Most of Illinois has experienced excessive rains, which have resulted in waterlogged soils and flooding. It is important to understand what is happening to plants growing in these conditions and what to expect later. It is a wait-and-see situation. Many herbaceous plants are experiencing injury symptoms now. Visible injury symptoms on trees and shrubs may not occur for a year or more.
Injury symptoms, which vary according to several factors, include decreased growth of shoots and roots, decreased transpiration rate, leaf chlorosis (yellowing), leaf epinasty (twisting), leaf abscission (drop), death of roots, increased susceptibility to attack by predators and pathogens, absence of fruiting, and death.
The main reason injury occurs is related to oxygen availability in the soil. In flooded or waterlogged soils, oxygen diffuses slowly and reduces in concentration to a few percent or zero. As oxygen is excluded from roots, there is decreased aerobic root respiration, root growth, transpiration, and translocation.
Factors influencing survival
Although survival is directly related to species’ tolerance of waterlogged soils, other factors are important—including the soil type; the time, duration, and depth of the water; the state of the floodwater; and the age and size of woody plants.
Tolerant species, such as baldcypress, littleleaf linden, redtwig dogwood, mulberry, silver maple, and willow, can live on sites in which the soil is saturated for indefinite periods during the growing season.
Moderately tolerant species, such as green ash, hawthorns, honey locust, pin oak, red maple, river birch, sweetgum, and sycamore, can stand saturated soil for a few weeks to several months during the growing season, but these species die if waterlogging persists or reoccurs for several consecutive years.
Weakly tolerant species, such as American holly, balsam fir, black walnut, burr oak, catalpa, hackberry, Douglas fir, eastern cottonwood, and red oak, can stand relatively short periods of soil saturation—a few days to a few weeks—during the growing season, but they die if waterlogging persists for longer periods.
Intolerant species, such as American beech, black locust, crabapples, eastern hemlock, flowering dogwood, paper birch, pines, redbud, spruces, sugar maple, tuliptree, white oak, and yews, die if they are subjected to short periods of 1 or 2 weeks of soil saturation during the growing season.
What to do now
Unfortunately, little can be done to prevent damage to plants growing in waterlogged soils. If a woody plant shows injury symptoms, such as leaf drop, do not immediately replace it. Some plants will show initial injury symptoms and then recover. Many woody and herbaceous plants, including turf areas, will not recover. Be patient. Whether your plants are simply waterlogged or actually growing in flood areas, it will take a while to see the full extent of plant damage.