A weed is a plant that grows where it is not wanted. Weeds compete with turfgrass for sunlight, water, and nutrients. Proper lawn care and maintenance are important factors in preventing weeds from becoming established and taking over a lawn. Weeds tend to infest sparse, poorly established lawns because sunlight can penetrate the soil surface, promoting the germination of weed seeds.
There are two chemical approaches to weed control, based on application timing: pre-emergence and post-emergence. Pre-emergence herbicides prevent weeds from becoming established, while post-emergence herbicides control weeds after they have emerged and become established.
Pre-emergence herbicides are primarily used to control annual grasses, such as crabgrass, goosegrass, and annual bluegrass, as well as certain annual broadleaf weeds, including common chickweed and henbit. Effective use of pre-emergence herbicides requires a basic understanding of the target weed’s life cycle and temperature requirements for germination. For example, annual bluegrass germinates when soil temperatures are between 55 and 75°F.
There is a narrow window of opportunity for applying pre-emergence herbicides. If the application is not made within this window, the herbicide will have little or no effect. Pre-emergence herbicides must be applied before the target weeds emerge.
Most winter annual weeds begin germinating in early fall, with growth continuing through late spring or early summer. In St. Lucie County, the best time to apply pre-emergence herbicides for control of winter annual weeds is from mid-September through the first week of October. For effective control of spring annual weeds, pre-emergence herbicides should be applied to turfgrass from mid-February through the first week of March.
In addition, approximately ¼ inch of rainfall is needed within seven days of application for the pre-emergence herbicide to be effective. If adequate rainfall is not expected, the herbicide should be thoroughly watered into the soil using a sprinkler.
It is important for pesticide applicators to accurately identify the turfgrass species before applying an herbicide. Not all herbicides are labeled for use on all turfgrass species. For example, atrazine can be used as a pre-emergence herbicide in centipedegrass and St. Augustinegrass but may severely injure bermudagrass and bahiagrass.
Always read and follow the manufacturer’s instructions and all directions provided on the pesticide label. Pre-emergence herbicides are available at local hardware stores and commercial landscaping supply retailers.
For questions about turfgrass-related topics, contact Grantly Ricketts at 772-462-1660 or by email at gricketts@ufl.edu
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