Question: During last week's intense rain, part of our yard became a lake! Many of our landscape plants were flooded with several inches of water. Will they survive?
Answer: Maybe - it depends on a number of factors. There is no question that excess water can be a serious problem for many woody ornamentals. Roots in flooded or water-logged soils are damaged and die from oxygen deficiency. In addition to this direct damage to the root system, flooding has also been associated with causing physiological changes in woody plants that influence their growth and other processes.
The feeder roots, which are non-woody and important for the uptake of both water and nutrients from the soil, are particularly sensitive and are frequently the first ones damaged by water-logging. Woody roots are more tolerant than non-woody roots to flooding. When feeder roots are damaged, they are unable to provide water to the top of the plant and a water deficit develops. Damage can be sudden or gradual
Most ornamental trees and woody shrubs cannot grow in water-logged soils for very long and can die if flooded for only a few days during the growing season. Visible symptoms are often not evident for an extended period of time, especially when the root damage is gradual.
Seedlings and new transplants are more sensitive to excess water problems than are established plants. This can be attributed to the lack of an established root system and to feeder root damage during transplanting. Needled evergreens are generally considered more sensitive to water-logged soils than broad-leaved deciduous plants. Symptoms of water-logging may not develop in a woody ornamental until water demands on the root system increase, typically during the hot summer months when the canopy is actively losing water through transpiration. This is sometimes exhibited as a sudden collapse of the tree or shrub.
Other trees may appear to lose vigor and slowly decline over a period of years. This can occur on trees that have been otherwise "healthy" for 10 to 15 years, but are growing in poor sites or heavy soils. Dormant plants generally appear to tolerate flooding longer than those in active growth. In addition to direct root damage, trees in flooded soils are predisposed to secondary pathogens.
EDITOR'S NOTE: For answers to local gardening questions, contact Master Gardener Rachel Gana at 642-8723 or e-mail her at: firstname.lastname@example.org.