, you may even manage to control the temperature in summer without having to invest in an air conditioning system, providing your extraction equipment has the right power to achieve this.

How to control excess heat when growing marijuana outdoors

Controlling environmental factors in outdoor cannabis grows is obviously much more complicated. Rain, pests, or, in this case, too high a temperature increase, are all out of your control when it comes to growing out in the open.

Although the cannabis plant needs many hours of direct sunlight to produce good yields, here’s a list of some measures that you can take to prevent excess heat in your outdoor grow:

• Use the right pot: avoid the use of black pots, or make use of fabric pots, which promote greater aeration of the substrate. It’s important to remember that you may have to move the plants at the hottest time of the day to help protect them from the sun.
• The time when you water your plants has great relevance. It is best to water in the early hours or at the end of the day, before nightfall. During periods with lower day temperatures, the plant transpiration processes become slower, so plants are able to retain water more efficiently.

• Protect your plants during the hottest hours: summer temperatures can exceed 35 degrees in some regions. An effective measure is to build a kind of ‘refuge’ where you can shelter your plants during those two or three hours of intense heat. All you need to do is tie up a piece of fabric or canvas so that you can create a shady space.
• Place a layer of topsoil on the substrate to help maintain the moisture in the soil a little longer and help isolate the roots from the surface heat. Some growers use hay or straw for this purpose.
• Try to prevent your plants from suffering stress caused by other reasons (dehydration, pests, fungi…). The stronger and healthier your plants are, the better they’ll recover if they suffer from thermal stress.
• Genetics matter, so choose heat resistant strains like the ones in Kannabia’s catalogue. These varieties have been developed to thrive in warm climates, which makes them less prone to suffer from thermal stress.

Kannabia’s most heat resistant strains

With these seeds, you’ll prevent your marijuana plants from suffering the effects of a heat wave, and you’ll have a better chance of filling your larder with scrumptious buds.

• Kritikal-K: a strain worshipped by those who live in the southernmost regions. A vigorous plant that withstands heat really well and produces an out-of-this-world yield. This is a must in the face of the ‘Mad-Max-like’ future that lies ahead with our rapidly changing climate!
• Big Bull: another all-rounder that withstands thermal problems without experiencing any stress. This also includes the end-of-summer rains, as long as they’re not too frequent.
• La Blanca: another perfect example of an old-school variety, but in this case from the White family. A really easy-to-grow plant that can withstand the most adverse heat conditions. In fact, she’s a classic from the toughest kind of guerrilla growing!

And, of course, any of our autoflowering strains, such as Baby Boom Auto, Mikromachine Auto, or Kickass Auto. These varieties are ultra resistant to temperature fluctuations and love being exposed to intense light so they can develop to their full potential. So, if you want to play it safe in the face of the heat waves that are drawing near, Kannabia’s autoflowering seeds are definitely the ones for you!