Effects of Drought Still Lingering
Janna Beckerman, Botany & Plant Pathology Department, Purdue University
Last summer’s drought will continue to cause problems on landscape plants—through 2013 and for the next several years. Drought damage develops in plants when dry soils prevent roots from absorbing the moisture necessary to replace water lost during transpiration (a process that a plant needs in order for photosynthesis to occur), resulting in stress. Under continued stress, leaves wilt, turn yellow to brown at the tips and margins, curl, drop or show all of these symptoms, before going dormant for winter, or dying.
However, for many plants, the symptoms of drought are not always obvious, particularly in conifers. Unlike shade trees, conifers don’t shed leaves (needles) in response to drought, and they have evolved to maintain their leaves/needles for two to seven years. Because the leaves/needles aren’t shed, the plant continues to transpire water even when there is none available, making the drought stress worse and possibly killing the tree or shrub. Trees that have lost most of their foliage often continue their downward spiral, and die during the fall and winter months.
Some trees and shrubs will wait until spring before giving up. Needles that are closer to the main stem of the tree or shrub can be two to seven years old; these old needles begin to turn color and die. At this point, all that is left is the current year’s needles, and they aren’t able to generate enough food to allow for new bud development, so by spring, many trees start to show even worse symptoms, or break bud and die.
Unfortunately, the symptoms of slow tree death aren’t usually obvious until after the damage is done, and when it is too late to save the tree or shrub. Even if the drought hasn’t killed them outright, stressed plants are predisposed to infection by pathogens and attack by insects. Although there are always exceptions, most conifers that have lost the majority of their needles won’t recover.
The management advice for severely drought stressed conifers is as Linnaeus suggested several hundred years ago: “If a tree dies, plant another in its place.” Except this time, be sure to keep it well watered, and if you can’t, avoid drought sensitive trees like white-cedar (also called arborvitae, Thuja spp), false-cypress (Chamaecyparis spp), and Colorado blue spruce (Picea pungens). Norway spruce (P. abies) is one of the most drought hardy of conifers; Eastern redcedar (Juniperus virginiana) is a very drought hardy, native conifer, which is also very drought resistant.
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