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What Happens to Trees in a Heatwave? How Heat Stress Affects New Jersey’s Trees

hot sun shining on heat stressed trees

Whenever there’s a heatwave, people are warned to stay indoors, drink plenty of fluids, and watch out for signs of flushed skin or elevated body temperature. But what about the warning signs of heat stress in trees during a heatwave? What can you do to help your trees cope with the high temperatures?

How Trees Stay Cool

While the effects of excessive summer heat are not as obvious or immediate in trees as they are in humans, trees can also suffer injury and even death due to a heatwave. While trees don’t have the option of ducking into an air-conditioned building, drinking a glass of cold water, or standing in front of a blowing fan, they do have a way to dissipate heat – it’s called transpiration.

Trees (and other plants in your New Jersey landscape) absorb water from the soil through their roots. That water is spread throughout the tree and, in hot temperatures, is released as water vapor from the leaves. This evaporative process cools down the leaf surface and surrounding air (that’s why it’s often a few degrees cooler beneath a tree).

What Happens When Temperatures Rise

The higher the air temperature, the more the tree transpires. Eventually, it reaches a point where it cannot absorb water from the soil fast enough to support the entire canopy. That’s when you’ll start to notice symptoms of heat stress.

When daytime temperatures consistently reach above 95 degrees, biological functions in the tree also begin to slow down. This allows the tree to conserve moisture by cutting back on transpiration, essentially putting the tree into a sort of dormancy where it exhibits even more signs of stress.

Unfortunately, this also reduces the amount of sugars and carbohydrates the tree can produce through photosynthesis and store in its tissues, weakening the tree and making it more susceptible to pest and disease problems.

Signs of Heat Stress in Trees

There are some early warning signs of heat stress to look for in your trees and shrubs so you can help them recover. The longer you wait to take action, the worse the outcome can be. Newly-planted shrubs and trees, especially, can die from heat stress.

  • Leaves wilt and branch tips droop
  • Rust-colored bumps appear on the leaves
  • Leaves or needles in the interior of the tree turn yellow
  • Green leaves start falling off the tree
  • Leaf edges become scorched (they look brown, crispy and dead)
  • Leaves may curl

Unfortunately, heat stress in trees can look a lot like drought (see below), as well as common diseases that plague trees in our area, so often it can be hard to diagnose. If you’re not sure what’s happening to your trees, ask the Certified Arborists at Alpine Tree to inspect them and make a proper diagnosis.

Heat Stress is NOT the Same as Drought Stress

Heat stress can occur in trees regardless of the amount of moisture in the soil. Even well-watered trees can suffer from the heat.

If you notice wilting, drooping, curling or browning leaves, don’t assume it’s due to lack of water. Check the soil first! Only water if the soil is dry around the tree’s root zone.

Over-watering will NOT help the tree recover (remember, it’s not taking up much water while it’s heat stressed). Instead, it can lead to root rot and fungal diseases which will further harm the tree.

What to Do For Heat Stressed Trees

If you notice any of the above symptoms, especially if the weather has been particularly hot or dry, there are a couple of things you can do immediately.

  1. Refresh any mulch over the tree’s root zone. If there is no mulch, add a 2- to 4-inch layer. This will help insulate the roots from the high heat and minimize moisture loss from the soil.
  2. Check the soil moisture level. If it’s dry, water as soon as possible.
    • Newly-planted trees need plenty of water. Be sure that you have a watering system or schedule in place for any trees that have been planted in the last two years. Click the link to learn more about how to care for a newly-planted tree.
    • For larger, more established trees, be sure to water the drip line, which extends out to the tree’s canopy (where the edge of the leaves are). Never water the trunk of a tree. Roots grow outward to help the tree grow and also to anchor the tree into the ground so that it’s less likely to blow over in storms. You want to make sure that you are watering those roots.

Ongoing Tree Care After the Heatwave

After the heatwave has passed, look closely at your trees to see if they are recovering. If you don’t notice any signs of improvement, contact a Certified Arborist for a consultation.

Keep in mind that heat stress can have lasting effects on a tree – some symptoms may not appear until years later. If your trees have made it through a heatwave, it’s doubly important to ensure that they receive regular and appropriate care and maintenance for at least the next few years.

As we mentioned, it can be difficult to tell if a tree is suffering from heat stress or from a disease or pest infestation. If you’re not sure, contact us at Alpine Tree to schedule a tree inspection. We can help diagnose the issue and help you keep your trees healthy.

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