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Extension > Garden > SULIS > Maintenance > Sustainable Lawncare Information Series > Weed Management > Types of Herbicides

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Types of Herbicides

Responsible Herbicide Application

The first step in using any pesticide product responsibly, including herbicides, is to follow the label directions exactly as printed on the product container. The label provides necessary information regarding proper product application including required personal protective gear (e.g., what type of gloves) and container disposal procedures (Fig. 10.8). Labels are legal documents which are enforceable by law should the product be used in a manner inconsistent with directions. As with fertilizers, extreme care must be taken to prevent the direct application of herbicides into surface water areas.

Figure 10.8. Always wear personal protective gear as listed on the product label. Protective gloves are worn during application with this particular herbicide.

Preemergence Herbicides

Preemergence herbicides affect germinating seeds. Consequently, preemergence herbicides are most effective against annual weeds that come back from seed every year. It’s important to remember that a preemergence herbicide does not kill the seed prior to germination. The seed must start to grow and sprout such that when the newly emerging root, shoot or both contact the herbicide they are killed (Figure 10.9). This activity occurs prior to the weed seedling emerging from the ground. Hence the term preemergence is applied to these herbicides. If the seed doesn’t germinate it will not be affected by the herbicide.

Figure 10.9. Preemergence: directed at control of the germinating weed seeds.

To be effective, apply preemergence herbicides before you expect weed seeds to germinate. For control of annual summer grasses such as crabgrass, apply preemergence herbicides between May 1 and May 15 in a typical year or once soil temperatures in the top one inch or so are consistently in the 50o to 55oF range as measured early in the day.

Postemergence Herbicides

Postemergence herbicides are used to kill weeds after the weeds are visible and growing (Fig. 10.10). Postemergence herbicides are usually most effective when absorbed through the leaves. When used according to label instructions, either liquid sprays or granular applications are effective. Postemergence herbicides are most effective if applied when weeds are young and growing vigorously, or in the fall when perennial weeds are storing food in their roots for the next year.

Figure 10.10. Postemergence control is directed at destroying visible, actively growing weeds.

Postemergence herbicides that can control annual, biennial or perennial broadleaf weeds in a lawn without harming the lawn grasses are termed Selective postemergence herbicides (Figs. 10.11 and 10.12). Since these types of herbicides cannot distinguish one broadleaf plant from another, they can severely damage or kill nearby trees, shrubs, flowers, and vegetables should they come in contact with those plants. Thus, they must be used with great care in mixed landscapes where trees, shrubs and flowers may be growing in or around the lawn and may or may not have roots intermingled with those of the grass plants.

Figure 10.11. Creeping Charlie with grasses before application of selective herbicide.

Figure 10.12. Creeping Charlie dying after application of selective herbicide.

Nonselective postemergence herbicides kill most green plants, both desirable and undesirable. These herbicides are used to control perennial grassy weeds that are not affected by selective herbicides. Spot treat infested areas using only enough product to wet the foliage. It is not necessary to thoroughly drench the area to achieve satisfactory control.

Figure 10.13. Broadleaf weeds and grasses before application of nonselective herbicide.

Figure 10.14. Nonselective herbicides potentially affect or destroy all green vegetation.

Paying Attention to Product Formulation

When applying herbicides such as 2,4-D, a common selective postemergence herbicide, the amine formulations do not easily vaporize and potentially drift offsite by wind. This offers greater protection against non-target plant injury. Hence they are less risky to use in warmer conditions. Vapors from ester formulations are more likely to injure nearby ornamentals and garden plants and if used, should only be applied during the cooler periods of the year thereby reducing the herbicide’s potential to vaporize and move off-site. Manufacturer’s product labels will identify which particular formulation was used in their product.

Table 10.1. Dicamba Herbicide – Use with Extreme Care Around Trees and Shrubs

The herbicide dicamba is potentially dangerous to trees and shrubs because it can move in the soil and be taken up by tree and shrub roots. Therefore, use extreme caution when applying any herbicide mixtures containing dicamba near root systems of trees, shrubs and other landscape plantings. Also, avoid application where mulches have been used around trees and shrubs. The tiny feeder roots will be actively growing into the mulch and can readily take up dicamba (and other herbicides) and/or be directly killed by the herbicide. Compacted soils, where there is extensive shallow root growth of trees as well as grass is another area where unintended injury from the use of dicamba (and other herbicides) can result to trees, shrubs, and other landscape plantings.

Where only a few scattered weeds exist, or where weeds are confined to small areas, hand removal or "spot" treating with an appropriate herbicide may be the most appropriate control measure. Application of a preemergent herbicide may only need to be done in those areas where crabgrass was found last year. These might include areas bordering sidewalks, driveways or curbs. Crabgrass may not be a problem in the remaining lawn area where it is cooler and the other lawn grasses are more competitive.

It is also important to remember that an occasional weed is not uncommon in lawns. Hand removal and tolerance of a few "weedy" plants, while maintaining an otherwise healthy lawn can significantly reduce weed control inputs.

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