Leaf Scorch

Scorchin' Summer Problems In The Garden By Dennis Hinkamp

This summer has been so hot and dry that even native plants are starting to reconsider their choice of residence. Other plants are experiencing leaf scorch. "Some trees whine and complain more than others," says Jerry Goodspeed, Utah State University Extension horticulturist. "They give the impression that they have never experienced a Utah summer. A little bit of hot, dry, windy weather, and their wimpy leaves begin to scorch. Summer leaf scorch normally doesn't show up earlier than usual this year."

Leaf scorch is technically a disease, although it is not caused by a living organism, he says. It is a symptom that appears on the leaves when the tree is not able to move enough moisture to cool the leaves. This normally happens when we have high temperatures, dry winds and low humidity.

Some trees suffer from leaf scorch more than others, Goodspeed explains. Trees with bigger leaves require more cooling and are more prone to this damage than smaller-leaf trees and shrubs. Poplars, cottonwoods, maples, walnuts, sycamores and ash are a few of the more susceptible trees.

"Besides the size of the leaves, a couple of these trees aggravate the problem because they lack any common sense," he says. "In the spring when the weather is cool and wet, they develop a lot of extra growth, foolishly believing that the weather will remain the same throughout the year. However, their root systems are not efficient enough to supply the water needed to cool all this new growth."

The symptoms of leaf scorch vary a little from tree to tree, Goodspeed says. Normally, a few of the leaves begin to turn yellow or brown between the veins of the leaf. This area may eventually die. Quite often a line or front (how far the water reached) appears on the leaf. As the summer progresses, the die-back on the leaf continues, leaving two or more lines or fronts on the leaf. The area closest to the leafstalk stays green, while the margins and tips of the leaves burn. Some leaves may turn yellow or brown and fall off.

Leaf scorch can affect one or two branches, or the entire tree may suffer some form of burn, he says. Sometimes one side of the tree is affected while the other side remains green and lush. Newly planted trees are susceptible to leaf scorch because the roots have not had time to fully develop. Damaging established tree roots either by cutting, changing the soil level or removing the water source can also increase the likelihood of leaf scorch. Over-watering can also cause leaf scorch as the root zone becomes waterlogged, depriving the roots of oxygen.

Unfortunately, there is no easy cure for leaf scorch, Goodspeed says. The good news is it usually is not fatal. A branch or two may be lost in severe cases, but normally the leaves are the only part affected. Be sure to check trees that are susceptible to leaf scorch and give them a deep drink of water 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 resupply the deep water most trees need.

To prevent damage to the roots or lower trunk area of trees, keep lawn mowers and weed trimmers away from them, he adds. Remove grass growing around the trunk, and then be sure not to over-water.