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

Although the impact of flooding on homes and other structures is almost immediate, the effect of flooding on trees is not as immediate in most instances. Over the past few days, I received many calls asking about how flooded landscapes will impact the health of trees.

There are several factors to take into account when considering the impact of flood stress on a particular tree. These include: 1) duration of flood, 2) species tolerance to flooding, 3) age of tree, 4) tree vigor, 5) season, 6) water temperature and oxygen level, and 7) freshwater vs. brackish water.

DURATION: The longer tree roots are exposed to flooding, the greater the potential for injury. The good news is that short periods of flooding during the growing season can be tolerated by most trees. In fact, with the exception of pines, most trees can withstand 24-to-36 hours of flooding without any long-term problems. However, if flooding is recurrent and keeps the soil saturated or prevents recovery from previous floodings, injuries will accumulate and serious damage may occur.

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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.
So, what course of action would I recommend regarding trees flooded? Of all the possible factors influencing survival rates of landscape trees and shrubs, flood waters containing salt water concerns me most.

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.

For more information contact:
Texas AgriLife Extension Service - Galveston County Office
5115 Highway 3
Dickinson, Texas 77539
(281) 534-3413, Ext. 1, 2

OR - visit our website

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