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heat stress on cannabis plants

Heat Stress

Problem: Your cannabis plant can only withstand a certain amount of heat and light. After a certain point, your cannabis will start exhibiting signs of stress on the leaves near the sources of light and/or heat. Your leaves will get yellow or brown brown spotting and may appear generally burnt in places when there’s too much light. It’s also common for leaves to curl up or down, fold inward like conoes or tacos, and for the serrated edges of leaves to start flipping up. What else can cause dry, crispy marijuana leaves?

This cannabis plant suffered from the grow light being too close along with major heat stress during a heatwave in Southern California

Important for Hydroponic Growers! High temps can trigger root rot, a serious problem that can kill your plants.

Cannabis will also display heat stress when grown outdoors in hot, dry weather, especially when not given enough water.

When the heat gets too high, the edges of the serrated leaves will begin to curl up even if there are no burns or other signs of light stress.

When the heat gets too high, the edges of the leaves will begin to curl up and the leaves will begin to “cup.”

Heat Stress

Very low humidity can make plants more likely to get stressed by the heat. Sometimes you’ll get symptoms that look like heat stress even if it’s not that hot, and the symptoms are worse because the plant is being affected by very low humidity! Dry, hot air will commonly tip up the edges of leaves like this:

Heat stress is even more damaging in the flowering stage since plant is no longer growing many new leaves. Indica-leaning strains are most prone to heat damage in the flowering stage. Heat damage during budding will reduce your yields by demolishing many of your most important leaves, while also causing buds to grow airy with ugly foxtails.

Even though the grow lights were turned off, this is what happened to an indica-leaning plant overnight after being exposed to 105°F (40°C) temperatures during a heat wave.

If flowering cannabis plants are grown under too-hot conditions for a long time, sometimes they respond by growing new buds on top of the old ones. When you see extensive growth on top of the buds closest to the grow lights, that’s a sign that the grow light is too close or the temperature is too high. Some people call the new growth (which often grows in spires) “fox tails.”

If it seems like your cannabis plants are completely ready for harvest, but they keep putting out new white pistils at the top of the plant, it might just be heat. If that’s the case, pay attention to the lower growth to decide when to harvest.

Heat during the flowering stage also causes fox-tails, which are airy and don’t have much substance to them. It’s basically the same response as growing new buds on top, it just looks a little different on some plants. The plant is basically “abandoning” the original heat-damaged bud to try to make a sad new one.

Example of unwanted “fox-tailing” caused by too much heat

Solution: Get a way to monitor temperature. Control heat by whatever means necessary using the steps outlined below.

Indoors, find a way to lower the temperature and/or increase the circulation in the grow room or grow area if heat is the problem. Having a small fan blowing over the tops of your plants will help prevent hot spots from forming directly under your grow lights. How far away should you keep your grow lights from your plants?

You may consider removing grow lights further away from the tops of the plants if heat is a problem.

When growing cannabis, it’s best to try to keep things at 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 roots cool!

If you can keep your roots cool, it will help your plant deal with heat affecting the top of the plant. If there’s some way to protect the roots from heat, do it!

When cannabis plants are recovering from heat shock, some growers recommend using seaweed kelp extract (often available as a convenient liquid fertilizer) to help plants recover from the stress and possible even protect plants from heat stress in the future.

Many indoor setups will require that you vent out hot air using a fan and/or an exhaust system. By creating good suction with an efficient exhaust system and adding a carbon scrubber, you can also pretty much scrub all smells from the grow room. Learn more about controlling odors in the grow room.

An oscillating fan will circulate air in the room as well as provide a gentle breeze for your plants, and a small one will cost less than $20.

Outdoors, you have less options to reduce heat during a heat wave, but you are able to monitor your local weather via weather forecasts.

It is possible to partially shield your plants when you know the temperature is going to get hot. You can also adjust your watering schedule to make sure plants at least have plenty of water.

Some things to try when you know the weather outside is going to be hot or dry:

  • water plants in the evening or early morning to help prevent water evaporation during the hottest hours
  • keep roots cool – for example by putting your potted plant in a ceramic pot to help insulate the roots from the sun. I’ve also heard of growers digging a hole in the ground to place their potted plant inside, because the ground is usually cooler than the air when the temperature gets high
  • kelp extract for roots – provide a small amount of liquid fertilizer that contains seaweed kelp extract (can help protect against heat stress)
  • increase shade to reduce the heat experienced by plants – you can use an old sheet or other cloth as a short term solution, or get a profesionally made “Sun Shade Sail” which is made particularly to create shade outdoors. It’s important to remember that giving plants shade for more than a few days will make them less “hardened” to the sun, and you may need to reintroduce full sunlight back slowly to prevent them from getting shocked from the light intensity
  • move potted plants – luckily with potted plants, it’s usually easier to move them out of direct sunlight during a heat wave
  • take extra good care of heat-stressed plants – when cannabis plants appear heat-stressed, try to baby them as best you can, and offer shade during the hottest days.

When growing cannabis outdoors, it can often take a few weeks for plant to recover after a hot or dry spell, so prevention is the best medicine for outdoor plants.

Too hot for your cannabis in the grow room? Learn how to save your plants from heat stress with a variety of techniques as well as certain supplements!

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.

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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

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. ]]>