Herbicide makes for popular garden 'tool'

KENNEWICK -- Ever since it first hit the home garden market, Roundup has been a favorite chemical "tool" of many gardeners because it kills tough perennial weeds.

It has remained a popular herbicide since 1974, when it was introduced to agricultural users by Monsanto.

Monsanto's patent on glyphosate, the active ingredient in Roundup, expired in 2000. Since that time other companies have been producing glyphosate (pronounced gly-fo-sate) under other various trade names.

When you read the labels of the many different glyphosate products, the ingredient that does the work of killing weeds is glyphosate or one of the salt forms of glyphosate. What you don't see listed on the label are the other ingredients that make up the different formulations of the product.

Manufacturers add various surfactants, defoaming agents, and other compounds that either make their product work "better" or make it easier for the users to handle. These materials are proprietary and are listed as inert ingredients. It's interesting to note that research at Purdue University and other universities comparing the effectiveness of different glyphosate formulations revealed no significant difference between the formulations tested.

So let's review how glyphosate works to kill plants. It's applied as a liquid spray to the leaves of green plants. It's absorbed into leaf tissues and then moves within the plant's vascular system (via phloem cells) to the growing points at the shoot and root tips. At the growing points it inhibits the production of an enzyme needed for the production of amino acids and proteins essential for plant growth. As a result, the plant dies.

Care must be taken when using glyphosate products for weed control because they are "non-selective." They can't tell the difference between a weed or a desirable plant and will kill or damage any green plant to which they're applied. Glyphosate should be applied "selectively" only to offending weeds.

Glyphosate is a useful herbicide tool, but sometimes treatments fail. Common reasons for failure include:

1. Getting too little chemical into the plant because there isn't an adequate amount of green growth treated. The weed must be actively growing with plenty of green leaf surface area to treat. Don't pull, mow or cultivate the weeds before treating. The more leaf surface treated, the greater the amount of the chemical that will enter the plant. Treated plants should not be under stress from drought, excess soil moisture or high heat.

2. Plants are dusty. Glyphosate binds with the dust particles and won't enter the plant. Wash the dust off your weeds or apply after rain has washed them for you. Leaves should be dry before application.

3. Poor coverage. The more leaf surface treated, the more effective the treatment.

4. Rain or irrigation water washes the material off. Don't apply glyphosate if rain or sprinkler irrigation is expected within six hours.

5. Some of the toughest weeds require repeat applications because of deep or extensive root systems. Check the best time of year for treating tough-to-control weeds with glyphosate. For example, late summer is the best time to treat Bermuda grass.

6. Dirty or hard well water can reduce the effectiveness of a glyphosate spray mix.

Sometime soon we'll talk more about glyphosate damage on desirable plants, as well as some of the newer Roundup home garden products. Until then, be sure to read (and follow) the label directions of any herbicide product you use in your yard and garden.

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