Research Shows Weeds Becoming Resistant to Glyphosate

What Gardeners Can Learn

Glyphosate is a broad-spectrum, non-selective herbicide commonly used on perennial plants including grasses, sedges, and broad-leaved weeds. Most gardeners have either heard of glyphosate or used herbicides that contain it in their gardens. It can be found in familiar products like Roundup, Rodeo, Touchdown, and Pondmaster. Due to its repeated overuse, researchers have recently found 13 species of weeds that are now resistant to glyphosate.

The Research

In 2007, an agricultural study conducted by University of Minnesota researchers concluded that 13 species of weeds are now resistant to glyphosate at normal field-use rates. When herbicides containing the chemical glyphosate were applied at recommended rates for field crops, the weeds failed to respond.

A decade ago, the only known resistant species was annual ryegrass and goose grass, and even then, those events occurred in far off places like Australia and Malaysia. Currently, glyphosate resistant weeds species can be found in almost every state in the U.S. and in several countries worldwide. The type of glyphosate resistant weeds in this study are not the type of weeds most of us have to contend with in our backyard gardens. Among the species researchers found to be resistant were giant ragweed, lamb's quarters, pigweed, mare's tail, and others. Still, there is plenty gardeners can learn from these important findings.

How Do Weeds Develop Resistance?

Weeds develop resistance to the forces of their environment through natural selection. When any one environmental force is repeated over time in the same geographic area, genetics eventually comes into play so plants can adapt and survive. In the case of glyphosate, plants that show even a obscure genetic resistance to the chemical will start to survive and propagate. Their offspring will be even more resistant to the chemical than the parent plants were. Because some weeds spread by pollen or seeds, the spread of herbicide resistant populations can infiltrate a large geographic area quickly.Another phenomena relating to natural selection may start to happen as well-one involving an entirely new and different set of weeds. As glyphosate continues to work on target weeds, different weeds (those that have always shown a natural resistance to glyphosate), are able to move in and repopulate the area.

What Gardeners Can Learn From This

The natural response to controlling chemically resistant weeds is to replace the first chemical with one that is more powerful. This can be slippery slope-not only for growers, but also for human health and the environment. The lesson gardeners can take away from this study is one of practicing good weed management in the garden.

• Whenever possible, avoid the use of herbicides. Garden organically and use cultivation techniques or mechanical means to control your weeds.
• Build healthy soil. Weeds tend to thrive in poor soil. Vigorous growing plants are much better equipped to out-compete neighboring weeds for available water and nutrients.
• Know your weeds. If you fail to understand their life cycle, you run the risk of poor control.

If you decide you need a chemical means of weed control, follow the label directions carefully and avoid repeated applications. Remember, if you find yourself applying herbicides year after year, in the end you may find yourself with nothing more than a bumper crop of chemical resistant weeds.