Hot Weather and Leaf Scorch on Trees

Hot Weather and Leaf Scorch on Trees

Marianne C. Ophardt
Washington State University Cooperative Extension
Area Extension Agent  

It takes longer for trees and shrubs to show signs of distress from lack of water or improper watering practices than other smaller plants in vegetable and flower gardens..  Leaf scorch is a possible sign of water stress.  Mild leaf scorch appears as the yellowing and then browning and drying of the tips and margins of leaves.  More severe scorch appears as the yellowing and then browning of tissues between the main veins of the leaf or large angular blotches of brown tissues between the veins. (In many cases no yellowing occurs at all, tissues just turn brown.) Extremely severe scorch involves the browning and death of entire leaves.

Leaf scorch is a physiological problem related to watering.  The leaves are telling us that they’re losing water at a faster rate than can be replaced by the plant.  Leaf tissues that are furthest from the veins (the margins and areas between the veins) are the first ones affected by the lack of water. Environmental conditions that increase the water demands on a plant tend to increase the severity of leaf scorch.  Hot temperatures, low humidity, high light intensity, and wind all increase the rate of water loss from the leaves.

Sometimes leaf scorch is simply due to a lack of adequate irrigation during the demanding summer weather.  However, anything that interferes with the uptake and transport of water to the leaves can lead to leaf scorch.  Compacted soils; girdling, choking, or underdeveloped root systems; root damage from excavation; and trunk injury from mowers and weed trimmers are often involved. Certain disease and insect problems may also be part of the problem.   Recently transplanted trees with unestablished root systems are frequent victims of leaf scorch.  

Some types of trees and shrubs, such as Japanese maple and flowering dogwood, are not well adapted to hot summers in an arid climate like ours.  To survive and thrive in our region, they do best where they’re protected from excessive heat, afternoon sun, and drying winds.  In more exposed locations or surrounded by reflected heat from buildings and pavement, they will often develop leaf scorch.  They also benefit by having their roots kept cool with a layer of bark mulch. 

Strangely enough, leaf scorch can also be caused by too much water. Saturated soils don’t allow roots to get the air that they need.  Suffocated roots die and the plant can’t take up water... creating the same symptoms that appear if there isn’t enough water.  Saturated soils can lead to fungal and bacterial root rots and eventual death.

If leaf scorch develops on a tree or shrub of yours, try to determine the cause.  Don’t simply start dumping more water on it, assuming that too little water is the problem.  Check the roots and trunks for problems.  Consider all the possible factors that might be involved, but don’t forget to check soil moisture too.

It’s easy at this time of year to be derelict in your watering duties. Contrary to the belief of many, most trees don’t have tap roots and aren’t able to make use of the water in the water table. Most tree and shrub roots that absorb water are located in the top 18 to 24 inches of soil.   Trees growing in our region depend upon us and irrigation water to supply their moisture needs.

During the hot part of summer, trees and large shrubs should be watered with “deep” watering about once a week.  More shallow lawn watering is inadequate to supply their needs. Use a soaker hose to provide this deep watering.  Soaker hoses are made of porous canvas, plastic, or rubber. They allow water to seep out slowly and are useful in watering trees, flowers, vegetables, and shrub beds. Don’tprovide the water at the base of tree trunks where it’s wasted.  Apply water to the area known as the “drip line”, the outer edge of the branch spread.

Special attention should be given to trees and shrubs planted within the last two years.  Make sure they’re getting enough moisture by checking their root balls.  They can’t “tap” into surrounding soil moisture until their roots grow out of the original root ball. It’s important to keep their root zone moist... but not wet.

How much water does a tree need to keep it “happy”. Research has shown that a mature silver maple can lose over 000 gallons of water a day through its leaves, and a mature oak tree can lose over 400 gallons a day.  However, rather than worrying about how many gallons of water to give your tree, you should be checking the soil moisture in the root zone.  Use a spade, shovel, or probe to see if the soil is moist in the top two feet of soil. 

Prevent hot weather stress in your yard and garden by watering the right way!