Three straight days of dangerous heat, and heat index values between 105° and 115° aren’t just hard on us—they’re hard on our landscapes, too.
During periods of extreme heat, like we experienced early this week across Nebraska, you may notice your trees and shrubs looking different than they did just a few days ago. Brown leaf edges. Curled leaves. Early leaf drop. Maybe a branch that suddenly looks thin. But before assuming the worst, remember that trees don’t respond to extreme weather overnight. Some changes appear quickly. Others may take weeks—or even years—to develop.
How Trees Respond to Heat
Earlier this summer, we explored Plant Available Water (PAW)—the water stored in the soil that plants can actually access and use. That concept becomes especially important during a heat wave. If you missed it, you can read that article here: The Soil Moisture Story. While a thunderstorm may leave puddles on the surface, what matters most to a tree is how much water has made its way into the root zone and replenished that underground reservoir. As soils dry, that reservoir begins to shrink.
Root Helpers are Real Fun Guys!
Trees rely on thousands of tiny root hairs to absorb water, but they don’t do it alone. They have a team of fun guys helping them out.
Ok… technically they’re fungi, but these underground partners deserve a little credit.
Roots work in partnership with beneficial fungi that form an underground network around the roots. These hidden helpers, called Mycorrhizal fungi, help gather water and nutrients from the surrounding soil in exchange for sugars produced by the tree through photosynthesis. Researchers estimate these fungal networks can expand a tree’s effective root system by five times or more! Now that’s fun. However, even this partnership reaches its limits during extended heat and drought. As soils continue to dry, the amount of plant-available water decreases until neither of these partners can efficiently pull moisture from the soil.
“Bounce Back”…or Recovery?
If dry conditions persist, the finest roots and fungal threads begin to die back and the tree’s ability to absorb moisture declines…just when it needs it most. This is one reason trees don’t simply “bounce back” after a single rainfall. The tree must rebuild the fine roots and fungal partnerships that make water uptake possible. This process can take days, weeks, or even longer depending on the severity of the stress.
Knowing what’s happening below ground can help manage expectations once we’re through a heat wave. Even after temperatures cool, symptoms may continue to develop for weeks, months, or even into the following growing season. Here are some of the most common signs of heat and drought stress.
Signs of Heat or Drought Stress
1.Leaf wilting, drooping, or curling. Wilting and drooping are much more common—and often more noticeable—in herbaceous plants like perennials and annuals than in trees and shrubs. However, some woody plants will temporarily curl or droop their leaves during periods of extreme heat. In the writer’s opinion, this may be the most misleading symptom. It’s easy to see drooping leaves and assume a plant needs more water…which can lead to giving plants too much water, and then that opens the door to even more problems! So, before you potentially overwater, do some further investigation. Wilting occurs when a plant loses water faster than it can replace it, reducing the water pressure inside its cells and causing sad, droopy leaves. This doesn’t necessarily mean the soil is dry—it may actually be the plant’s protection mechanism kicking in. During the hottest part of the day, even if adequate soil moisture is available, plant leaves may droop, wilt, or curl. This reduces the amount of leaf surface exposed to the sun, which helps slow water loss through transpiration and helps the plant better withstand the day’s heat! If you notice drooping or curled leaves during the afternoon, wait before you try to intervene. Instead, wait until temperatures begin to cool and check the soil moisture a few inches below the surface. Many herbaceous plants will recover by evening as temperatures drop. If the soil is dry below the surface—or if the leaves remain wilted into the evening or the following morning—it may be time to water. Persistent wilting, especially when accompanied by symptoms like leaf scorch, premature leaf drop, or branch dieback, is a sign that the plant is experiencing more than just a temporary response to the heat.
2. Leaf scorch. Leaf scorch happens when leaves lose water more quickly than the tree can replace it. Leaves may become dry, turn brown, and become brittle. In deciduous trees, damage is often most noticeable in the upper canopy, on the sunny (southern) side, and in areas most exposed to winds. For your evergreens (such as pines and spruces), leaf scorch typically appears as brown or purple-brown discoloration of the needle tips. Needle browning increases as stress becomes more severe.
3. Premature Leaf Drop and Reduced Growth. This may be the most alarming symptom of heat and drought stress, because it may look like a tree is dying. However, early leaf drop is often a protective response during prolonged heat or drought. Some deciduous trees shed part of their canopy to reduce water loss through transpiration, so they have fewer leaves to support. With fewer leaves to support, the tree can instead conserve moisture and direct its energy toward survival, rather than growth. That shift in priorities doesn’t end when the heat wave does. One of the most surprising things about heat and drought stress is what’s known as a
biological lag effect. Simply put, the damage you see next spring may actually have begun the previous summer. Trees under stress often produce fewer new shoots and buds, and because many of next year’s leaves and branches begin forming during the current growing season, the effects of drought may not become apparent until the following spring. A tree that appeared to recover in late summer may leaf out sparsely or show sections of branch dieback the next year. If growing conditions improve, the tree may stabilize and continue the season with its remaining foliage. Some species may even produce a modest second set of leaves—referred to as a flush of leaves— though many simply focus on recovering and preparing for the following year.
4. Branch Dieback. As you now know, not all drought damage appears immediately, and branch dieback one of those “later” signs. Often a sign of more severe or prolonged stress, branch dieback may not become noticeable until the following growing season. If conditions become severe enough, the tree may stop supporting smaller twigs and branches to preserve the health of its main trunk and larger limbs. Those unsupported branches eventually die back, often becoming evident months after the heat wave has passed.
Understanding how trees respond to heat is the first step toward helping them recover. In many cases, patience—not panic—is exactly what a stressed tree needs. Have a tree or shrub you’re worried about? We’d love to see what you’re noticing. Send us a photo to info@greatplainsnursery.com. We’re happy to help you investigate whether your tree or shrub is showing normal heat stress or signs of a bigger problem.
