Trees and Flooding - Revisited (6/9/11)
Tables 1 and 2 are adapted from the USDA Natural Resources Conservation Service (North Dakota) document “Tree and Shrub Characteristics.”
Unfortunately, not every tree or shrub species is listed. Nevertheless, the information in the tables clearly shows that there are many more flood-sensitive species than flood-tolerant species. As I mentioned in an earlier article, the low-oxygen environment that flooding brings will kill some trees outright, while others will suffer a great deal of stress. This stress can weaken trees and make them more susceptible to root diseases or stem borers.
While there is very little that we can directly do about flooding, we can help surviving trees recover afterwards. Some recent research suggests that applying a nitrogen-based fertilizer will help trees get better. A general recommendation is to fertilize the soil around trees at a rate of 1-3 pounds of nitrogen per 1000 square feet of ground surface, per year. The fertilizer can be applied through one or several applications, but it is important to avoid overfertilizing or applying fertilizer during mid-to-late summer (July through mid-September) as this may decrease the tree’s ability to harden up for winter.
Trees that normally grow along rivers are generally adapted to occasional flood events. Floods that occur during the growing season are definitely more stressful than those that occur during the dormant season. And unfortunately, flooding sometimes kills trees outright. Nevertheless, by minimizing post-flood stresses to the surviving trees, we can help them recover and hopefully thrive as well as they did before inundation.
Joe Zeleznik - Extension Forester