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OHIO'S 1999 DROUGHT LESSONS MAY BE WORTH STUDYING HERE
I hope it's raining torrents as you're reading this. I hope the predictions of drought in Minnesota are wrong. Farm crops suffer in a drought and so do our gardens and trees.
Drought was severe in Ohio and neighboring states in 1999 and both the USDA and the National Weather Service are warning that states further west (that's us) may experience drought this summer.
Here are some of the ways drought can affect trees:
Pines, spruce, arborvitae and other conifers that appeared normal last fall may begin to show drought symptoms this spring. Symptoms can range from needles becoming off-colored to needles becoming brown and dropping off. Needle discoloration, browning and needle drop can occur over entire branches in a random pattern on a tree or over the entire tree. In some cases, particularly on spruces, only needles on last year's growth will appear to be affected. Some of this browning can occur suddenly.
A particular problem of conifers is that, while deciduous trees have the ability to "evade" drought by dropping their leaves and shutting down for the winter, conifers lack such a mechanism to escape the effects of low soil moisture in the fall and winter. When temperatures rise in the fall, winter and early spring to levels sufficient to stimulate transpiration, conifers lose moisture from their needles. If transpiration exceeds the rate of absorption from the soil, physiological stress develops.
The rate of absorption can be hindered by frozen soils or by the lack of soil moisture. If soil is frozen, physiological stress may produce "winter burn." This is not an uncommon occurrence and conifers usually recover as soil thaws. However, if stress is due to lack of soil moisture, recovery may not occur. Trees dehydrate, needles eventually turn brown, drop, and the tree dies.
While this may be happening now, it is also likely that it occurred late last fall. Dead conifers that are kept cold by winter weather will remain green and appear to be living. However, as weather warms in the spring, their true nature is revealed and the needles on these "alive-looking" dead trees will turn brown very rapidly and drop.
Winterburn on conifers in April and May is fairly common and can be misleading. Most trees and shrubs, particularly yews and arborvitae, recover from freeze/frost injury without much help. Conifers that turn totally brown, unfortunately, are dead and will need to be removed.
It is rare for established trees and shrubs to die after a single encounter with an environmental problem. Death usually occurs only after repeated or prolonged environmental assaults. In most cases, it's not one problem, but several problems that have a cumulative effect over time in pushing a plant over the edge. This makes diagnosing the exact cause of death a challenge since there is seldom a single problem at work.
In drought-plagued Ohio, conditions last year were perfect for the development of damaging populations of spruce spider mites, both in spring and fall. The mites benefited from the previous mild winter as evidenced by the lack of over-wintering eggs - adults in some populations just kept feeding. The dry spring, particularly in southern and central Ohio, kept mites from being washed off trees. Mites did not disappear into the summer egg stage until July, and dormancy only lasted until the first of September. Once eggs hatched in September, the drought once again gave the mites a reprieve from the washing effects of rainfall.
Long-term effects of Ohio's 1999 drought on plant health remains uncertain; however, there can be some reasonable speculations regarding certain plant pests and diseases. Several species of conifer bark beetles common in Ohio prefer to colonize stressed trees. As their name implies, the beetles burrow into the bark to lay eggs. This produces characteristic "shot-holes" in the bark. Heavy infestations of these beetles were seen late last summer in central and southern Ohio. If conifers remain stressed by the impact of last season's drought, it would seem likely that infestations will continue to be common this growing season.
The Ohio experts speculate that deciduous trees will be more susceptible to several fungal diseases associated with stressed trees. These include Verticillium wilt of maples and anthracnose, a common Minnesota tree disease.
In a drought period, trees should be watered deeply once a week. Let a hose run slowly for a half hour or an hour at the base of a tree to ensure that water reaches the root zone.
The Ohio horticulturists report that some Ohio homeowners over-compensated by over-watering their trees - "they killed their trees with kindness." Too-frequent watering can kill roots through a lack of oxygen and make roots more susceptible to root-rotting fungi.
In a drought period, be careful not to over-fertilize trees and shrubs. Drought stress cannot be corrected through fertilization and high nitrogen levels may actually enhance stress.
So, the bottom line is: pray for a normal year of rainfall.
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