How to identify and cure cannabis heat stress

How to identify and cure cannabis heat stress

Cannabis heat stress refers to the complexed issues which can arise when cannabis is grown in conditions which are simply too hot for the plant biochemistry to cope with.

Fundamental cell functions become increasingly difficult to carry out as temperatures rise. The result is deteriorating plant health until the heat stress can be stopped.

If cannabis heat stress can't be addressed promptly then you will see both yield and quality of your harvest compromised. In the worst cases your plant simply may not survive.

Understanding cannabis heat stress to better prevent it
Cannabis heat stress symptoms
Identifying and curing indoor cannabis heat stress issues
Identifying and curing outdoor cannabis heat stress issues
Cannabis heat stress FAQ
Tips and tricks to address cannabis heat stress
Tackling cannabis heat stress

Understanding cannabis heat stress to better prevent it

Cannabis heat stress may have several contributing factors. Sometimes the heat stress and poor environmental conditions may cause other issues e.g. with nutrients. It can be a complicated picture to resolve and difficult to remedy.

Low Humidity
In high temperatures, one method cannabis tries to naturally combat the stress is with increased transpiration. Transpiration infers that water needs to be present in the plants environment and root zone in order to allow transpiration. But in low-humidity environments transpiration becomes much more difficult.

Low humidity

Nutrient Burn
If a cannabis plant is undergoing heat stress it helps if the plant is in good health to cope with the stress. However, if your plant is already suffering from nutrient issues (e.g. nutrient burn) then this will compromise your plants ability to manage the additional difficulties that come with heat stress.

Nutrient burn in cannabis

Similarly, if your plant is suffering from nutrient deprivation and then has to deal with heat stress you can expect to multiply your problems significantly. Keeping the cannabis plant in the nutrient sweet spot (without over feeding or under feeding) from seed to harvest is the ultimate aim for any grower.

Too Much Light
Anyone that has placed their plant too near the grow light may have seen growth stop and possible bleaching near the tip. Cannabis can only tolerate certain levels of light intensity before the plant becomes overloaded.

Kerosene Krash with lightburn

If you do have too much light, with PPFD levels exceeding 1300-1500 then you can expect issues unless you have CO2 supplementation.

Check with your light manufacturer for recommended hanging heights and ensure that you’re not pushing your plants a little too hard. Excessive PPFD (light intensity) in combination with heat stress will bring a sharp end to any growth in your grow room and will severely compromise harvest quantity and quality.

Poor Ventilation
One common cause of cannabis heat stress is poor levels of grow room extraction. The two main reasons to have good ventilation are to remove heat and to ensure constant supply of fresh air for photosynthesis and respiration. Check to ensure your extraction fan is adequate for your grow tent and upgrade if necessary.

Auto Lemon Kix with ventilation

Growers in hot countries should be aware of ambient external temperatures. If the outside air temperature is e.g. 35ºC then no matter how powerful your extraction fan you will never get temperatures low. In these circumstances the best option is air conditioning.

Equipment Failure
Failure of e.g. a grow room temperature sensor which controls your extraction fan can lead to severe issues. So can failure of the extraction fan itself, or some other key equipment failure.

In a small grow room it should be easy to check that all your equipment is in working order. Some serious hobby growers even keep replacements for items such as carbon filters, pH meters etc for the inevitable day when they need to replace them.

Cannabis heat stress symptoms

The conditions required for heat stress can vary from one strain to another. Certain jungle sativas, for example, can have greater resistance to heat than an indica in a high humidity environment.

Note that some nutrient issues, or other environmental problems can give similar symptoms to heat stress. Be careful not to misdiagnose and mistreat the issues or you can make matters worse!

Stage Symptoms
Cannabis seedling heat stress

● Slow growth of the seedling.

● Newer leaves tend to look pale and withered.

● Stunted seedling growth. In severe cases the seedling will die.

● Seedlings are extremely vulnerable and lack the strength of more mature plants to fight heat stress.

Vegetative cannabis heat stress

● Curling leaves which may look dry with withered leaf edges.

● Spots on the leaves which may be dry and brown.

● Tips of the leaves may look burnt and curled. Eventually the browning works its way down the plant.

Flowering cannabis heat stress

● Canopy leaves may look bleached and yellowed.

● Leaves may curl rather than be flat.

● Foxtail buds, long airy buds that emerge unusually from the top of other buds.

● Unusual amounts of white pistils before harvest (when you might expect more orange hairs) is a sign of heat stress in bloom.

