Floodwaters Can Affect Health of Landscape Trees, Plants
Dr. William M. Johnson, County Extension Agent-Horticulture
Texas AgriLife Extension Service
Galveston County Office
5115 Highway 3
Dickinson, Texas 77539
SPECIES: Some species of trees are better able to adapt to flooded conditions. Trees that have evolved in a flood plain ecosystem have mechanisms to cope with the periodic flooding that may occur and are better able to handle flooding. However, urban areas that end up flooded are not usually forested by trees that are adapted to flooding. There are some notable urban exceptions: Arizona ash, green ash, Chinese tallow, hackberry and even silver maple are all considered relatively tolerant to flooding stress. In contrast, pines are all relatively intolerant of flooded conditions.
AGE: Adult trees in their prime tolerate flooding better than overmature trees or seedlings of the same species. Therefore, some species rated as flood-tolerant may be quite sensitive in the seedling stage. Seedlings often die because they are pushed over, buried in mud, or uprooted.
VIGOR: Tree vigor at time of flooding has a major influence on tolerance to floods. Vigorously growing, healthy trees withstand flooding better than less vigorous trees. Tree vigor may be irrelevant, however, if the tree is totally submerged for an extended period of time.
SEASON: Flooding during the growing season usually is more harmful to woody plants than flooding during the dormant season. The timing of a flood in relation to the stage of growth also can be critical. For example, trees are most susceptible to injury by flooding in early spring just after the first flush of growth. The timing of a spring flood influences species differentiation. For example, since Arizona ash flushes earlier than pecans, an early flood might be more damaging to Arizona ash while a later flood might be more injurious to pecans. The impact of floods caused by Tropical Storm Allison was lessened since trees were well into their active spring growth cycle.
TEMPERATURE AND OXYGEN: All other factors being the same, cooler flood water is less injurious to trees than warmer flood water due to the capacity of cooler water to hold more dissolved oxygen. Also, rapidly flowing water (with higher oxygen content) is less harmful than stagnant water.
FRESHWATER VS. BRACKISH WATER: Brackish
means mildly salty and a common example in our area is when salty water
from the Gulf is pushed inland through freshwater bayous. While brackish
water is not nearly as salty as water in the Gulf, most landscape plants
are sensitive to flooding by brackish water. The severity of salt damage
to plants depends upon the amount and duration of exposure, and the salt
concentration of the brackish water.
Even so, I strongly recommend allowing 6-to-8 weeks time before removing a tree to provide sufficient time to better gauge the damage sustained and the likelihood of recovery. An important exception to this recommendation involves large trees that are not solidly anchored, or otherwise pose a hazard to home, traffic, play grounds, etc.
For those with a measure of patience, I suggest waiting for spring for the tree to “talk to you” about its “feelings”: Does it have a full complement of leaves? Do the leaves have an overall green as opposed to green with brown-tips?
As can be seen, determining the flood tolerance of a species of tree and the actual impact of a flood on trees in a given area are influenced by a complicated and diverse set of factors. How well a tree copes with flooding depends upon how vigorous the tree was prior to flooding and how long the flooding occurs. Well-maintained, healthy trees can and usually recover quickly when flooding is relatively short in duration and flood waters do not contain any salt water from a storm surge.
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by Master Gardener Laura Bellmore, under the direction of William M.
Johnson, Ph.D., County Extension Agent-Horticulture & Master Gardener