How To Protect Your Cannabis Plants From Heat Stress
Growing cannabis can be a tricky proposition. However, there are some tried and true techniques to preventing damage to the plants from overheating.
Are your plants looking a little worse for wear? Despite following an accurate feeding, watering, and light schedule, you are frustrated to discover discoloured and deformed leaves on your cannabis plants. Don’t worry, you’re probably doing everything else correctly, it could just be the signs of heat stress.
Cannabis is a hardy and resilient plant, and many original landrace varieties adapted to survive in harsh regions such as the Hindu Kush mountain range and the tundras of Central and Northern Asia. Despite the resilience of this plant species, it can only deal with a certain amount of heat before its physiological systems can’t take it anymore.
HOW CAN YOU IDENTIFY HEAT STRESS?
One of the telltale signs of heat stress is when the tips of fan leaves begin to curl upward. They’ll also exhibit a generally dry and withered look. This factor indicates that heat is the most likely culprit, removing the possibility of most nutritional deficiencies. As well as looking generally unwell, leaves will develop large brown spots or blotches that manifest in irregular shapes, primarily along the edges of the fingers of fan leaves. These unsightly scars are usually accompanied by yellow patches of discolouration. These symptoms mainly affect leaves located near the top of the canopy and around the perimeter of the plant—areas in the direct line of fire.
If you detect this set of symptoms in the vegetative phase, you can be sure that heat stress is at the bottom of it. Indoors, this could be caused by allowing the tops of your plants to grow too close to a powerful light source. Outdoors, it could be the result of a particularly brutal heatwave or an extremely hot and dry climate.
Heat stress manifests differently during the flowering phase. Sometimes, much to the shock of many growers, new buds can start to erupt out of the tops of older ones. This can cause what is known as a foxtail, a pillar of small buds and sugar leaves. This is a survival mechanism initiated by the plant as it attempts to form new buds capable of reproducing and generating seeds.
Below, we’ll cover indoor and outdoor methods to prevent and treat heat stress.
DEALING WITH HEAT STRESS INDOORS
Heat stress can affect an indoor grow in multiple ways. For one, growers who live in hot climates may find it hard to regulate indoor temps during the dog days of summer.
Regardless of climate, grow tents can become very warm if certain measures aren’t put in place. The heat generated from different light sources can pound down upon leaves and subject them to significant stress. A lack of adequate fans and an exhaust system prevents convection currents that would otherwise cool down the interior of the tent. Here are some tips to help you avoid heat stress indoors.
PLACE FANS IN THE GROW SPACE
This is a simple and cheap solution to start cooling down your growing environment. Fans disrupt still, hot air and will effectively create a convection current that will help to cool things down. The wind generated by fans will also gently stress your plants into developing thicker and stronger stems.
USE AIR CONDITIONING
This might seem obvious, but it’s definitely a viable solution. Indoor growers have the advantage of taking complete control over the microclimate of the grow tent, provided they have access to the right equipment. This is a fairly expensive option, but can be a real life-saver if you live in areas such as Spain or the southern United States. Place an air conditioning unit in your grow tent to keep cool air circulating.
CHANGE THE POSITION/TYPE OF LIGHTING
If your plants begin to exhibit signs of heat stress, consider how close they are to the light source. If only the top fan leaves are showing symptoms, then readjust the position of your lights so that they hang further away.
If changing the position makes no difference, then you might need to change the type of light you are using. Most lights put out a fair amount of heat, and if you’re growing in a confined space in a warm climate, things are going to heat up quickly. LEDs are a good option for growers dealing with these conditions. They put out a lot less heat than HID light sources; plus, they are often much cheaper to run.
INSTALL AN EXHAUST SYSTEM
An exhaust system is like a reverse fan. It works by sucking stale and hot air out of the grow space. If you have fans running at the same time, these will serve to replace old air with fresh and cool air. If you’re running a deaerator, it’s advised to fit it with a carbon filter that will remove any suspicious scents from blowing out into the surrounding area.
USE SUPPLEMENTS TO TREAT DAMAGED PLANTS
Some growers find success using supplements to treat the symptoms of heat stress. These may be effective, but will only work long-term if the environment is cooled down using the techniques mentioned above.
