What are Herbicides?

Herbicides are useful, effective tools in the control of weeds in turf areas. Herbicides are not a miracle cure, as they provide only short-term relief from weeds. The best approach is an integrated regime of proper mowing, fertilization, irrigation, and the use of other cultural practices to maintain a vigorous turf.

Selective: A selective herbicide controls certain weed species without seriously affecting the growth of other plant species. The majority of herbicides used for weed control are selective herbicides.

Nonselective: Nonselective herbicides control green plants regardless of species or weed type. These are generally used to kill all plants, such as in the renovation or establishment of a new turf area, for spot treatment, or as a trimming material along sidewalks, landscape beds, and wooded areas, etc.

Contact: Contact herbicides affect only the portion of green plant tissue that is contacted by the herbicide spray. These herbicides do not translocate or move in the vascular system of plants. Therefore, these will not kill underground plant parts. Contact herbicides often require repeat applications to kill any re-growth from these underground plant parts.

Systemic: Systemic herbicides are translocated within the plant's vascular system, which transports the nutrients and water necessary for normal growth and development. Systemic herbicides generally are slower acting, killing plants over a period of days.

Pre-emergence Herbicides

Pre-emergence herbicides affect seeds while they are germinating. To be effective, the herbicide should be applied two to three weeks before weed seeds begin to germinate. Consequently, pre-emergence herbicides are most effective against annual weeds, rather than biennial or perennial weeds.

Post-emergence Herbicides

Post-emergence herbicides are used to kill weeds after the weed plants have emerged from the ground. To be effective, most post-emergence herbicides must be absorbed through the leaves; consequently, liquid sprays generally work better than dry, granular materials. Post-emergence herbicides are most effectively applied when weeds are young and growing vigorously.

Selective post-emergence herbicides are usually used to control annual, biennial, and perennial broad-leaved weeds because they will kill many broadleaf plants without damaging the grass plants. These herbicides can also severely damage or kill trees, shrubs, and flowers; thus, they should be used with great care near these plants.

Post-emergence herbicides may be applied any time the weeds are actively growing, the air temperature is 60–80 degrees F, there are no winds, and there is no rain in the forecast for 48 hours. The most effective control of perennial broadleaf weeds is obtained when applied in early fall (August 15–October 15) or in spring (May 1–June 1). For some weeds, repeated application at 20 to 30 day intervals may be required for control.

Nonselective post-emergence herbicides kill all plants, both desirable and undesirable. These herbicides can be used to spot treat perennial grassy weeds that would not be affected by selective herbicides. To spot treat an area, thoroughly wet the weed foliage with the herbicide solution.