With summer lurking and temperatures rising, we are all aware it’s time to start protecting ourselves against the harsh sun. Some more than others, we are all familiar with and eager to avoid the dangers of high heat and stay safe during summertime.
Plants are no different than us and will also suffer from excessive heat. High temperatures across long periods will cause them stress and harm. But, unlike us, plants can’t just find a cooler spot, so we need to aid and protect them.
To avoid irreversible damage, here’s all you need to know about heat stress in plants, from how to identify it and how to treat it, to what plants are the best at withstanding scorching temperatures.
What Is Heat Stress in Plants? Why Is it Harmful?
As the name suggests, heat stress is a stress response plants will display when exposed to excessive heat, both air and soil heat, as well as harsh sunlight.
This can go from a simply sad-looking, dehydrated plant to irreversible damage. It can force leaf drop and dormancy, stunted growth, low-quality crops, and poor re-seeding for the next season. Too many attempts at saving itself throughout the summer and your plant will become chronically conditioned, altering its ability for it to grow and develop normally.
Furthermore, your plants will become more susceptible to further damage from other diseases, as they become weakened and more vulnerable.
What Temperature Is Too Hot for Plants?
For the majority of plants, any temperatures above 85 ºF (29 ºC) start becoming uncomfortable. 90 ºF (32 ºC) is considered the plants heat stress baseline temperature. On the other hand, houseplants, due to the lack of air humidity and airflow inside, are more susceptible to heat, and they’ll begin to feel uncomfortable around 75-80 ºF (24-27 ºC) when in confined conditions.
It’s important to point out that more than the temperature itself, what makes heatwaves dangerous for plants is how prolonged their exposure is to these extreme conditions.
Plant Heat Stress Symptoms
The best way to give your plants a fighting chance is to be on top of weather changes. You’ll have to be extra vigilant when temperatures begin to creep up in your area and look for symptoms as early as possible. This can be complicated, as symptoms vary for every plant and will often look similar to other issues and diseases. Here are some things to look for:
One of the first signs will be wilting. As the heat begins to evaporate large amounts of moisture, your plants won’t be able to actively replace all the lost water, and they’ll end up with droopy, sad-looking foliage. Other plants will also “close” their leaves to reduce moisture loss.
As your plant gets more affected, a significant amount of leaves will drop in many plants, mostly trees and shrubs, due to an inability to use available water fast enough. In some cases, flowers will also drop if already existent or will simply not develop, leading to reduced yields.
This is a process in which the extreme heat will push plants to the end of their life span, forcing them to flower and produce seeds earlier than would be expected. This phenomenon will happen mostly to cool weather crops and can’t be stopped or reversed. The best course of action is to harvest these crops before they are affected.
Intense heat and sunlight can give your plants a sunburn, just as they would to you. Leaves and fruit can get burned, crack and develop bleached spots. damage is usually worse for newly transplanted plants or young plants that are yet not fully established. Especially for transplanted plants, you can reduce shock by gradually transitioning the plant instead of all at once.
Blossom End Rot
This issue is caused by a calcium deficiency which in itself will happen due to inconsistent moisture in the soil, as plants go through wet and dry cycles instead of having consistent growing conditions.
Will Plants Recover From Heat Stress?
Recovery will depend on your care and treatment, the plant’s general health and sturdiness, but most importantly how long were plants subjected to and the severity of the heatwave.
Temporary temperature peaks and moderate intensity are usually manageable, and most plants will overcome this type of heat stress, as the majority of plants will control water loss and achieve thermal balance.
Young, sick, or newly transplanted plants will have a harder time recovering and some can’t recover whatsoever, much like plants subjected to intense heat for long periods.
How Long Does it Take a Plant to Recover From Heat Stress?
As stated above, there are a lot of factors that come into a plant’s recovery from heat stress. It can be from a few days to a few months, depending on the severity of the stress and some may not recover at all.
The best way to assure your plants recover fast is to act as soon as possible when symptoms begin to appear and to practice preventive measures.
How to Treat Heat Stressed Plants
Here’s what you should and shouldn’t do to assure a successful plant heat stress recovery for all your greenies.
Indoor plants are easier to manage, as their environment can be controlled.
You’ll have to increase watering frequency, as moisture will evaporate faster in potted plants. You can also increase humidity by misting, but make sure to do so with a very fine mist and in the morning.
Move your heat stress plants away from light sources. If you use a grow light, update your setup by reducing the amount of heat your plants are receiving. You can also install light blockage, like curtains to filter sunlight further.
Installing cooling devices is a great way to avoid indoor stuffy conditions. You can use an AC or fan, but make sure the wind is not directed at your plants but at an empty space. The idea is to drop the temperature, and the last thing your plants need is further stress.
Outdoor plants are more left to the whims of the weather and thus harder to treat.