Seaweed or kelp extracts work well to remedy some of the effects of heat stress. They are loaded with minerals and nutrients that make plants more resistant to high temperatures. Then there’s silicon, a compound that increases the resilience of cell walls, makes plants more resistant to heat and cold stress, and helps them absorb key nutrients such as zinc, copper, and phosphorous more easily.
HOW TO PROTECT AGAINST HEAT STRESS OUTDOORS
Outdoor growing is more difficult to control than the alternative. Crops are subject to the whims of the elements, as well as countless forms of pests and infections that can damage vitality and yield. Heat stress is no exception. Indoor growers have the advantage of full climate control, whereas outdoor growers have to deal with heatwaves, monsoons, and everything in between.
Heat stress is a prominent threat to those growing in warmer regions closer to the equator. Although increased levels of sunlight can be advantageous, too much can lead to heat stress symptoms. Here are some tips on how to protect your outdoor crop.
TIME YOUR WATERING WELL
During a heatwave, try to water plants either in the mornings or evenings when the temperature is less intense. Watering your plants in the middle of the day might seem like a good way to cool them down, but it can actually be harmful. Water droplets can magnify sunlight and cause increased heat to be directed toward plant tissue.
POTS ARE MORE PORTABLE
If you’re growing outside in a warm climate, it’s a good idea to grow in large containers or pots as opposed to directly in the ground. This makes your plants portable and gives you the option of physically relocating them to a sheltered spot if the sun gets too intense.
SET UP A TEMPORARY SHELTER
Keeping your plants under a shelter all day long will inhibit photosynthesis and have a detrimental effect overall. However, making a temporary shelter using cloth or a tarpaulin can help to protect your crop during the peak heat of the day. Erect the shelter during the hottest hours, and let your plants bask in sunshine before the heat sets in and after it tapers off.
OUTDOOR PLANTS CAN BENEFIT FROM SUPPLEMENTS, TOO
Seaweed or kelp extract can help increase plant resilience both indoors and outdoors. Use the formula according to the manufacturer’s instructions and give your plants some extra protection using these nutrient-rich substances.
START WITH A HEAT-RESISTANT STRAIN
There are thousands of cannabis strains available on the market, and each one has been selectively bred over time to display specific desirable traits. Some breeders have managed to create cultivars that are particularly resilient to high temperatures. You can significantly reduce the risk of heat stress by starting with one of these varieties. Here are three heat-resistant strains we recommend.
Fruit Spirit is a hardcore strain that laughs in the face of high temperatures (within reason). This beast is the offspring of legendary parent strains White Widow and Blueberry. She features a closely balanced genetic makeup of 60% sativa genetics and 40% indica. Fuelled by 18% THC and a medium CBD level, she offers a high that relaxes the body and invigorates the mind. Her name is a tribute to the sugary and fruity tastes of her flowers, which dance across the tongue when smoked.
Fruit Spirit grows well in both indoor and outdoor environments. Indoor plants reach a height of 80–120cm, feature a flowering time of 8–10 weeks, and yield up to 425g/m². A single plant grown outdoors can reach up to 220cm in height and produce a harvest of up to 525g.
Cannabis is a hardy plant, but it can also be damaged by too much heat exposure. That said, there are some easy ways to prevent heat damage.
Autoflowering Plant Heat Stress Symptoms
Your plants can only withstand a certain amount of heat and light. After a certain point, your autoflower will start showing signs of stress on the bud or leaves. Your leaves can get yellow or brown, appear burnt or bleached, it’s also common for the leaves to fold in a taco shape all of those symptoms and more we’ll explain here.
1. What is Heat Stress?
Heat stress can happen indoors or outdoors, it occurs when your autoflower is exposed to high temperatures for a long period of time.
This can cause a wide variety of problems, stunting growth, affecting yield and ultimately killing your plant.
2. What Causes Heat Stress?
Heat stress indoors is a result of not adjusting your climate for optimal growing. Outdoors it’s harder to control but in both situations, heat stress is caused by the same factors: low humidity, high temperatures, and high light intensity.