● Severe heat stress in bloom can have irreversible effects.

Identifying and curing indoor cannabis heat stress issues

Indoor growers have quite a challenge to provide optimised grow room conditions. They must fill the grow room with the necessary equipment and lights but also ensure that fresh air is introduced to remove heat and stale air. Get it wrong and there will be inevitable issues.

Many growers aim for a complete air change in their tent every 4-5 minutes. But if you live in a hot country, or experience a heat wave, the external air may also be hot. If that’s the case your only realistic option is an air conditioning unit to reduce temperatures nearer your target of around 22ºC-24ºC (around 70-75ºF).

If your ambient temperatures are low, you may not need air conditioning, and may be able to reduce canopy temperature levels simply by raising the height of the light and increasing extraction/ventilation.

LED light burn

Tips and recommendations to address indoor cannabis heat stress issues:

  • Consider increasing the distance between your canopy and the lights

  • Use oscillating fans to move air around grow room - avoid still/stale air pockets

  • Consider upgrading to LEDs which produce comparatively less heat than HPS

  • Consider the use of air conditioning if temperatures reach excessive levels

  • Upgrade your extraction fans so that fresh air fills your room at least every 5 minutes

  • Keep grow room temperatures below 25°C, many serious growers constantly monitor temperature and humidity

  • If necessary, avoid growing during the summer months and grow indoors during cooler seasons instead. Without air con, summer indoor growing is simply impractical for many.

  • Consider switching your grow lights on at sunset and switching them off at sunrise. This can allow the plants to avoid ‘lights on’ during the hottest part of the day

Identifying and curing outdoor cannabis heat stress issues

Dealing with excessive heat in an outdoor grow presents some extra challenges. With indoor growing, if all else fails, a powerful air conditioning unit can restore your temperatures to optimum. But in the great outdoors your choices are more limited, but you still have options.

Dealing with a serious heat wave is difficult, sometimes it can be a matter of damage limitation rather than seeking a cure. Planning ahead can allow you to choose more heat resistant strains or locations which offer some midday shade.

Cannabis showing heat stress symptoms

  • If growing in containers, try to keep them cool. You may be able to move the containers to a cooler area, perhaps in the shade. Keep the containers cool and avoid putting them directly on e.g. a hot tiled surface. Avoid black containers which absorb heat and cook the roots, you want to ensure a cool root zone.

  • Large containers can retain more moisture, making life easier for the roots. If you do have black plant containers, try covering them with some white wrapping to reflect the light and maintain cooler temperatures.

  • Water in the morning. Avoid making your plants wait through a full, hot day until you water them. Ensure they start the day well with a good watering. If necessary, water them at the end of the day too.

  • Seaweed and kelp. These natural nutrient sources are rich in minerals and can help your plants cope with heat stress.

  • Offer protection from the sun. A simply awning or cover pulled over your plants to create shade in the hottest parts of the day can reduce heat stress levels for your plants

  • Choose heat resistant outdoor strains that are adapted to life in hot climates. This may include certain sativa strains for hot climates with high humidity levels. But it’s important to add that some tough indica strains (e.g. Mazar, Banana Blaze, Master Kush, Auto Blackberry Kush…) will also perform well in hot climates with a low humidity

Cannabis heat stress FAQ

What are the best heat resistant cannabis strains?

Strains for hot climates with high humidity include Desfrán (and Auto Desfrán) / Power Plant (and Auto Power Plant) / Skywalker Haze (and Auto Skywalker HazeAuto Daiquiri Lime / Auto Euforia.

Strains for hot climates with low humidity (and possible cold night-time temperatures): Mazar (and Auto Mazar) / Banana Blaze (and Auto Banana Blaze) / Master KushAuto Night QueenAuto Blackberry Kush (possibly Blueberry / and Auto Blueberry).

Can transplanting your seedlings help to recover from heat stress?

Moving a cannabis seedling to a larger container may also present certain risks. If your cannabis plant is recovering from heat stress it may be wise to allow some time for the plant to recover from the main symptoms, such as drooping leaves/branches before attempting to transplant.

When transplanting be very careful not to damage the rootball, this will only add to your problems. When you transplant, the stress of the process can be lessened if you use beneficial mycorrhiza in the new soil.

This will help your root zone to maintain maximum health. Ensure the new soil is well watered so the plant adapts to its new home quickly.

Can heat stress cause a cannabis plant to produce seeds?

