Flooding impacts trees
September 6, 2011
Many properties throughout the Midwest that never have experienced flooding in the past may well be under water for extended periods this summer due to a combination of weather events.
What are the implications of flooding on trees and other landscape plants? According to Eric Berg with the Nebraska Forest Service, the resilience of trees to flooding varies by species, age and health of the tree before flooding, whether flooding occurs during the growing season, duration of flooding, either soil erosion or buildup of soil over the roots, the type of soil deposited (often the soil deposited by floodwaters is highly saline) and other circumstances.
Symptoms of flood damage include leaf yellowing, early leaf drop, formation of small leaves, watersprouts at the base of the tree and dieback at the crown. Symptoms can occur quickly or over a period of time. Once trees are stressed, they are far more vulnerable to secondary damage from fungi, insects and disease.
The initial running water from flooding is far less likely to cause damage than standing anaerobic water since tree roots need to obtain oxygen. Standing water may also contain toxic amounts of carbon dioxide, methane, hydrogen and/or nitrogen. If standing water can be redirected, that will help the roots begin to recover. If more than 3 inches of soil has been deposited, it is best to remove excess soil from the root area. If there is minimal soil but it’s encrusted, it may help to scrape the soil to improve oxygenation. On the other hand, if roots have been exposed, they should be covered to the original soil depth to prevent damage.
Most trees should return to health if floodwater recedes within a week. It’s best not to make immediate decisions. Chlorosis from wet soils will be reversed as the soil dries out. Leaning or broken trunks or limbs should be removed for safety but branches that appear to be dead may simply be defoliated. Pruning live wood is best avoided as that can further deplete food resources. Longer term recommendations include aerating the soil over the root system, avoiding fertilizer for a year or more since that can further stress injured roots, properly mulching the root system once soil has returned to normal and, surprisingly, making sure to water trees thoroughly if extended dry periods follow since the roots may be injured and less able to absorb water and nutrients.
Trees most susceptible to damage from flooding include conifers like pine and spruce, fruit trees, beech, black walnut, ironwood, linden, tuliptree and yellowwood.
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Some of the trees most able to withstand periodic flooding include baldcypress, bur oak, cedar, cottonwood, hackberry, red and silver maple, white and green ash, sycamore and willow. For more information about recommended species, tree health and tree care, visit nfs.unl.edu.