Ever wonder why one or two or even a section of trees die in a hedge row of arborvitae? It's a common occurrence, but it's still surprising because arborvitae have few insect or disease problems in our region.

If a dying arborvitae is young, planted in recent months or even in the past two years, the cause could be the failure of the plant to survive and grow well after planting. This is usually because of how the tree was planted or the follow-up watering practices. Many of the larger arborvitae being planted in our area are "balled and burlapped" plants from the Willamette Valley area. The soil in their root balls tend to be a heavy clay. When simply placed in the planting hole without the removal of burlap, twine and soil, the roots tend to stay in the root ball and not grow into the surrounding soil. One common reason for this is that most burlap (with a green tint) has been treated with copper to prevent rot. It also prevents new root growth.

Another reason is that clay surrounding the roots is a much different texture than the soil found in our region. This difference interferes with water movement and root growth. The best way to plant trees and shrubs with clay root balls is to remove the burlap and twine around the root ball. Then as much of the heavy clay as possible should be gently washed away from the roots. This must be done with great care, as you don't want to break or rip the roots away. This is done so the roots will be in contact with the surrounding soil. The clay is also "heavy" and stays wet much longer than the rest of the soil.

Another critical factor is to make sure the root balls are not planted too deeply. The top of the roots should be just under the soil surface. They don't tolerate planting too deep or heavy mulching, deeper than 4 to 5 inches.

Container-grown arborvitae don't have to contend with clay soil, but they still can have problems with the difference between the texture of potting mix in which they're growing and the soil in the planting hole. It's also very important to loosen encircling and tightly matted roots and wash off as much of the potting soil as possible.

New plantings of arborvitae require careful watering. Roots should be kept moist but not saturated. If the roots dry out, they will die. If they're excessively wet, they will die because they can't get enough oxygen.

What about the older arborvitae that have been dying in recent months? The causes are pretty much the same ... too little water or too much water. Because they're "evergreens" that keep their green scaly leaves through winter they still need water when temperatures are above freezing. Most gardeners don't water in the late fall and winter months. Before this past winter, area evergreens were subjected to at least two successive years of winter drought followed by very hot, demanding summer conditions. This has led to significant drought stress in many arborvitae and other evergreens.

Wet soil also can lead to root and collar rot. I often see this happen if established arborvitae are placed on drip irrigation. Too often the emitters are placed near the base of the tree. This is what's needed for newly planted trees, but on established arborvitae the fine roots capable of absorbing the water are quite a way from the trunk. Saturated conditions at the trunk base can lead to collar and trunk rot -- a sure way to kill an arborvitae.

* Marianne C. Ophardt is a horticulturist for the Washington State University Cooperative Extension Office in Benton County.