Heat stress, and indeed any form of stress, can cause some plants to become hermaphrodites (‘hermies’) and produce male flowers. Male flowers release pollen and this can cause seeds to be formed.

When is it too late to revive a cannabis plant from heat stress?

Extreme heat stress

Much depends on the severity of the heat stress. If your plant was in vegetative growth and has had time to recover then you may be able to see a relatively normal bloom. If the heat stress was severe and hot during mid bloom there simple may be little that can be done to rescue your plant.

In such cases, view any buds that you are able to harvest as a bonus and focus your attention on avoiding the situation in the future - if possible. Prevention is always better than cure when it comes to cannabis cultivation problems.

When and how to water cannabis plants suffering from heat stress?

Don't stress the plant further by shocking the roots with ice cold water or putting it directly below an air conditioning unit. Add water with a temperature of around 20ºC slowly once your plant is in a cooler environment, taking care not to over-water your plants. You may want to let your plant recover in a cool area for a day or two.

Tips and tricks to address cannabis heat stress

Many growers encounter cannabis heat stress and manage to work their way through the problems. Sometimes it will be an easy fix, such as raising the height of your light and increasing extraction.

Other times the problems will require longer term solutions, such as avoiding growing at certain times of the year, or installing air conditioning.

The following advice is based on several decades of experience and customer feedback, it may be useful advice for you.

LED vs HPS. Most serious growers are giving due consideration to the cost of upgrading their grow room to a quality LED fixture. Yes, it’s probably the most expensive grow room improvement you can make. But it’s also one of the best.

Cannabinoid levels increase measurably, heat stress and inherent IR heat decreases significantly. Heat production goes down, energy usage reduces. If you can afford it, and if heat is an issue, upgrade to a good quality LED as soon as you can.

Air cooled HPS vs regular HPS. If you cannot afford the LED upgrade just now, consider switching your HPS to an air-cooled HPS. An air cooled HPS encloses the light in a sealed air stream which can significantly reduce grow room temperatures.

Lighting time. Grow lights on at sunset, lights off at sunrise is good advice. It allows you to avoid your plants having to cope with the combined effects of ‘lights on’ during the warmest part of the day.

Conversely, if you suffer from cold grow room temperatures (e.g. during winter), you may want to grow autoflowering cannabis seeds under a 24/0 light schedule to maximise temperatures.

Extraction. Many growers routinely use extraction rates that are below optimum. Ensuring good ventilation helps remove hot air and reduce grow room temperatures.

Raise light height. Simply raising your light, especially if it’s an HPS, can help reduce canopy heat stress/temperatures.

Air con and fans. If all else fails, do what the professionals do and use air conditioning!

Tips for outdoor growers:

Create shading for your plants. When growing in soil in an open field, try to build a basic shelter with some poles and netting. Usually a white net/cloth works just fine to protect the plants from the severe heat and intense sunlight during summer days.

Purple #1 in a shaded garden

Move the plants to a cooler spot in your garden. When growing in containers, try moving them to a spot where they receive a little less sunlight or where they are protected from direct sunlight during the middle of the day (when the sun shines the brightest)

Protect your rootzone. You can do this by adding a layer of mulch or white (light reflective) stones at the base of the stem. This will help to create a buffer-zone, so the upper part of the soil dries out less quickly. When growing in containers it is best to use breathable pots like smartpots, rootpouches and airpots.

In the case they are black, try wrapping them (not tightly though, they still need room to breathe) with white plastic in order to reflect the intense light instead of absorbing it.

Consider innovative watering solutions. If you are growing in extreme heat you may want to consider grow methods with some level of increased sophistication/automation rather than simple soil-growing. One challenge here is to keep your water temperature cool too.

A good solution for most growers is the usage of a system like Autopot, this semi hydro-system can be used in combination with Smartpots which allows the plants to drink whatever they need each day completely by themselves (without needing to be connected to power).

By using a large reservoir they can survive during the toughest summer days and will draw water/nutrients when required.

Frisian Dew outdoor grow

Tackling cannabis heat stress

Heat stress can't always be beaten, sometimes an extreme heatwave can simply be too much to deal with. But with some creative thinking and up-front planning you may be able to avoid cannabis heat stress in future.

Changes to your grow room and the timing of your grow can make a big difference. So can the type of cannabis seeds you use, choose the best seeds you can source and good luck with your grow!

How to identify and cure cannabis heat stress
July 2nd 2021
Categories : Cannabis Plant

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