By Mark Gleason
Plant Pathologist
Iowa State University Extension
In 1840, Iowa was covered by a deep-pile, wall-to-wall carpet of prairie grass. Bits of deciduous woodlands were scattered around. Conifers (needle-bearing trees and shrubs that produce cones) were scarce. Other than white pine and red cedar, Iowa was pretty much a conifer-free zone.
Fast-forwarding to today, Iowa is awash in conifers of all types – pines, spruces, firs, junipers, yews – in yards, parks, windbreaks and wildlife plantings. Although few of these species are native to Iowa, or even to the Midwest, most have adapted and even thrived.
But occasionally we get a year that pushes these immigrants to their limits and beyond. This year certainly qualifies as a stress test for many conifers. With record-breaking rainfall during the first half of 2008, we are finding out how long conifers can tread water.
As a group, conifers hate wet feet. Most do not tolerate long periods when the soil is waterlogged – especially during the growing season. The problem is simple: their roots can’t breathe in water-filled soil. Like us, they can’t hold their breath very long.
Once the roots run out of oxygen, they decline rapidly. Soon, they start to die. First to go are the tiny root hairs that do most of the work in absorbing water and nutrients. In a short time, the tree’s needles start turning yellow or brown. In severe cases, conifers can die after a few days in waterlogged soils.
Waterlogging injury can show a spectrum of symptoms. Yellowing of the interior needles on blue spruce, tip dieback on concolor fir and 100 percent browning of white pine have all been common in Iowa over the past six weeks.
Why is the damage so variable? Part of the reason is genetics. Conifer species vary widely in their tolerance to stresses like waterlogging. The same soaked-soil scenario will have far different results depending on which species are experiencing it. For example, white pine is far less tolerant than red cedar to long periods of wet soil.
Even individual trees within the same species vary in stress tolerance. This variability is not really surprising when you think of how differently individual people respond the same stress. You might think that all blue spruces are alike, but individual trees that are produced from seeds are as diverse as people.
The level of stress on a tree’s root system also can differ from place to place. Two trees separated by just a few feet can experience profoundly different levels of waterlogging stress due to changes in soil type, drainage, elevation and exposure to runoff. Not all of these changes are obvious to us, since we cannot see below the soil surface to where the roots are living.
A common comment by clients bringing samples of waterlogging-damaged conifers to the Iowa State University Plant and Insect Diagnostic Clinic (351 Bessey Hall, ISU, Ames, IA 50011) is that “it can’t be waterlogging injury because the tree was growing on a slope.” Their perception is that the slope will carry excess water away. This is true for surface water flow.
But roots respond to internal soil drainage, too. A heavy soil, high in clay content, can remain waterlogged for long periods, even after surface water has disappeared. As a result, it’s not uncommon to find conifers suffering from low-oxygen stress even on hillsides.
Every now (2008) and then (remember 1993?), some of Iowa’s conifers will suffer from low-oxygen conditions due to excess rainfall and flooding. What is the prognosis for these injured trees? The answer is: it depends.
Many waterlogging-damaged conifers have suffered only sub-lethal stress. In other words, they took some root damage but managed to survive. It is possible that these trees saw a bright light and Elvis in his Las Vegas period during these crises, but nobody knows for sure. Many of these trees will gradually recover, although it may require several years to resume a completely healthy appearance.
The best practice for dealing with these injured trees is often to do nothing drastic. No fertilizer spikes, deep-tine aeration, other exotic horticultural therapies are likely to help with recovery, and some may even be harmful. Ironically, watering can be part of the solution. If we go into a prolonged dry period, a thorough watering once a week can help trees continue to develop new roots and avoid drought injury.
Some trees will not recover, unfortunately. We call these trees “toast” due to their uniformly brown, crispy appearance. When replacement becomes the best option, it is advisable to plant a different species than the dearly departed. Consult your local nursery or ISU Extension office for advice on selecting tree species with relatively high levels of tolerance to wet soils.