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There are several heat stress warning signs to look for in your trees and shrubs so you can help them recover. If you are watering your trees/shrubs but keep seeing signs of heat stress, this could be a problem. [Sources: 8, 11]
The first sign of heat stress is usually wilted leaves, so if your trees show any signs of stress, make sure they get enough water. Overwatering won't help the tree recover (remember, it won't absorb as much water during a heatwave).
Shallow watering will result in a shallow root system, which is not conducive to tree stability. [Sources: 0, 8, 15]
It is best to water each tree deeply so the roots can absorb and retain water. Water is usually absorbed by the roots of each tree and flows through the plant until it reaches the leaves.
This water eventually evaporates from under the leaves into the air in a process called evapotranspiration. [Sources: 5, 15]
This water is distributed throughout the tree and is released as water vapor from the leaves at high temperatures. In the warm season, the process of water passing through the trees is accelerated, as a result of which the tree absorbs less water and dehydrates. During particularly hot or dry periods, water evaporates faster and trees become dehydrated. [Sources: 5, 8, 15]
Trees struggle as temperatures rise and water becomes scarce. If the temperature is maintained above 90°C for a long time, plant growth slows down, and some plants begin to show signs of stress.
Above 104 degrees Fahrenheit, many plants survive but show different signs of heat stress depending on plant type, plant maturity, and factors that often result from high temperatures such as drought or wind. Therefore, they tend to suffer from heat in one way or another. [Sources: 1, 10, 13]
While some plants, like succulents, adapt well to heat by retaining water in their fleshy leaves, most plants lack this luxury. Certain types of plants, such as conifers and succulents, can be further damaged by any treatment, even the use of soap or neem oil in extreme heat.
Areas of discoloration on the leaves of new seedlings, trees, or other plants that move from the inside out may indicate sunburn. During extreme heat, this can lead to sunburn on previously shaded leaves. [Sources: 1, 3, 10]
Often, heat stress kills newly planted shrubs and trees if left untreated. Newly planted trees and shrubs (less than two years after planting) can be particularly affected by heat and drought.
Newly planted trees are especially at risk during prolonged droughts, but even trees that have survived harsh conditions in the past can wither or even die from prolonged drought and heat. Mature trees may suffer from heat stress and drought-like smaller plants, but symptoms of this stress may appear later. [Sources: 1, 4, 6, 12]
Unfortunately, heat stress in trees can be very similar to drought (see below) as well as common tree diseases in our area, so it can often be difficult to diagnose. But it can also be a problem for those of us who live in areas where the warm season is shorter.
While the effects of excessive summer heat on trees are not as noticeable or immediate as they are on humans, trees can also be harmed and even killed by heatwaves.
High temperatures can cause a lot of nasty health problems and symptoms, but if you prepare for the heat and give your trees some health care on a regular basis, they can make it through the summer unscathed. [Sources: 5, 8, 11]
Because your tree/shrub is stressed by the heat, it will have a harder time fighting off these insects and diseases and will likely suffer a lot if you don't take action.
Take a close look at your trees to see if they are under heat stress. Heat stress from a plant can also be recognized by leaf fall, especially in trees. When the water runs out, you will see the leaves begin to curl up and dry up, which is a sign that the trees are in serious trouble. [Sources: 5, 9, 10, 11]
Heat-stressed trees lose water faster than they replenish. Without it, they cannot transport nutrients from the roots to the leaves. And, as we've already learned, trees need water to take root in the ground. Instead, it causes root rot and fungal diseases that further damage the tree. [Sources: 0, 8, 15]
Young plants with less-developed root systems need more water to absorb water from the soil than mature plants with larger root networks. Trees planted in the past two to three years need more water than older trees because the roots are still growing underground.
Because of the deeper roots, trees take longer to get wet than most other plants, so symptoms tend to appear later or may not appear until months or years later. [Sources: 1, 6, 15]
Limited rainfall creates dry soil that can stress the tree or shorten its lifespan. When a tree lacks water, it may appear shriveled in an attempt to conserve water. Despite the fact that water deficiency is only a dangerous effect on the plant, this article will focus on the effect of heat on trees. [Sources: 0, 2, 14]
Without water to dissipate heat or "cool" breathing, sometimes radiant heat from the environment can increase the heat load of the tree and push the associated water demand to the limit of what is available.
Excessive heat stress releases large amounts of water, triggers metabolic problems, and can cause heat damage just above the ground/tree (root collar/stem base area). This requires energy, which is released in the form of heat, which takes heat away from the plants, much like a swamp cooler makes a home more comfortable. Plants, especially trees, need a lot of water for cooling.
The entire surface of the leaf, Goodspeed said. [Sources: 7, 14]
Looking at a picture of a large tree, each 18°C increase in temperature, starting from 40°C, where water is densest, physically doubles the plant's respiration and water loss.
An increase in temperature increases the vapor pressure deficit between the leaves and the atmosphere, and also increases the rate of water loss from other tree layers. [Sources: 14]
Leaves that quickly die from heating are usually held on the tree by hard xylem tissues and do not have an effective abscissa zone. While trees can't squeeze into an air-conditioned building, drink a glass of cold water, or stand in front of a fan, they do have a way of dissipating heat—this is called transpiration. [Sources: 8, 14]
And yes, trees have a heat problem here in northeast Ohio, too. While you may know how to recognize the signs of heat stress in people - redness, flushing, hot skin, high body temperature, etc. - you may not know how to recognize the signs with your trees. [Sources: 2]
##### Sources #####
[0]: https://www.gcanews.com/ways-to-help-heat-stressed-trees/
[1]: https://extension.oregonstate.edu/gardening/flowers-shrubs-trees/heatwave-garden-how-identify-prevent-heat-stress-plants
[2]: https://www.independenttree.com/the-effects-of-heat-stress-on-your-trees/
[3]: https://www.parkwestinc.com/blog/how-to-identify-heat-stress-in-turf-trees-and-plants/
[4]: https://www.thespruce.com/prevent-heat-stress-in-garden-plants-2539775
[5]: https://www.urbanforestprofessionals.com/blog/how-to-identify-heat-stress-in-trees/
[6]: https://droughtresources.unl.edu/heat-drought-hard-on-trees
[7]: https://extension.usu.edu/archive/how-to-recognize-plant-heat-stress
[8]: https://alpinetreenj.com/heatwave-heat-stress-affects-trees/
[9]: https://www.kiro7.com/news/local/local-trees-still-feeling-burn-after-surviving-intense-historic-heatwave/Q7WACHQA5ZEKBDXVDQ4X23YTOI/
[10]: https://www.gardeningknowhow.com/plant-problems/environmental/heat-stress-plants.htm
[11]: https://rtectreecare.com/stop-heat-stress-in-its-tracks/
[12]: https://www.weed-pro.com/blog/bid/144854/Signs-of-Heat-Stress-in-Shrubs-Trees-How-To-Help-Them
[13]: https://www.heartwoodtree.com/uncategorised/are-your-trees-suffering-from-summer-heat-stress/
[14]: https://extension.psu.edu/heat-stress-in-urban-trees
[15]: https://organicplantcarellc.com/trees-suffering-summer-heat-stress/
[16]: https://plant-pest-advisory.rutgers.edu/managing-heat-stress-in-nursery-and-landscape-plants/
