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ISSUE 8  June 22, 2000

TREES

Flooding of Trees

    Much of the Red River Valley has seen flood conditions over the last week. Will these conditions cause
tree damage? Most trees can withstand a few days of flooding during the growing season. Since the effects
of flooding on trees involves a complex interaction between flood characteristics, soils, and individual tree
tolerances, predictions of tree survival from flood conditions are difficult to make. The complexity of the
interactions has resulted in contradictory information from different studies about tree response to flooding,
but some generalizations can be considered.

    Flood characteristics that effect tree survival include:

(1) Timing of flood - floods during the growing season are generally more damaging to trees
than floods before trees leaf out in the spring.

(2) Duration of flood - trees are damaged more by floods that last for longer periods of time.

(3) Depth of flood - damage is generally greater on trees with standing water around them
than trees in saturated soil. Damage is usually greater on trees where the foliage is submerged
and tends to increase as more foliage is covered with water.

(4) Oxygen content of water - Cold water is usually less damaging than warm water and
moving water less damaging than stagnant water because of a higher oxygen content.

(5) Potential for physical injury - young trees are especially susceptible to physical injury
from fast water currents and floating debris.

(6) Water contaminants - runoff containing herbicides and other chemicals from lawns,
agricultural fields and other areas can damage trees. Impact from these chemicals depends
on the concentration and type of chemical.

    Flooding can modify soil characteristics in many ways. Some modifications include:

(1) Soil deposits around a tree can smother tree roots or current can wash away soil, exposing roots.

(2) Flooding can cause anaerobic conditions. A lack of oxygen can cause poor root growth and eventual death of a tree can occur after extended flooding. Hydrogen sulfide and ethanol can damage roots as it builds up through anaerobic decomposition.

    Tree health, size, age, and species can all affect the ability of an individual tree to survive flooding.

(1) Vigorous trees tend to be damaged less by floods than stressed trees.

(2) Since large trees tend to have more of their crowns out of the water, they tend to

      survive better than small, submerged trees.

(3) Young trees and very old trees tend to suffer more damage from flooding than other trees.

                    (4) Some tree species are more tolerant to flooding than others:

Tolerant trees: Boxelder, Bur Oak, Cottonwood, Linden, Willow

Somewhat tolerant: American elm, Black Walnut, Downy Hawthorne, Eastern Red-cedar, Green Ash,
Hackberry, Honey-locust

Intolerant: Lilac, Pine, Rocky Mountain Juniper, Spruce

Most trees can withstand a few days of flooding during the growing season if they are healthy and the water
does not contain tree injuring contaminants.

Marcus Jackson
Extension Forester
cell phone: 701-799-9872


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