Heat stress in plant

While it is not uncommon to see plants wilting and dying in the summer heat, there are a couple of ways to preserve the plants’ health before it’s too late.

First and foremost, the best way to help your garden survive hot weather is by providing adequate moisture, which also means not to drown your plants by overwatering.

“In the heat of the summer, the best way to water efficiently is to use drip irrigation. All of the garden centers and home improvement stores have kits and additional parts,” said Dr. Gary Bachman, GRP, CPH extension/research professor of horticulture for Mississippi State University.

Avoid watering a plant’s leaves to minimize disease risk. If you have a sprinkler, put it on a timer for the early morning or dusk hours, so it will dry quickly.

“Watering in the morning is best as it allows the water to move into the roots zone and getting the plant hydrated before the heat of the day,” Bachman said.

Stephanie Huckestein, instructor at Indoor Plants Hahn Horticulture Garden at Virginia Tech, said plants need more water in the summer than other times of the year.

“Plants more readily transpire (a plant or leaf to give off water vapor) and respire during the hot summer months, so more water is needed for most plants,” Huckestein said.

Many home gardeners like growing plants in containers, and it is even more important to make sure the containers are well hydrated each day.

When selecting a location to put a garden, it is best to choose a location with morning sun and afternoon shade. The afternoon hours of sunlight can be very harsh on plants.

“Not all plants are tolerant of shade. The afternoon sun is more intense than morning sun, so if a plant needs part sun, it is best to provide that sun during the morning hours, so better to locate them in an east exposure instead of a west exposure,” Huckestein said.

There are many plants native to warmer zones that do well in summer weather, and they are able to tolerate hotter conditions.

Experts recommend spacing out plants so they do not compete for water.

Pulling weeds helps the plants because the weeds compete for nutrients and water. Once the weeds are gone, gardeners should add some extra mulch or pine straw to keep heat out and moisture in.

“Applying a layer of mulch no deeper than 2 or 3 inches will help conserve precious soil moisture and cool the soil during the hottest parts of the day. Remember that mulching while conserving moisture does not negate the need to water consistently,” Bachman said.

A shade cloth could help some plants but may hurt other plants that require more sunlight. If you don’t want to install a shade cloth in your garden, give your plants some extra nutrients.

“Coffee grounds, fertilizers and compost add necessary nutrients for optimal plant health and a healthy plant is more tolerant of adverse conditions than one that is not so healthy,” Huckestein said.

Not all plants have the same fertilizer requirements, so check the plant’s requirements before applying fertilizer.

Bachman said it is very important to add slow release fertilizer, like Osmocote, as this will help establish a good nutritional base before the heat of the summer.

“There aren’t really any fertilizers that you can apply in the hot weather months, except for using some liquid feed to maintain good growth and flowering, like Miracle Grow, during regular watering,” Bachman said.

According to Bachman, one of the hardest situations for the home gardener is a flowering plant that starts to decline. Bachman said he gets numerous frantic calls asking how to save those type of plants.

“My advice is to remove and use this as an opportunity to try something new. This saves the home gardener from the angst of worrying about the plant,” Bachman said. “Now this advice is for flowering annuals and such that are readily replaced, not trees and shrubs.”

Cold temperatures can also cause the same type of tissue damage. In the fall, cold nights cause wilting and eventually the cold causes cell damage. Brown leaves and dead plants mark the end of the season.
A couple other factors are over-fertilization and disease. When you apply too much fertilizer you can create an imbalance in the plant’s growth rate forcing the roots and stomata to work themselves to death; the root structure may not be able to support the plant growth and the lack of water movement causes wilting.
Some plant diseases cause wilt (often seen in tomatoes); there are usually other signs associated with this type of wilt and sprays and powders that can help when you identify it. If your soil is good, your water practice is good, and the plant doesn’t recover under cool conditions, the wilting may be due to a fungal disease. Researching the plant, and it’s susceptibility to wilting diseases, can help you take appropriate control measures.

