Urban and Landscape Sites The best approach to managing flood stressed trees is to enhance tree vigor by proper tree maintenance and protection from additional stresses. Tree vigor can be enhanced by fertilizing with a low nitrogen fertilizer, aerating the soil, mulching, and watering if soil conditions become excessively dry. Dead or severely cankered branches should be removed. Prune only when bark surfaces are dry or during the dormant season. Newly transplanted or mature, high value trees may need protection from leafspot diseases such as anthracnose and from insect defoliators and various sucking insects such as aphids or scales. It should be noted that leafspot diseases are not severe every year. Trees need protection during spring seasons that have frequent rainfall at budbreak and during leaf expansion. Refer to the enclosed USDA Forest Service brochure entitled, How To Identify and Control Dogwood Anthracnose, for more information. Although the above mentioned publication deals specifically with dogwood anthracnose, the cultural control recommendations are applicable for leafspot diseases of trees and shrubs in general. A fungicide not mentioned, thiophanate methyl, is labeled for control of anthracnose on shade trees and woody ornamentals. Several tradenames of fungicides that contain thiophante methyl include Cleary 3336, Topsin M, Domain, Fungo and Zyban.1 Fungicides should be used only to supplement a cultural control program. Read fungicide labels carefully to determine registered uses and application rates. Forest Stands Any harvest or salvage activities should create a minimum of damage to remaining or regenerating trees or disturbance to the site itself. Soil compaction, rutting, bark removal, and branch breakage can all act as additional stress on other trees in the stand. Salvage activities such as "sanitation cuts" can be beneficial by removing breeding material of stem boring insects. (See Management Implications section for additional salvage considerations). Insecticide use is rarely practical or ecologically sound in forested situations. Hazard Evaluation of Flood Damaged Trees As noted above, flooding results in some trees being stressed, physically damaged, and/or insect and disease infested. These trees possess defects that decrease their structural integrity, making them more prone to windthrow and structural failure. Defective trees located in high use areas such as yards, parks, or other recreational areas are hazardous and pose safety risks to people and property. Mature, well-established trees are more tolerant of flooding than over mature trees or seedlings of the same species. If flooding is recurrent or uninterrupted and keeps soils saturated, serious damage to trees may occur. |
Flooding during the growing season typically is more harmful to trees than flooding during dormant periods. Flood-stressed trees exhibit a wide range of symptoms including yellowing leaves, defoliation, reduced leaf size and shoot growth, crown die back and sprouts along the stem or trunk. Symptoms may progress into tree decline and death, reoccur for several years and then eventually disappear, or subside by as early as next year indicating rapid tree recovery. Flooding reduces the supply of oxygen to the soil and roots and usually results in growth inhibition and injury to flooded trees. Deposits of silt or sand as shallow as three inches can be injurious, especially to newly planted trees. Tree roots also must contend with high concentrations of toxic compounds that accumulate in waterlogged soils. Strong currents and soil particles suspended in flood waters can erode soil from around the base of trees exposing tree roots. Exposed roots are vulnerable to drying and mechanical injury and their occurrences may make trees more vulnerable to wind-throw.
Flood-stressed trees are prime candidates for attack by secondary organisms. Several opportunistic disease-causing fungi and insects invade trees that are weakened or stressed. Minimizing additional stress or injuries should be a priority on high value trees for one to three years after flooding to reduce the chance of attack by insects.
“The best approach to managing flood-stressed trees is to enhance their vigor by following proper tree-maintenance practices and eliminating additional stresses. Dead or severely cankered branches should be removed as soon as possible. Aerating the soil, mulching and watering during extended dry periods are recommended tree-care practices that can help enhance vigor, but they are not rescue treatments for severely injured trees. Trees developing substantial die back and decline symptoms or those possessing defects that prone to wind throw and structural failure should be removed from the landscape immediately.
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