What can you do about flooded trees?

Author(s): Graham Herbst
Tuesday, March 16, 2021
Cottonwood tree tipped over after prolonged flooding event in south sioux city.

Water is critical to the growth and survival of trees. We often discuss what happens when trees and other plants become dry, but what happens when trees are submerged during short or prolonged periods of flooding? This publication addresses common issues experienced when there is an overabundance of water present near trees.

First considerations – safety!

Assessing risk in trees is complicated and a consultation with a certified arborist shouldn't be ruled out.

Riparian trees are those that grow naturally along flowing water bodies. A significant amount of a tree’s living tissue is under the soil and out of sight. Since the root system is the portion of a tree most adversely affected by flooding, signs of flooding damage in the canopy are usually delayed. Changes in the foliage are often the most obvious symptoms to property owners, but the foliage damage itself is not usually a big problem. Damage to the roots, trunk, and branches has more consequences for the long-term health of a tree. Brief flooding periods (a few days) will not severely impact most healthy trees. In a long-term flooding situation, you should expect that some level of damage has occurred to the root system as a result of low oxygen conditions. Floods can carry debris downstream that collide with above-ground parts, causing permanent physical wounds.

Assessing risk in trees is complicated, and this publication will help you identify some of the most common warning signs to watch for in your flooded trees but should not be used as a substitute for professional consultation with a certified arborist.

Symptoms of flood damage

Trees are large organisms with massive stores of energy in their roots and branches that make them capable of living for years after life-threatening events like a major flood. Most of the symptoms and problems associated with flooding damage listed below will be apparent in 1-6 months following the flooding event if they are going to occur at all. Flooding damage will also cause recovery stress to affected trees in the years to come and predispose them to insect and disease problems. This period of recovery (or decline) may cause heavy seed production and the growth of vigorous but weak branches called suckers or water sprouts. These are signs of stress, but don’t indicate whether the tree will recover or not. Some of the most common warning signs to look for when assessing flooded trees are:

Damages caused by flooding may not be readily apparent and may take several growing seasons to become obvious to an untrained eye. 
  • leaning trunk
  • mound of soil on the opposite side of the tree lean
  • exposed roots
  • cracks in the soil surrounding the root flare
  • early fall coloration
  • leaf yellowing, browning and/or wilting
  • leaf drop
  • branch or twig dieback

Along the spectrum of potential impacts, many factors contribute to the extent of stress and damage in flooded trees.
Smaller impact Greater impact
Short duration flooding Long duration flooding
Well-established trees   Young trees and overly mature trees
Late-season flooding Early season flooding
Floodplain tree species Species not tolerant of flooding
Stress/injury is only water-related Additional physical damage from debris
Flooded by running water            Water is hot, stagnant (lacking oxygen: aquatic hypoxia)
Minor change in topography Significant scouring of soil around trunk (root exposure)
Minor soil deposition     Significant quantity of add’l soil over original grade (>2”)
Tree remains growing upright     Tree develops a lean, or begins to lean more over time
Surrounding trees remain standing Many surrounding trees die or fall over

Caring for flooded trees

While heavy rains can saturate soil like a flood, the inundation period is usually very short and only rarely will heavy rain cause instability or leaning in a tree.
  • Carefully remove excessive sediment (more than 2 inches) from around the trees without crushing exposed roots or damaging the tree further. Be sure to do this work as soon as possible, but wait for the ground to dry out and stabilize enough for large equipment to be safely operated.
  • Fill in low spots with soil from nearby or with similar composition and texture and replace mulch in areas where it has been washed away.
  • Monitor flooded trees regularly for symptoms listed above as well as unrelated insect and disease problems.
  • Unless a soil test of the flooded area indicates a nutrient deficiency needing to be addressed, do not fertilize flooded trees. This can cause excessive top growth while suppressing a tree’s natural defenses and the recovery of its root system.
  • Any large flooded trees that are close by valuable structures or high-traffic areas should be evaluated by a certified arborist. Since these trees have the potential to cause harm to people and property, proactive risk mitigation by pruning or removal is more important.
  • Core aeration and vertical mulching on high-value trees surrounded by turfgrass may be helpful. Ask your arborist about these services.
  • Flooding can kill many of a tree’s small feeder roots and cause an imbalance between the roots and the canopy. Monitor soil moisture after the floodwaters recede to ensure that flooded trees aren’t further stressed by long, dry periods as they try to recover.
  • When planting or replacing trees in flood-prone areas, choose species that are adapted to flooding conditions and purchase them from a reputable nursery.

Helpful links

International Society of Arboriculture - https://www.treesaregood.org/findanarborist/findanarborist

Nebraska Arborists Association - http://growth.nearborists.org/certifiedarborists