How flooding affects trees
How flooding affects a tree is determined by the time of year flooding occurs, tree species, soil type, how long roots are submerged in water or saturated soil, contaminants in flood waters and sediment deposited over roots.
There is also concern about flood waters physically undermining the soil around tree roots. This would increase the risk of trees leaning or becoming more susceptible to wind throw and being uprooted in a storm.
Flooding harms trees by depleting oxygen levels in soil. Roots need oxygen for growth and respiration. When oxygen is depleted in flooded or saturated soils, this leads to root death, build-up of toxic compounds in a tree and reduced nutrient uptake.
However, when flooding occurs during the dormant season when trees are not actively growing, they tolerate flooded or saturated soil longer than when flooding occurs during the growing season. While dormant, many trees can handle at least a week to several weeks of flooding.
Unless soils remain saturated due to additional flooding or rainfall as trees break dormancy and begin growth, most trees will tolerate this dormant season flooding.
Another issue is sediment left behind after flood waters recede. As little as a few inches of sediment will also restrict oxygen levels in soil. Sediment should be removed close to original grade as far out from the tree as possible. Trees’ roots extend at least two times the height of a tree and sediment needs to be removed far beyond the drip line.
Do not wait too long to remove sediment. Some trees respond by quickly growing new roots into the sediment. Most trees begin root growth when soil temperatures reach about 40 degrees Fahrenheit. If sediment is removed after new roots have grown into the sediment, this can be a major stress for trees.
If sediment is less than a few inches or cannot be removed for some reason, consider harrowing to avoid a crust developing on the sediment. This can allow oxygen from the atmosphere to enter soil.
If flood waters eroded soil around tree roots, carefully cover any exposed roots with soil back to the original grade to reduce root drying and root death.
If a medium- to large-sized tree is leaning after flood waters recede, it best to remove the tree. The tree could have major root failure that leads to tree death or causes trees to fail in future wind storms.
Inspect trees for mechanical injury. Tree trunks and branches may have been damaged by floating debris. Correctly prune broken branches back to where they attach to another branch or the trunk. Do not use wound dressing or tree paint.
With a sharp knife, carefully removed torn bark from the trunk but take care not to create a larger wound than was already created. Again, do not cover or treat the wound with any type of product.
There is no way to know what contaminants were in flood waters or in sediment. If there were contaminants that might affect a tree, only time will tell.
But keep your safety in mind. Flood waters can contain contaminants like as E. coli bacteria. When removing debris from around trees and working with trees affected by flood waters, wear rubber gloves and be sure to often wash hands thoroughly with hot water and soap.
Kelly Feehan is a community environment educator for Nebraska Extension.