Cooperative Extension Service 
Institute of Food and Agricultural Sciences
Hendry County Extension, P.O. Box 68, LaBelle, Florida 33975-0068   Phone (863) 674-4092

Hendry County Horticulture News

Herbicides for Home Lawns

Herbicides can provide a safe and effective method of weed control in home lawns if homeowners have a basic understanding of the weeds that they are trying to control and the limitations of the herbicides being used.  It is essential that  product label instructions are followed. Label instructions include proper timing of application, proper rates, and dispersal methods. The timing of herbicide application during the weeds growth cycle also is important. For example, weeds not controlled prior to seed head formation are harder to control and are able to deposit new seeds for future problems.

Herbicides may be classified by their spectrum of effect and mode of action.  Most of the commonly available herbicides may be either selective or non selective in nature and are either contact or systemic in their mode of action.

A selective herbicide controls certain plant species without seriously affecting the growth of other plant species. Most herbicides are selective herbicides.  Non-selective herbicides control green plants regardless of species. These are generally used to kill all plants, such as in the establishment of new turf or as spot treatment along sidewalks, and beds.  Roundup is an example of a non selective herbicide.

Contact herbicides affect only the portion of green plant tissue that is contacted by the herbicide spray. These herbicides are not translocated or moved in the vascular system of plants. Therefore, these will not kill underground plant parts, such as rhizomes or tubers. Often several applications of contact herbicides are needed to kill the regrowth from these underground plant parts. Examples of contact herbicides include Basagran, Finale and Diquat.

Systemic herbicides are translocated in the plant's vascular system. The vascular system transports the nutrients and water necessary for normal growth and development. Systemic herbicides kill plants over a period of days. Examples of systemic herbicides include Roundup, 2,4D,  Banvel, Image), and Vantage.

Two categories of herbicides with regard to timing of application are also important to understand.

Pre-emergence herbicides are applied prior to weed seed germination. Knowledge of weed life cycles is important, especially when herbicide application timing for pre-emergence control is attempted. If chemical application is after weed emergence, pre-emergence herbicides are generally ineffective. This narrow window of application timing is a disadvantage for many lawn care companies and homeowners, who often wait too late in the spring to correctly apply the pre-emergence herbicide. A general rule of thumb for spring pre-emergence herbicide application is February 1- 15 in our area.

If goosegrass is the primary weed species expected, wait three to four weeks later than these suggested application dates, since goosegrass germinates later than most summer annual grasses. For pre-emergence control of winter annual weeds, apply herbicide when nighttime temperatures drop to 55° to 60°F for several consecutive days, late Oct. to early November in our area.

Aeration and de-thatching should be done before a pre-emergence herbicide application. Pre-emergence herbicides form a uniform soil barrier. Disturbing the treated soil may disrupt the herbicide barrier and bring a fresh supply of weed seeds to the surface, reducing herbicide effectiveness. Adequate soil moisture before and after application is necessary to activate most pre-emergence herbicides. Pre-emergence herbicides are generally effective in controlling weeds from 6 to 12 weeks after application. For season long control, an additional application should follow 9 weeks from the initial one.

Post-emergence chemicals are active on emerged weeds. Normally, the younger the weed seedling, the easier it will be controlled. Post-emergence herbicide effectiveness is reduced when the weed is under drought stress, has begun to produce seeds, or is mowed before the chemical has time to work (several days after application).

Many herbicides are formulated with a fertilizer as the carrier. These mixtures or so-called "weed-n-feed" products enable a one pass application of both products. When using these products, it is important to determine if the manufacturer's recommended rate of the product supplies the amount of fertilizer needed by the turfgrass and the amount of herbicide that is required for weed control. Supplemental applications of fertilizer or herbicide  may be required if the fertilizer/herbicide product does not supply enough fertilizer or herbicide to meet the fertility needs of the turfgrass or the amount of herbicide needed for weed control.

Ii is important to note that certain herbicides are intended for use with specific types of grasses.  Atrazine - a popular pre-emergence herbicide for use on St. Augustine lawns will cause injury to bahiagrass.  Be sure to read the label carefully.  For more information on this topic please contact the Hendry County Extension Office.

Turfgrass fertilizer/herbicide products should be used with caution near ornamentals. Products that contain dicamba, metsulfuron or atrazine can be absorbed by the roots of ornamentals and cause severe injury. Do not apply products that contain these over the root zone of ornamental trees and shrubs.

A basic knowledge of herbicide action should help you achieve more effective results in controlling turf weeds.  Good luck and good gardening.

Gene McAvoy is the horticulture agent with the Hendry County Extension Service.  Direct your horticulture questions to PO Box 68, LaBelle, FL 33975,  e-mail - or phone 863-674-4092 or  863-983-1598.  You are also welcome to visit the Hendry County Extension Office at 225 Pratt Blvd., LaBelle.   Office hours are from 8:00 - 5:00.

The Institute of Food and Agricultural Sciences is an Equal Employment Opportunity - Affirmative Action Employer authorized to provide research, educational information and other services only to individuals and institutions that function without regard to race, color, sex, age, handicap or national origin.

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