By Island County Extension Agent, Donald B. Meehan
USING SOIL STERILANTS AROUND THE HOME LANDSCAPE
This is the time of the year when many of you are considering finding a way to keep the weeds in the driveway, backyard, and landscape from taking over this summer. Some of you have probably heard about soil sterilants, or, as they may more appropriately be termed, soil-residual herbicides. These herbicides are designed to eliminate all vegetation in an area and may seem like the quickest and easiest thing to do. Before you make that purchase or begin applying soil sterilants take the time to finish reading this news column, you may want to change your mind.
The use of soil-residual herbicides in residential areas is an area of frequent concern because these herbicides can kill or injure desirable vegetation. Investigations revealed that damages to desirable vegetation were caused by chemicals being used, in every case, in ways inconsistent with the label.
Soil sterilants were originally developed to provide long-term vegetation control on industrial sites, along highways and railroad rights-of-way, and similar areas. Many of the products were never intended to be used in residential areas or landscape areas.
The problem with soil sterilants begins with the persistence, often higher rates of application, and indiscriminate root absorption by plants. While there are no records of the total annual loss of landscape vegetation from sterilants, it must be substantial. Because of the long life of these herbicides, damages are frequently experienced over a period of several years which adds considerably to the cost of reclamation of the contaminated property for re-landscaping.
Products frequently used for soil sterilant work include chemicals such as diuron, simazine, bromacil, prometon, monuron, ureabor, tebuthiuron and often mixtures of some of these products. In most cases, a statement is made on the label, to the effect:
"IMPORTANT - injury to or loss of desirable trees or other plants may result from failure to observe the following: Do not apply, or drain or flush equipment on or near desirable trees or other plants, or on areas where roots may extend, or in locations where the chemical may be washed or moved into contact with their roots. Do not use on lawns, walks, driveways, tennis courts or similar areas....."
Some soil-residual chemicals cannot be used without risk of serious plant loss in the average landscape area because roots of trees in areas of frequent shallow watering or on shallow soils, extend many feet beyond the drip line. One mature Elm tree 70 feet high can have feeder roots extending a radius of more than 150 feet from the tree center. A Spruce with a branch spread of 15 feet will have some roots extending 6 to 10 feet beyond. In many cases, the chemicals do not move into the root zone but rather, the roots extend into the treated area at a later time. Damages may take several years to appear following the application of the soil sterilant.
Tree roots also tend to grow abundantly between the soil surface or gravel base and asphalt. Thus, these roots are very shallow and subject to contact with herbicides used under asphalt.
Trees and other desirable vegetation in the Pacific Northwest have been killed in some of the following ways with soil sterilants: treating cracks in sidewalks, streets and curbing; asphalt interface on streets; and applying the wrong material under asphalt.
Herbicides such as dicholbenil (Casoron, Norosac) are registered for controlling weeds under asphalt. Dichlobenil inhibits new plant growth and exhibits minimal absorption or movement into tree roots. In other areas where total vegetation control is desired use of mulches is a safer option. Herbicides such as glyphosate (Roundup), which may be applied as a directed spray, or short-term soil residual herbicides that inhibit root and shoot growth, are preferable to compounds that move easily in the plant.
Ultimately, READ and FOLLOW label directions on the materials selected and know the growth habits of the surrounding vegetation to determine if roots extend into the area to be treated or will grow into that area in the future.
Other questions which need to be asked are:
1. Is the site sloping and will the herbicide move downslope?
2. Does the label state different herbicide rates based on soil texture and organic matter?
3. What action does the herbicide exhibit on the plant and how will it affect desirable vegetation?
4. Will there be a possibility of changing the landscape or planting new plants?
A positive answer to any of these question should be reason enough to look into the other weed control methods and possibly avoid use of any of the soil sterilant type herbicides.
Homeowners need to be aware that state law requires all persons using pesticides such as herbicides and insecticides must follow labels instructions. Failure to do so could result in a $1,000 dollar fine. Soil sterilants, because of their residual effects, can pose serious problems for homeowners with neighbors located close by.
My thanks to WSU-Extension Weed Specialist, Robert Parker, for much of the information in this news column.
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