There are a few things you can do to reduce harmful wilting. The best control is maintaining consistent soil conditions. If an organically-enriched soil is constantly at an appropriate moisture level through efficient watering and mulching practices, the plant will have the water when it needs it.You won’t need to worry about plant damage even when the plant looks stressed during the heat of summer.
Knowing your plants and selecting heat and sun loving plants for hot and dry conditions is good. Plants that have a natural tolerance to heat stress may show no sign of wilting even in extreme conditions. Xeric plants are in this category. Plant tags, plant catalogs, and online resources will often identify how much sun and how much water a plant requires. Matching the plant with the appropriate location is always the best planting practice.
Most importantly, don’t automatically reach for the garden hose when you see wilting. In addition to the root issues I discussed above, spraying the leaves can cool the air temperature and cause the stomata to open. If there isn’t enough moisture accumulated in the roots or soil, you will force the plant to lose water from its cells and that may be enough to cause irreparable damage.
Once you understand wilting and realize it’s nothing that requires overreaction, you can relax and actually enjoy watching the natural process. Seeing a plant recover from the sad state of wilt to a fully erect display of health in just a few hours is an amazing thing.

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.

Learn 5 secrets to controlling heat in the grow room!

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.

Learn everything you need to know about controlling temperature in the grow room

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.

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Tips For Care Of Plants And Flowers In Hot Weather

When the weather suddenly skyrockets with temperatures above 85 F. (29 C.), many plants will inevitably suffer from ill effects. However, with adequate care of outdoor plants in extreme heat, the effects of heat stress on plants, including vegetables, can be minimized.

How Plants Cope with Heat

So how do plants cope with heat once temperatures begin to soar? While some plants, like succulents, are well equipped with handling heat by conserving water in their fleshy leaves, the majority of plants do not have this luxury. Therefore, they will normally suffer from the in heat some way or other.

Generally, heat stress of a plant will show itself by wilting, which is a sure sign that water loss has taken place. If this is ignored, the condition will worsen, as the plants will eventually dry up, turning a crunchy brown before dying. In some cases, yellowing of the leaves may occur.

Heat stress of a plant can also be recognized by leaf drop, especially in trees. Many plants will actually shed some of their foliage in an attempt to conserve water. In excessively hot weather, many vegetable crops have difficulty producing. Plants like tomatoes, squash, peppers, melons, cucumbers, pumpkins, and beans will usually drop their blossoms in high temps, while cool-season crops, like broccoli, will bolt. Blossom end rot is also common during hot weather and most prevalent in tomatoes, peppers, and squash.

How to Care for Plants in Hot Weather

The care of plants and flowers in hot weather is pretty much the same with exception to container plants, or those that have been newly planted. Of course, additional watering is a given, with new and potted plants requiring even more irrigation. In addition to watering more often, mulching plants can help conserve moisture and keep plants cooler. The use of shade covers, especially on vegetable crops, may be helpful as well.

Container plants will require daily watering, even twice a day in high temperatures. These plants should be given a thorough soaking until water can be seen coming out from the drainage holes. Placing water granules in pots also helps. As these will gradually soak up excess water, in times of dryness, the granules will slowly release some of this water back into the soil. Moving potted plants to a shadier location during the heat of the day is also recommended.

How To Revive Sick Cannabis Plants

Your plant is wilting, you notice the leaves turning yellow or forming unsightly spots, or maybe it’s refusing to grow altogether. There are many reasons why cannabis plants can become sick, from issues with watering to pest infestations, inadequate lighting, heat stress, and more.

No matter the reason for your plant’s sickness, the first thing you’ll want to do is diagnose the problem. When you have addressed the cause(s) of your plant’s condition, you obviously want to revive it as fast as possible. Here are some things you can do to help plants recover from infestations, illness, and more.


If your plant appears to be dying or suffering hard, it is unlikely that a minor issue is occurring. Most of the time, a rapid descent in the health of your plant signals a fundamental issue or invasion. This can involve problems with environmental conditions, microscopic infestations, and other culprits.


If you’re growing indoors, the first step to reviving your plants is to check the temperature and relative humidity of your tent or grow room. The ideal temperature for cuttings and seedlings is between 20–25ºC. As the plants get older, they can tolerate a bit more, up to 28ºC. Everything above this is excessive and causes stress, which will make it much more difficult for your plants to recover.

