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Large, brown trees are not normally what we want to see in our gardens. Unfortunately, we see all too many as the summer goes on. Leaf scorch is a common physiological problem for trees. It is most noticeable on trees with large leaves, including maple, oak, ash and beech. The symptom is less noticeable on other trees, but it can severely affect most woody plants.
Leaf scorch results from an acute water shortage in the plant, because the leaf tissue dies from no water. Trees are essentially a giant plumbing system. It takes a great deal of energy and time for trees to move water from the soil into the branches and to the leaves. If water does not move rapidly enough, the leaf cells near the major vein retain adequate moisture, but the interveinal areas scorch, turn brown and die. Evergreen trees and shrubs show needles with a purple or light tan color. If moisture stress continues, the entire leaf and eventually the woody part of the plant dies.
Leaf scorch is most noticeable in periods of hot, dry weather, particularly when accompanied by hot, dry south winds. These blasts of air remove water from the leaves faster than the tree can replace it. Various trees manifest leaf scorch differently. Often leaves are affected on only one side of the tree while the rest of the tree remains normal. This occurs because the scorched side of the tree is limited by the root system. Streets or other kinds of paving impede the proper development of roots. Additional causes include injury to the roots including pipeline construction, grade changes or soil compaction.
Paving over the roots of existing trees is another cause of scorch. Additionally, reflected heat is a serious problem for trees planted near buildings or other structures. Additional causes of leaf scorch include poor water penetration, particularly on hillsides or slopes, salts or use of herbicides. Ironically, even overwatering is a major cause of leaf scorch. Excess water keeps out the oxygen and rots the tiny feeder roots that absorb the water. Without feeder roots, the plant cannot take up water.
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.
Leaf scorch is controlled by relieving the water shortage as quickly as possible. Once the leaves are scorched there is little you can do to correct the problem until the following growing season. Make adjustments so the scorch does not get worse. Investigate the true reason for the scorch. Root health is a major problem and should be checked whenever problems are noticed on top of the tree. Make sure there are no grinding roots, wires or other interferences with normal growth.
Vertical mulching may help on high value trees. Holes are drilled 12 to 18 inches deep around the root zone of the tree. They are filled with a porous mixture of parlite, peat moss, vermiculite or other loose material. Vertical mulching opens up the soil and allows more oxygen to penetrate. It also allows excessive moisture to drain away from the root system in heavy soils.
Tree selection is important in preventing scorch. Do not plant trees in small, restricted areas where growing conditions are not adequate. Trees with large susceptible leaves should be avoided in anything but the very best growing sites. Roots do not seek out water, they only grow if ample water is available. Apply water deeply and infrequently so it penetrates 18 inches or more into the soil.
Diseases and insects, including aphids, scale and others, rob trees of needed moisture. Bores prevent translocation of water up the tree. Fungi, including phytophthora, verticillium and other wilt diseases, cause scorch. Cytospora and fireblight are other diseases that induce scorch symptoms, and all trees suffering from iron chlorosis are more likely than a healthy tree to be scorched. Chlorosis is a major cause of scorch, particularly in silver maple.
Scorched trees are an all too common problem. They are seen throughout the valleys and are a sign of trees protesting because of improper planting, improper soil, incorrect watering or other growth factors. Don't let your trees fall victim to these problems. Check the causes carefully and correct them before your trees are permanently damaged.
