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3 Tips for Overcoming Crop Heat Stress

“While heat is a necessary component of healthy cannabis plant growth, overheating can cause adverse reactions that will detract from proper development and ultimately, the quality of your yield,” experts note.

Let’s take a look at the causes and symptoms of heat stress and then consider some possible remedies.

How and Why

Cannabis likes it warm, but not too warm. An ideal grow space will be between 70 and 75 degrees Fahrenheit during the day and between 50 and 60 degrees at night.

When temperatures get too high, a number of warning signs will emerge. Leaves may show yellow or brown spotting, or they may actually appear burnt in places. Leaves may curl up or down or fold inward.

When plants experience excessive heat for a long time, new growth patterns emerge. They may, for instance, generate new buds on top of old ones in what are sometimes called fox tails. This is the plant’s attempt to abandon a bud that is failing to thrive because of heat exposure. The result is an airy growth that lacks substance.

It can be easy to miss the signs of heat stress or mistake them for something else. Overfeeding, for instance, can also cause leaves to curl, although typically this condition won’t come with that same burned-out look. It’s important for a grower to carefully differentiate when examining a distressed plant.

While overheating can severely damage a cannabis grow, there are a number of relatively simple measures that a technologically equipped grower can take to both prevent and remediate heat stress.

1. Monitor and Manage

Monitoring is a key step in heat management. An environmental management system should be implemented to track temperature, as well as humidity, nutrients, and other key factors. Equipped with such a system, a grower who cannot be on-site 24/7 still can observe temperatures and even make adjustments with a smartphone app.

Low humidity can worsen the symptoms of heat stress, so keep an eye on this measure as well. Here, again, a system of sensors and monitors, along with remote management tools, can help a grower contain the issue relatively easily.

2. Add Fans

The next step is to consider circulation and make sure the heat is distributed evenly throughout the room. A small fan blowing over the tops of plants can help keep hot spots from forming under grow lights. Experts recommend using a carbon scrubber to avoid pumping undesirable aromas outside.

3. Adjust Position

Position is also important. How close are your lights to the tops of your plants? Sometimes a small adjustment is all that is needed to bring the temperature down. Some trial and error may be required, but it’s worth the effort to find a position that gives the plants maximum light without the risk of overheating. In addition to watching for signs of heat stress, a simple manual check can indicate whether lights are positioned too closely to the plants. Hold your hand under the light at plant height for a minute. If the heat is too warm for comfort, you probably need to increase the distance.

Likewise, it can be helpful to position plants in a way that allows free flow of air around the base of the plants. While heat stress shows up in the leaves, it can start in the roots. Keeping the roots cool is an important part of overall environmental management.

Heat stress can damage crops if left unchecked, but it can be easily prevented. Technology gives the savvy grower an edge here. Sensors and monitors, paired with remote management, enable the grower to keep heat at appropriate levels without always having to be on-site.

Read on to learn how to identify, prevent, and remedy crop heat stress in your growing operation.

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.

Low Humidity

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.

High Temperatures

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.

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 leave