Water preferably in the morning or evening and do so deeply, so not only the first layer of soil is moist. You’ll also have to adjust to how hot it is, as water will evaporate faster, but beware of overwatering, as plants will hurt from the added stress factor.
One of the best things you can do for your plants is to physically protect them from the harsh sun. Shade cloth and other alternatives will create a cooler, more pleasant environment. With potted plants, you can move them around, finding them shadier and cooler spots around the garden as well.
Adding fresh mulch will help cool soil temperatures, as well as retain moisture in the soil and prevent excessive evaporation.
You can prune dead flowers and other dead material to relieve your plant's burden. Just be careful not to over-prune to prevent further sun exposure and to not remove material that is still alive, as it will stress your plant further.
How to Protect Plants from Heat Wave
As with many plant issues, prevention is better than treatment. Preparing your plants for success and making sure they’re happy and healthy before a problem arises will greatly reduce the risk of irreversible damage.
Throughout the entire summer, gain the habit of watering in the morning, deeply and as often as your plants need to never have overly dry soil.
This strategy should be employed always, but especially as temperatures begin to creep up. Mulch covers a world of hurt, especially when it comes to dry soil, retaining moisture, and keeping plants' roots cool.
Transplanting and Pruning
If summer is beginning to get too hot, refrain from transplanting or even planting during the hottest times of the day. Your greenie will almost certainly become a heat stress plant. Also, pruning trees and shrubs before a heat wave will cause stress and reduce shade cast by foliage, so refrain from doing so.
Although reducing competition is always great for your plants, removing weeds before a heatwave will reduce the chances of your plants being outgrown and outlived by weeds, as many can tolerate and even thrive in adverse weather.
Avoid fertilizing when your plants are stressed, as this will only make it worse. Also, new growth is more susceptible to intense heat and will quickly succumb.
Try to choose plants that have a higher tolerance to scorching temperatures. This will prevent heat stress on plants and a lot of work and hurt for everyone.
Top Tolerant Plants for Hot Weather
If you still want a vibrant garden but hate to see your plants hurt under the summer unforgiving sun, here are some plants that are less susceptible to heat damage.
Outdoor plants need to be resistant enough to endure the peak of the afternoon sun.
Succulents are some of the best plants to endure extreme heat, as they preserve water in their leaves.
Much like succulents, these desert plants are hardy and thrive in hotter environments, due to small water needs and moisture reserves.
Besides their many benefits to your garden and crops, these bright flowers tolerate both intense sunlight and heat, but also poor soil conditions, making them perfect for less comfortable weather.
This is pretty much a super plant, as it can endure extreme heat, but also extreme cold, drought, and everything in between. Plus, no pests will get near it.
Indoor plants can be moved around, but will still need to be resilient and heat-hardy.
Although not too heat-friendly, they will endure small periods of hot conditions without suffering damage. Also, they are not too demanding and will purify your air.
This succulent can endure heat, thanks to its water reserves. Its fleshy leaves will not take much damage from high temperatures, as long as it’s not under direct sunlight as well.
Much like Spider plants, ZZ plants will endure short periods of extreme heat without suffering damage. Plus, they are extremely easy to care for and virtually impossible to kill from neglect.
This tropical plant will thrive in hot temperatures and will suffer more from low than high temperatures. Much like the ZZ plant, it is almost impossible to kill this greenie due to its overall hardiness.
Sun is the source of life! But we have to be careful with its majestic power. Sunburn is an unpleasant cost of enjoying the bright side a little too much. If you don't leave your plants vulnerable, they'll be very thankful. After all, they can't just put on their shoes and leave!
How Often Should I Water My Garden in 100-Degree Heat?
When temperatures soar in the peak of summer, you should water at least 2 to 3 times a week, preferably in the morning. This will vary from plant to plant and should also be paired with other measures, like shading and mulching.
How Often to Water Flowers in 90-Degree Weather?
Flowers should be watered around every 2 to 3 weeks during normal summer heat. Now, whenever temperatures get excessive, aim to water more often and more deeply, around 1 to 2 times a week.
What Temperature Is Too Hot to Water Plants?
Around 90º F (32 ºC), water will evaporate faster than normal, and plants won’t be able to make use of it. They’ll wilt and begin to adopt some protective strategies to preserve moisture.
When Is it Too Hot to Water Plants?
Aim to water early in the morning or evening, to avoid excessive evaporation. This is the best way to ensure your plants are still receiving enough water despite the heat. You’ll have to be patient until temperatures drop again to resume watering your plants normally.
What Temperature Is Too Hot for Tomato Plants?
Despite being summer plants, tomatoes will suffer from the heat. Temperatures should stay below 75 ºF (24 ºC) during the day and 70 ºF (21 ºC) at night. Anything above 90 ºF (32 ºC) and their yield will be affected.