The past couple of weeks, I have been asked to look at numerous trees and shrubs that have dead, dried up leaves. The plants may just have the outer edges turning brown (appear to be burned) or the leaves are completely brown except for the main leaf veins. These symptoms would indicate a condition called “Leaf Scorch”.
Both of these conditions are called leaf scorch, caused by a tree or shrub’s inability to take up sufficient water to meet its needs under harsh summer weather conditions. Water is taken up by a plant through its fine feeder roots and transported through the vascular system to the leaves. When it is unable to take up enough water, the leaf tissue that is farthest from the major veins will dehydrate first. I usually see this condition on newly planted trees or trees with impaired root systems, dues to improper planting or poor soil conditions.
Leaf scorch can also be caused by soil compaction, transplant shock, nutrient deficiency, salt toxicity and herbicide injury. The loss of leaves is seldom immediately fatal, but conditions causing leaf scorch should be corrected if possible--over time, they can cause the decline or death of the tree or shrub. Plants under stress are subject to secondary problems such as attack by insects or diseases.
Once leaf scorch has occurred, there is no cure. The dehydrated portions of the leaf will not turn green again, but with proper water management, the plant may recover.
Prevention of scorch needs to begin with winter watering. A deep soaking once a month, when there is no snow cover, will help prevent root die-back due to dehydration. The roots of mature trees extend outward several times the height of the tree, and this entire area needs water. To water in winter, choose a day when the air temperature is above freezing and water early enough in the day for the water to soak in before the nighttime freeze. Water that freezes on the surface will cause root suffocation and make the problem worse.
During the growing season, water deeply and as infrequently as possible. This can be difficult when trees are located in or near lawns, but a deep and infrequent watering schedule will benefit your lawn as well, encouraging deep root development. Trees typically need less frequent irrigation than lawns do, but the two can coexist. Remember that roots need oxygen, and this means the soil must be allowed to dry out somewhat between watering. If you are using a drip irrigation system, you may not be providing enough water for the entire root system of a large tree. Trees need water under the entire tree (or drip line) not just around the trunk.
Finally, if scorch occurs, resist the urge to continually apply more water. Just keep up with the deep and infrequent schedule. In some years, especially if conditions are especially hot and windy, some scorch is likely, but do not panic and kill off your trees with an excess of kindness by over watering.
In addition, keeping the trees and shrubs in your landscape healthy and vigorous will help them be more resistant to environmental stresses and pests. Often landscape trees are planted into soils with low fertility. Signs of low nutrition are poor tree growth, pale green or yellow leaves, and mottled patterns between the veins, dead spots, stunted leaves or early loss of leaves. When possible, conduct a soil test to determine whether the soil at the planting site is deficient in nitrogen, phosphorus or potassium.
Ideally, soil test results should be used to determine fertilization rates, especially for phosphorus, potassium and micronutrients. However, if test results are not available, fertilizing with a 2-1-1 or 3-1-1 ratio material at a rate supplying 2 to 4 pounds of actual nitrogen (N) per 1,000 square feet per year should promote good plant vigor. If more than 2 pounds of quickly available N (from a source such as ammonium nitrate) is applied, it should be split into two applications, perhaps in April and October. Certain plants such as broadleaf evergreens, dwarf conifers and alpine plants should receive about half the rate of most deciduous plants. Excessive fertilization of pines often creates large gaps between branch whorls.
Fertilize shade trees throughout the area occupied by the root system. This usually includes the soil up 1.5 to 2 times the diameter of the branch spread. In most landscape situations, 95 percent of a tree’s roots are found within 18 inches of the surface. Fertilizer applied to turf surrounding a tree will often supply most of the minerals required to keep the tree healthy. When possible, chop leaves in place with a mulching mower, allowing the particles to filter down into the turf where the minerals they contain can be recycled.
In general, surface application of fertilizer is an efficient and effective way of getting nutrients to tree and shrub roots. In many cases, fertilization of turf supplies enough nutrients to meet the needs of trees in the area.
It should be noted that, fertilization is not a panacea. Do not expect it to solve problems associated with careless planting, improper watering or poor drainage, and when planting new trees be certain that they are suited to the climate and location.
Fertilizing Shade Trees, Christopher J. Starbuck, University of Missouri Extension.