The following is by Mark Gleason, plant pathologist at Iowa State University Extension:
The 2008 growing season was a bad time to be a conifer in Iowa. The Iowa State University Plant and Diagnostic Clinic was inundated with suffering conifers from windbreaks, commercial landscapes, and backyards. What happened?
Rain is what happened: too much of it. The six months from January through June saw record rainfall totals in Iowa – the most since record-keeping began in the 1850s. Late May through mid-June was especially sodden, as much of the state saw damaging floods. The result was wet soils, week after week.
What’s wrong with lots of water in the soil? After all, trees need plenty of water to stay healthy. But 2008 was a case of too much of a good thing.
During the seemingly endless rains of May and June, soils in much of the state became saturated with water – and stayed that way. Even when trees weren’t engulfed in floods or giant puddles, they were rooted in sodden soil.
When soil is saturated, the normally air-filled spaces become water-filled instead. This profoundly changes the roots’ environment. Roots get almost all their oxygen by absorbing it from the soil, and the soil oxygen is replenished from the atmosphere.
Once soil water replaces soil air, the ballgame changes. Water carries only a tiny fraction of the oxygen found in the air. In prolonged periods of saturation, that smidgen of oxygen is further siphoned off by soil microorganisms. Faced with oxygen starvation, roots begin to slow their activity.
Tree roots, like people, go into distress pretty rapidly when their oxygen is cut off.
In extreme cases like 2008, the roots can die. The first roots to bite the dust (or mud) are the ones that work the hardest: the tiny roots that do most of the work absorbing water and nutrients from the soil. As a last gasp, some roots start making their own alcohol; this can be a fatal goodbye party, since alcohol also poisons the roots.
Damage to conifers was extremely varied. Common symptoms included dead branch tips, yellowed needles, top dieback, and dieback of scattered branches. Some fungal diseases were stimulated by all the rain, too, but the vast majority of the conifer problems had a physical cause – root damage due to low-oxygen stress.
Often, the problems showed up in late July or August, even though the worst of the wet spell was long gone. Why the delay? A tree can tolerate some level of root damage. But in warm, dry spells in mid- to late summer, trees call for water to stay cool. If the roots are damaged, the tree can’t take up water fast enough to avoid overheating, and the foliage overheats. So spring’s root damage became summer’s fried foliage.
How come conifers got nailed worse than broad-leaved trees? Most conifers hate wet feet, and the longer the soil stays saturated, the worse they do. At the ISU Clinic, we saw hundreds of samples and photographs of damaged blue and Black Hills spruce, white fir, arbor vitae, and other species.
On the hopeful side, most of these damaged conifers are not at death’s door. For many, the prospects of eventual recovery are good – as long as the weather cooperates a bit. Meanwhile, avoid the temptation to help nature along by fertilizing, since you are likely to harm rather than help the tree’s recovery. Watering should be done only when the soil is dry, and should add only one inch of water (rain included) per week.
With luck and patience, 2009 will return us to green conifers instead of brown ones.