Differences exist among non-selective herbicides

Although you often hear the terms "non-selective herbicide" and "soil sterilant" used interchangeably, the two are distinctly different. Soil sterilants--more appropriately called soil fumigants--are just one type of material that falls into the general category of non-selective herbicides. They are materials that are gaseous in their active state and move through the soil as a vapor. A non-selective herbicide is simply a herbicide that is active on all plant types. These products vary in their physical state, method of application, behavior in the soil, route of entry into the plant and mode of action in the plant.

In contrast to non-selective herbicides are selective herbicides, which--at labeled use rates--are only active on certain types of plants. A common example of a selective herbicide is 2,4-D, which is active only on broadleaf-weed species and has little or no effect on grasses.

The terms non-selective and selective can be misleading, however. For the purposes of this article, we'll use selectivity to describe the nature of a herbicide's chemical activity. After all, selectivity also is a function of application technique; you can make a selective application with a non-selective herbicide by directing the herbicide only to target species. An example of this would be spot-treating weeds in a landscape bed with the non-selective herbicide glyphosate.

You can categorize non-selective herbicides as soil fumigants, non-residual herbicides and residual herbicides. We'll base our discussion here on these categories (see table, "Categories and examples of non-selective herbicides," page 26). Keep in mind, however, that categorizing the activity of herbicides is not a clear-cut process; it is anattempt to simplify our understanding of these materials. For specific information regarding the use of any product, you must refer to the label.

Soil fumigants Soil fumigants are broad-spectrum materials. They act on existing vegetation, weed seeds and, to a varying extent, on fungi, nematodes, insects and earthworms, depending on the material you use. In their active state, fumigants are vapors. Therefore, you must take steps to ensure the vapors remain in the soil long enough to work effectively. For example, you must inject gaseous methyl bromide into the soil and quickly cover it or inject it into already covered soil.

However, you don't apply all fumigants in a gaseous stage. Some change into the vapor after you've applied them. Metam sodium, for example, is a liquid formulation that you can apply in a variety of ways. You can spray it on and water or till it in. Or you can apply it with a sprinkler can or through a specially configured irrigation system. Dazomet is a granular material that you apply with a drop spreader and till it in. You keep metam sodium and dazomet in place by watering the soil sufficiently to form a crust. Another application technique with metam sodium and dazomet is to use a tiller equipped with a roller that rolls faster than your tractor's ground speed. As the roller passes over the moist, tilled soil, it smears the surface and forms a crust.

Once in a gaseous state, fumigants will move upwards through the soil, rather than further into the soil. This is why it is important that you get them to the desired treatment depth when you apply them.

After the materials have had sufficient time to work, which will vary according to the material and site conditions, you can shallowly till the soil surface. Doing so enhances the release of these materials from the soil. It is important that you perform this follow-up tillage more shallowly than your tilling was when you originally incorporated the material. This is because if you till soil too deeply, you'll bring up untreated soil that may contain the very weed seeds and soil organisms you were trying to eliminate.

You can plant as soon as the fumigant has dissipated from the soil. This usually occurs 2 to 21 days after application, depending on the material and environmental conditions. Dissipation occurs more quickly when soil temperatures are higher. Product labels provide the necessary information on how long you must wait until you replant, based on practices and conditions.

Non-residual herbicides Non-residual herbicides do not leave an active residue in the soil. Therefore, they have no pre-emergence activity and will not harm nearby desirable plant material through root pickup. We further classify non-residual herbicides into contact or systemic herbicides. Contact herbicides affect only the treated part of the plant, while systemic herbicides are translocated within the plant and can provide complete control.

The term contact herbicide is not precise, but it is easier to use than "a herbicide that translocates very little, depending upon the species and environmental conditions." For example, diquat dibromide, which is similar in activity to paraquat, must move through the outer leaf covering, the cuticle, then into specific locations within individual cells in the leaves and stems. It then undergoes a specific chemical reaction before damage occurs. The injury from contact herbicides tends to be limited to those above-ground parts of the plant that you treated. Contact herbicides are useful where the treated vegetation is in the seedling stage or when you need quick burn-down. Fatty acid-based herbicides and diquat usually cause complete burn-down within 2 days.

Where you need complete control of vegetation without soil-active herbicide residue--such as controlling weeds in a seedbed before seeding or controlling well-established weeds near desirable landscape plants--use a non-residual, systemic herbicide such as glyphosate or glufosinate. Glyphosate does not act as quickly as contact herbicides or glufosinate, but it does translocate through the entire plant to provide complete control. Glufosinate is considered a localized systemic herbicide because it moves only a short distance within plant tissue. However, glufosinate strongly disrupts amino-acid metabolism, and it affects an area larger than the immediate zone of its activity as an enzyme inhibitor. Thus, glufosinate provides more thorough control than contact herbicides but less control than herbicides that are translocated throughout the treated plant.

Do not mechanically disturb plants treated with systemic herbicides for several days after treatment. This allows the herbicide to move through the plant as much as possible.

Systemic herbicides work more quickly when the plant is actively growing. In conditions where plant growth is slow, such as during drought or low temperatures, systemic herbicides will take longer to work. In situations where you must maintain bare ground near desirable plants, you can tank-mix systemic herbicides with pre-emergence herbicides that provide control of weeds coming from seed but that do not act on existing desirable plants. In this situation, systemics will control the weeds present, and the pre-emergence herbicide will provide the residual control that systemics lack.

Residual herbicides You should use non-selective herbicides with soil-residual activity in situations where you want completely bare ground--often called industrial weed control. Examples include vegetation control around buildings and equipment, along fence lines, unpaved parking lots, pipelines and storage tanks. Usually the intent of such a treatment is to make one application in the spring that will provide bare ground for the entire season. The workhorse herbicides for such treatments often are materials that are non-selective with soil-residual activity, such as bromacil, imazapyr, prometon, sulfometuron methyl or tebuthiuron. Imazapyr is an effective foliar herbicide, while sulfometuron methyl will provide effective post-emergence control of vegetation that is not yet well-established. Bromacil, prometon and tebuthiuron have very little foliar activity, so control of established plants is quite slow. Where well-established vegetation is present, and you want quick symptom development, use glyphosate or one of the contact herbicides.

Non-selective, residual herbicides are potent tools. They can accomplish a great deal for you, but mistakes can be costly. Do not use these materials near desirable vegetation. When working anywhere near desirable trees and shrubs, keep in mind that the roots of trees and shrubs commonly extend two to three times the canopy width. It is best to rely on a mixture of systemic, non-residual herbicides and pre-emergence herbicides on industrial sites where sensitive, desirable plants are nearby. Non-selective, residual herbicides should be a part of your tank mix where they are useful but only at the lowest practical rates.

Non-selective herbicides are a useful tool. To best use them, gain as full an understanding as possible of how they work. The best place to start is the label. After reading the label, ask questions of your university and extension representatives, product representatives and your colleagues using the materials.

Art Gover is supervisor at Pennsylvania State University's Landscape Management Research Center (University Park, Pa.).

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