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Intense summer heat is wreaking havoc on many trees

Published: Friday, July 3 2015 6:52 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) 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)

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.

Additional conditions that cause leaf scorch include poor water penetration into the soil. This problem is worse on hillsides or slopes. Salt in the soil or water causes chemical drought. Certain herbicides also cause leaf burn on the plants.

Avoid the temptation to drown the plants. Overwatering causes leaf scorch because the water displaces the oxygen in the soil and can rot the tiny feeder roots that absorb the water.

Without feeder roots, the plant cannot take up water.

Iron chlorosis is very prevalent this year and contributes to leaf scorch in susceptible trees. Silver maples, flowering pears and other trees have bright yellow leaves with green veins.

These are highly susceptible to scorching, and they sometimes develop black spots. These are more likely to show additional problems because the leaves are already stressed.

Salts from roads or other sources cause burned edges on the leaf. Herbicides may burn the leaves or interfere with the normal growth of the plant. When this happens, the plant does not take up sufficient water and shows burned symptoms.

Diseases and insects, including aphids, scale and others, rob trees of needed moisture. Borers prevent translocation of water up the tree. Fungi, including phytophthora, verticillium and other wilt diseases cause trees to scorch.

Check susceptible trees and give them a deep watering every two or three weeks. Let the water dribble around the base of the tree until it penetrates one or two feet into the soil. Normal lawn watering is shallow and does not supply the deep soil moisture most trees need.

Larry A. Sagers is a horticulture specialist for the Utah State University Extension Service at Thanksgiving Point.

Garden tips

Garden Talks in the Park are complimentary garden talks at Brigham Young Historic Park on the southeast corner of State Street and North Temple. "Roses, Everybody's Favorites, The Skinny on Keeping Them Fat and Happy!" is the topic on Aug. 15 at 7:30 p.m. No tickets are required, and all ages are welcome.

Thanksgiving Point is offering a basic landscape design course on Aug. 14, 21, and 28, 10 a.m.-noon or 6-8 p.m. The class includes a consultation on your plan by a USU Extension Service master gardener. Cost is $40. For more information, or to register, call 801-768-4971 or log on to www.thanksgivingpoint.org.

Thanksgiving Point is also offering a class on "Spectacular Spring Flower Bed Designs," Aug. 14, 21, and 28, 2-4:30 p.m. Cost is $43. For more information or to register, call 801-768-4971 or log on to www.thanksgivingpoint.org.

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