The Purdue Plant and Pest Diagnostic Laboratory

P&PDL Picture of the Week for
November 15, 2010

Drought conditions continue to create problems

Janna Beckerman, Associate Professor, Botany & Plant Pathology Department

Drought conditions continue to create problems for native and non-native trees and shrubs throughout Indiana. Many evergreens (white pine, Colardo blue spruce, fir, arbovitae), deciduous trees (maple, ash, oak) high-maintenance ornamentals (Japanese maple, Japanese stewartia, rhododendron), and even normally low-maintenance plants like Spirea are exhibiting symptoms of leaf scorch, dieback and in some cases entire plant death. Indiana was considered abnormally dry according to the National Drought Monitor for the month of August, September and October. The drought spread northward into central and northern Indiana as the months progressed and intensified in the southeast region of the state. By the end of the month 80% of the state was in drought status with 29% in moderate drought (D1) and 19% in severe drought (D2) status.

Normally, established, native plants have evolved to survive seasonal and annual variations in water supply, however even these species are exhibiting symptoms of stress in response to the severity of the drought conditions, including early leaf drought. 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, or show all of these symptoms and die. Stressed plants are predisposed to infection by pathogens, attack by insects, and additional injury from other site and environmental factors. At this stage, most broadleaved trees have dropped their leaves; however even these trees are still suffering from drought stress. In conifers, needles turn yellow or brown and drop and entire branches will die back. Due to the waxy, protective layer on conifer needles, these symptoms may not develop until many months after the initial stressful event, and many appear until next spring. By the time these symptoms develop, it may be too late to save the tree or shrub. Note-- drought symptoms on evergreens should not be confused with normal fall needle drop of older, inner needles, which is also occurring now.

To minimize the impact of drought, mulch around the base of the tree, taking care to avoid “mulch volcanoes,” and keeping the mulch away from the tree’s stem. Two to four inches of mulch increases soil moisture retention, prevents weed development and competition, and minimizes the likelihood of lawnmower and weed whip injury. However, more than four inches of mulch actually prevents roots from obtaining necessary moisture by blocking rainfall or irrigation, and provides attractive cover to rodents that may feed on young trees or shrubs. Other techniques to manage drought is to avoid planting shallow-rooted species (sweet gum, silver maple) in areas of low moisture or on sandy soils. Water ornamental trees about once a week to moisten soil 6 to 12 inches deep into the root zone (approximately 1” of water). For clay soils, watering two times per week with only 0.5” soil will prevent flooding, and allow water to percolate to the root zone. Watering now is essential to tree and shrub health, and will minimize the impact of the 2010 drought and how well your trees perform in 2011 and beyond.

For more information, see: http://www.ppdl.purdue.edu/PPDL/weeklypics/10-18-10.html about the interaction between drought and winter injury.

 

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Viburnum

Viburnum drought stress

Maple

Maple drought stress

Hens and chicks

Drought on hens and chicks

Purdue Plant & Pest Diagnostic Lab Purdue Cooperative Extension Service