Sunday, October 12, 2008

Landscape - Dieback in Needled Evergreens I

I recently had a question from a landscaper about dieback in evergreens. This year we are again seeing the loss of many evergreens. The following is the first posting on the subject using material from the Ohio State University.

Each year various narrow-leafed evergreens such as pines, Taxus and spruce are affected with needle yellowing and browning, dieback, poor vigor or death. These are problems often associated with one or more environmental stress factors. An explanation of some of these stress factors follows.

Wet Soil

Excessive amounts of water can result in a saturated soil, reducing oxygen levels to a point where small roots weaken or die. This root decline can be sudden or gradual and the roots may be invaded by various soil-borne fungi. Continuous wet conditions lead to progressively worse situations. If the top of the plant is unable to obtain the necessary water and nutrients, it declines or dies. However, the evidence of death (needle browning) often occurs at a much later date. An example is Taxus, or yew, planted in a heavy clay sub-soil with no sub-surface drainage. In fall, winter and spring, water accumulates and literally drowns the roots. The tops of the plant may not succumb until the following spring or summer, when hot weather first arrives and stresses the plant. Some evergreens appear to lose vigor and die back after 15 to 20 years. This often is the result of injury to the root system from moisture stresses. Heavy soils may limit development of the root system; root damage easily upsets the top-to-root ratio. Dieback and poor growth are often evident. Changes in sub-soil drainage caused by construction will often cause roots on older plants to die back.

Drought or Dry Soil

The lack of water for long periods may result in symptoms similar to those caused by excess water. Clay soils often pull away from the roots as they dry, drying or breaking the fine roots. Drought stress may be especially noticeable in the summer on evergreens planted on well drained sites (sand or gravel), or where roots are in the top layers of heavy compacted soil. Excessive needle drop and poor vigor are often evident as a result of drought stress.

Reprinted from "Yellowing, Dieback and Death of Narrow-Leafed Evergreens" Factsheet HYG-3034-96, by Stephen Nameth, Nancy Taylor, and Jim Chatfield, the Ohio State University Cooperative Extension.

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