Intense summer heat is wreaking havoc on many trees

Published: Monday, Aug. 13 2012 8:57 a.m. MDT

Leaf scorch, like on this linden, becomes evident when temperatures rise and humidity drops. The problem gets worse when plants are buffeted by hot, dry South winds.

Larry Sagers

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Last week's column covered what the heat is doing to your vegetable garden.

Unfortunately, the rest of the landscape has no immunity from this problem, so this week's focus is on trees. Numerous trees with burned leaves are gracing local landscapes. Since, in my many year of observing trees, I do not remember brown listed as a normal leaf color, we need to find an explanation.

Trying to unravel the what, when, why and how of what to do about scorch is sometimes confusing, but I will try my best to explain it. The following are general recommendations, and you might have to do more research to help a specific tree in your landscape.

First a definition of what is leaf scorch. Leaf scorch is a common physiological problem that affects many trees. It starts as a browning around the edges of the leaves, and as it becomes more severe, it affects more of the leaf surface and eventually may turn the entire leaf brown.

It is most noticeable on trees with large leaves, including maple, poplar, horse chestnut, catalpa oak, ash and beech.

The symptom is less noticeable on other trees, but it can affect most woody plants.

The when depends on growing conditions.

Leaf scorch becomes evident when temperatures rise and humidity drops. With the unseasonably hot, dry weather this summer, trees are under severe stress.

The problem gets worse when plants are buffeted by hot, dry South winds. These blasts of air remove water from the leaves faster than the tree can replace it.

On trees with large leaves, the areas between the veins turn brown and die, but the cells near the major vein are getting enough water so they stay green.

Evergreen trees and shrubs with needles show the scorch symptom by turning a purplish or light tan color.

If moisture stress continues, the entire leaf and eventually the woody part of the plant dies.

The why has more complicated physiological explanations.

The simple one is that trees cannot move water from the roots to the top of the tree fast enough. There are numerous reasons why this happens.

The soil often has enough moisture, but the tree has trouble absorbing it into the roots. Make certain to keep the soil moist but not waterlogged, as too much water kills the absorbing roots so the tree cannot get the water from the soil into the tree.

The final concern is what to do to take care of the trees. Different tree species manifest leaf scorch symptoms differently.

Sometimes leaves are affected on only one side of the tree while the rest remain normal. This symptom is frequently seen on trees in parking strips. If one side of the tree is covered with pavement, the tree's root system is severally restricted.

Without enough roots to pick up the water, the leaves at the top of the tree become water-stressed and the tree leaves then scorch.

This acute water shortage in the plant causes the leaf tissue to die from lack of water.

Think of your trees as a collection of pipes that conduct water from the roots to the leaves. If anything interferes with the pipes picking up the water in the soil or if the water flow is restricted anywhere between the roots and the leaves, the leaves scorch.

Keep lawn mowers, weed whips or other equipment away from the tree base as they can damage the water-carrying capacity of the bark and cambium.

Reflected heat from vinyl fences, buildings or other structures increases leaf scorch.

Anything that interferes with water uptake also increases scorch susceptibility.

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