How long can trees survive flooding before injury results? As you might expect, this has become an all too frequent question lately as torrential rains and bloated rivers continue to plague many regions in Iowa. Fortunately for most trees, the prospect for survival and continued growth is good. Even flood-sensitive trees will escape injury if flood waters recede in seven days or less. But, if flood waters cover roots of sensitive trees for longer periods, injury symptoms such as leaf chlorosis (yellowing), downward curling of leaves, leaf drop, and branch dieback may occur. And in a few extreme cases, entire trees may die.
Which trees are intolerant of flooding? Some of the more common species used in Iowa are sugar maple, white oak, yellow buckeye, tulip tree, black walnut, redbud, linden, red oak, and most pines and spruces. Researchers have found these species suffer severe injury or die if flood waters persist over their roots for one month or less. Sensitive trees along the Mississippi River will probably be pushed to their limit as flood waters are predicted to persist for quite some time. Thankfully along smaller rivers and tributaries, flood conditions usually last only a few days and should pose little danger to flood-sensitive trees.
Moderately flood-tolerant species that have actually survived an entire growing season of flood conditions include hackberry, hawthorn, osage-orange, boxelder, river birch, American elm, and sycamore. Species classified as being flood-tolerant, surviving as many as two growing seasons with their root systems under water include silver maple, sweetgum, red maple, green ash, honeylocust, eastern cottonwood, and baldcypress.
Flood waters will eventually recede but soils will undoubtedly remain wet for a long time. Saturated, poorly-drained soils may pose the greatest hazard for trees, particularly if this waterlogged condition persists for an extended period. If oxygen cannot penetrate to roots, trees may exhibit symptoms associated with flooding. Warm, dry weather is the only cure for this chronic and potentially deadly soil condition. Another hidden danger resulting from flooding is the deposition of sediment over tree roots. Silt and sand deposited to a depth greater than 3 inches also may impede movement of oxygen to tree roots, especially on small or newly-planted trees. When possible sediment should be removed.
Except in cases where flood waters persist for months or where trees have been injured by the sheer force of rushing water, most trees experiencing flood conditions should survive. If flood- sensitive species begin to show flood damage symptoms, recovery may begin once soil oxygen levels return to a more favorable state. During this recovery period it is important that any additional stresses be eliminated. In addition, if dead or dying branches are noticed in the tree crown, they should be removed as quickly as possible. And beware of so-called "tree experts" recommending rescue treatments for affected trees. Fertilization is not a cure or remedy for root injury caused by flooding! Finally, avoid planting sensitive species in flood-prone areas in the event of future flooding events.
This article originally appeared in the July 14, 1993 issue, p. 112.