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Index : Trees
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- 33. Sun-Scorched Mesquite & Palo Verde - Top
- Our recent rains, although a blessing, have done little to ease the the long-term drought we are experiencing. Normally hardy desert dwelling trees, such as mesquite and palo verde are showing the ill effects of our extended drought.
In times of adequate rainfall, native stands of mesquite, palo verde and others are able to grow sufficient leaves to provide a canopy of shade for branches and trunks. Not so in times of drought. Leaves are sparse, allowing too much direct sunlight to contact the branch wood. When this happens, a condition known as "sun-scorch" occurs. The sun's rays heat the wood to such an extent that the cambium, or sapwood layer under the bark is burnt and dies. This is much the same process as when we are overexposed to the sun and our skin becomes sun burnt. But unlike people, damaged tissue doesn't just peal away and replace itself with new skin or cambium. The damage is permanent!
Trees suffering from sun-scorch have branches that become cracked, with patches of pealing bark. Long patches of gray bark develop and are usually surrounded by a small crack. This is where the dead tissue has pulled away from the living bark.
Sun-scorch occurs mainly on the tops of branches, where exposure to sunlight is most direct and intense. It also is most common on branches that grow horizontally. Such branches are exposed to the strongest sunlight, beating down from directly overhead.
Secondary infections frequently develop on sun-damaged wood. The most common is a fungus disease called sooty canker. Spores of the fungus which happen to land on cracked and damaged bark can germinate and begin to grow. As the disease develops, a black sooty substance is formed under pealing bark. The black, soot-like substance is actually a mass of fungal spores.
Bark beetles and wood boring insects may also attack sun-damaged wood. The larvae of wood boring beetles tunnel under the damaged outer bark, and can move into health surrounding wood. There feeding further destroys the wood cambium.
Branches which have been sun-scorched usually die. As a result, there are even fewer leaves to protect the remaining branches, and so the process continues.
Sun-damaged limbs should be pruned out, back to healthy wood. To determine where the sun damage ends and healthy begins, scrape the bark with a sharp knife. If the sapwood underneath is brown and dry, that part of the branch is dead. If the sapwood underneath is green and moist, it's alive.Prune the branch back to the main trunk or to a healthy branch.
Reflective paint can be applied to branches that are in a position to be exposed to strong sunlight. If pruning out of dead or damaged limbs has further exposed healthy wood to strong sunlight, these limbs can be painted with a solution of white latex paint to reflect the sun's rays. Mixing one part white latex paint with nine parts water will create a white wash that can be applied to the exposed wood. Although a temporary detraction to the appearance of the tree, the white wash wares off in time.
To prevent future sun-scorch injury, encourage new leafy growth by watering effected trees thoroughly. If you have valuable trees, especially those near your home, not currently on drip irrigation, consider watering them. One deep soaking every 2 months, from early spring to early fall will stimulate abundant leafy growth. Watering can be skipped in the summer, if monsoon rains are adequate.
A light spring fertilization along with a deep watering will further encourage a leafy tree canopy. Spread ammonium sulfate (21-0-0) evenly under the branches, at the rate of one-half pound for every 100 square feet. For instance, a mesquite tree with a 20 feet branching spread from side to side would cover approximately 300 square feet. This area then would require the application of 1.5 pounds of ammonium sulfate or similar analysis fertilizer. Ammonium sulfate is available at most area garden centers.
Besides non-irrigated desert trees, sun-scorch can also occur on some irrigated trees species with sun-sensitive bark. These include; all citrus varieties as well as apple, peach, plum, and apricot. Keep pruning of citrus to a bare minimum. When pruning deciduous fruit trees such as those mentioned, allow some interior growth to remain for shading of the major branches.
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Written by John Begeman, College of Agriculture and Life Sciences, the University of Arizona, 520-626-5161.
Material originally appeared in Arizona Daily Star gardening column, on March 26, 2000
- Updated: March 26, 2000
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