Likewise, the humidity levels of your room must be kept within a certain range depending on the phase of growth. An optimal humidity level for flowering plants is 40–50%. Plants in the vegetative growth phase can tolerate a more humid environment, from 40–70%. If the humidity is too high, you need to look into better ventilation for your grow space. A dehumidifier is the best, albeit expensive option here. Your sick plants will have a hard time recovering if their environment is not stable and optimal.


Despite cannabis loves plenty of light and warm temperatures, if you grow outdoors in the summer, heat stress and excessive sun can be a problem, especially for plants recovering from illness. If you have your plants in pots and they look stressed from too much heat, move them to a shadier location. Less heat and direct sun will make it easier for sick plants to get back up to strength.


Cultivators normally keep their wattage levels as high as possible to encourage plants to grow faster. More light means the plant is working harder and will likely produce a greater yield. On the other hand, a plant that is working extra hard is more susceptible to deficiencies and other problems. One way to give your sick plant a break is to decrease the light intensity. Move your lights higher up and further away from your plants, or decrease the wattage.

When you grow indoors with your lights on a timer, you can also cut down on the daily light hours your plants receive. When you reduce the light hours for the vegetative phase to only 17 or 16 a day, this will give your plants more time to “rest” and recover.


Many problems with sick cannabis plants can be due to overfeeding. When your plant can’t take up the nutrients that you provide, salts and minerals will accumulate in the soil over time. This will change the pH level at your plant’s root zone, making it more acidic—beyond the small pH window that cannabis has for healthy growth. As a result, your plant is not able to take in nutrients, even if they are present in abundance. When this happens, further feeding only makes it worse.

In almost all cases where your plants show signs of nutrient deficiencies or nutrient burn, you should give your plants a solid flush. Flushing means that you rinse out the excessive salts with pure, pH-balanced water to restore the optimal pH of the growing medium.

To flush your plants, drench the growing medium with water numerous times. It should be ample enough that liquid comes out from the bottom of the container each time. For example, if you grow in 7l pots, flush your plants with 14l of water. When you grow in soil, your water should have a pH of about 6.5pH. After the flush, you can begin giving nutrients again, starting with ½ or ¾-strength doses. You can slowly work your way up from here to avoid putting plants under any additional stress.


Repotting your cannabis plants into new, larger containers with fresh soil can also help bring them back to life. Choose a container that has plenty of room for the roots to grow. If your plant is severely damaged due to overwatering (root rot) or various fungi, consider trimming its foliage. When the roots have fewer leaves to support, they can recover faster.


Plant infestations from spider mites, fungus gnats, fruit flies, and other insects are all too common when you grow cannabis. When you have finally gotten rid of the pests, you want to make absolutely sure that they don’t return. Pest infestations can really ravage a plant, so it certainly needs optimal care and time to be revived.

In terms of keeping the pests away, there are natural insecticides like neem oil that can be highly effective. You can even use it as a foliar spray, applying it to your leaves every 2 or 3 weeks. However, be careful during the flowering phase as you do not want the overbearing taste of neem oil on your buds! For fungus gnats, you can also set up yellow sticky traps, which will catch most of them.


There are certain supplements you can give to your sick plants to reduce stress, support their development, and increase their resistance.

Compost Teas

For those growing in soil, compost teas are an excellent supplement to support the recovery of sick and stressed plants. Compost teas can make your plants grow faster and more robust, making them less susceptible to diseases and deficiencies. Some cultivators make their own compost teas at home, although they can also be purchased at most well-sorted grow stores.


Silica has properties that strengthen the cell walls of your plants, which makes it helpful for increasing their resilience. Furthermore, it makes certain minerals and nutrients more available while protecting the roots as well.


Seaweed contains minerals and other micronutrients, and has been shown to help reduce plant stress. While its mechanism is somewhat unknown, seaweed has long been part of the weed grower’s arsenal.

With the above tips, we hope you are able to revive your precious cannabis plants and make it to a hefty and healthy harvest.

Happy Growing!

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.

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