Let’s examine heat stress factors and learn what solutions can be applied to each situation.
The ideal humidity for an autoflower is 60% depending on the stage it’s in. Very low humidity can make plants more likely to get heat stressed. Sometimes you’ll get symptoms that look like heat stress even if it’s not that hot, they can be worse because the plant is being affected by very low humidity.
Low humidity won’t stress your plant, it can affect growth and yield but there won’t be any apparent symptoms unless it is combined with high light intensity and/or high temperatures.
If you’re growing indoors, there are a couple of solutions. You can use a humidifier for a long term solution or you can place buckets filled with water inside or around your grow tent, always checking the hygrometer to keep humidity at an optimal level.
Outdoors there’s not much you can do, try to place your plant in the shadow for a few hours a day, we also recommend to water more times with less water throughout the day, this should keep the roots cooled down.
The optimal temperature to grow autoflowers is around 25 Celsius (77f). Heat stress is primarily caused by high temperatures. Your plant will always show you when she’s not happy, it’s essential to keep an optimal climate for your autoflowers, keeping in mind that flowering plants are even more susceptible to heat stress.
Plants in the vegetative stage usually start to fold leaves inward in a taco or cup shape and can start to damage leaves even if the temperature is not that high. Combined with low-humidity it can wreak havoc.
As said before, plants in flowering are even more susceptible and if a lot of leaves are damaged it will respond to overheating by growing buds with less potency and eventually with abnormal growth of buds. In result they will look like what is known as foxtail. It does not always look like that, sometimes it’s just an abnormal growth of bud.
What happens is the plant is trying to abandon the heat-damaged bud and start a new one. At this stage you should see a lot of white pistils growing.
Solving this primarily revolves around increasing air circulation in your grow space, if this is not possible an oscillating fan blowing on top of your plants might be a good alternative. If you’re growing outdoors you should try to cool down your roots. Instead of watering your plant once a day, you should water it multiple times with less amount of water. This way you keep the medium moist and it can help to cool down the roots.
Tip: If the damage is already done, use seaweed kelp fertilizers to help your plant recover, they contain cytokinins which help reduce stress.
High Light Intensity
Light burn or light stress only occurs indoors, it can happen when transferring a plant from a weak to a strong light or if the light source is too close to your autoflower. It also can happen to older leaves that have been exposed for a long time but that’s not common.
Usually grow lights come with height recommendations for both stages (vegetative and flowering), you should always experiment and test what works better but you should never start with placing the lights too close.
The most common symptoms are yellowing, burned leaves and bleached buds (when the bud starts to turn white). The first signs a plant is getting too much light is when the leaves start pointing up (sometimes you don’t even see any symptoms until the yellowing starts).
Often the leaves start turning yellow but the veins stay green and may appear pale. If it goes for a long time, leaves will start to taco, the tips start to turn brown and crispy, and eventually start breaking to touch.
Tip: Light burn should not be confused with a nitrogen deficiency, we recommend paying attention to the small differences. Nitrogen deficiency starts from the bottom and moves up, nitrogen-deficient leaves will fall on their own. Light-burned leaves are hard to pluck-off and in most cases, the yellowing will occur on top of the plant.
Bud bleach is most common with LEDs, basically happens when the bud gets too much light and starts turning white at the top, the affected part will lose potency and it’s smell.
If your plants are getting too much light, try moving your lights further away, removing some of the lights or look into a dimmer to control the intensity of your grow light. To prevent bleaching you should look into low stress training to prevent your plant from stretching too much.
Unfortunately, there is no way to bring your plants back to normal other than let them grow it out or harvesting before they die in the worst cases. The best way to prevent this is by taking all precautions before it happens.
3. In Conclusion
When making changes to your plant’s environment it’s best to make changes as slow as possible. To keep a good climate for your autoflowers you need to prevent a sudden change of humidity, temperature, and light. Keep in mind these 3 factors as they are tied together when talking about climate.
A thermo hygrometer is an instrument that measures temperature and humidity, always use one when growing indoors. It costs around 15 bucks and it can help save your harvest if you’re having the problems we discussed above.
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