A weed is defined as a plant
that is not valued where it is growing, either because it unsightly or it damages desirable plants or both. The Florida Department of Agriculture has stated that herbicide application is the most effective and only practical way to control weeds. Manual weed control (pulling weeds by hand) is much less effective and is highly inefficient. Many weed varieties will regenerate from seeds or fragments of the plants that were not removed with the target plant.
Furthermore, the countless man hours required to sustain any semblance of manual weed control are economically unfeasible. Herbicides on the other hand effectively eliminate the weed population and prevent new weeds from germinating for months when properly applied. Furthermore, the labor needed to apply herbicides is but a fraction of what is required to pull the same weeds by hand. Some herbicides are “selective” because they can be sprayed over a mixture of targeted (weeds) and non-targeted (desirable) plants without killing the non targeted varieties.
Weeds that grow in pavement,
plant beds, or utility beds are relatively easy to access for chemical elimination because they are not growing anywhere near non targeted plants or turf. Plant and utility bed weeds can be sprayed with non selective (kills all plant varieties) herbicides if the applicator is careful. However, weed varieties growing in the lawn are impossible to target without also targeting the lawn itself. These weeds are so woven into the grass that they must be treated with selective herbicides and are therefore somewhat more difficult to control.
Unlike St. Augustine varieties, which produce 99.9% sterile seed and must be farm grown as sod, invasive grassy and broadleaf lawn weeds are prolific because the seed they produce germinates readily and survives at a high rate. Most are deep rooted, drought resistant varieties that thrive under the very same conditions that will decimate a St. Augustine lawn. Their seeds are usually spread by wind and animals, although they can be inadvertently transported by shoes or wheels. Most varieties of both grassy and broadleaf lawn weeds will also spread and root aggressively along the ground. They are most active during the cool season, while the lawn is dormant.
Most broadleaf varieties will be crowded out by a healthy St. Augustine lawn during the summer, only to reappear in the fall. Carpet grass, southern crabgrass, and even some Bermuda grass will invade and spread steadily even in healthy St. Augustine grass. When the lawn becomes less healthy, these invasive grasses will establish and spread much more rapidly, permanently replacing the St. Augustine lawn in large areas. Most varieties of these grasses will brown out more quickly and deeply than the St. Augustine grass even as the adjacent St. Augustine grass remains green. Although some selective herbicides for St. Augustine lawn will damage the invasive grasses, the only effective way to eliminate them is to apply a non selective herbicide to them and then re-sod the area.
Broadleaf lawn weeds are not in the grass family with St. Augustine and will therefore succumb more readily to chemicals that don’t kill the lawn. Perhaps the most popular and least effective control for leafy lawn weeds is the “weed and feed” mixture of fertilizer and granular selective herbicide. As a practical concession, this mixture combines a low grade fertilizer with a single narrow spectrum (only kills a few weed varieties) herbicide in order to keep the price somewhat affordable. Unfortunately, price and ease of application are a trade off because neither the deficient fertilizer formulation nor the narrow spectrum herbicide works particularly well. The herbicide simply does not eliminate or even affect dozens of the most damaging broadleaf weed varieties here in southeast Florida and Palm Beach County.
Obviously, the most effective way
to restrict the proliferation of broadleaf weeds in the lawn is to assure all the conditions required to keep the lawn healthy and strong. This begins with reliable irrigation and regular applications of high quality fertilizer. When leafy weeds are present and spreading, a wide spectrum (kills nearly all weed varieties) blend of herbicides can be sprayed on the areas of lawn most threatened by damage in order to eliminate as many weed varieties as possible. Less resistant weed varieties will begin to yellow and die out within a week or two.
However, stronger varieties can take from four to six weeks to manifest duress and die out, allowing the St. Augustine lawn to mend. Finally, although selective herbicides help the lawn by controlling harmful weeds, it must be noted that they are not good for the turf by and of themselves. Too many treatments will adversely affect the lawn’s health and, ironically, could result in an even worse weed infestation. Applications should be limited to one or two during the cool season. Keep in mind that there is no way to eradicate every single lawn weed. If a leafy weed infestation is sparse, the lawn will usually take care of it by itself once warmer weather begins.
Thomas J. O'Hara, President
O'Hara Landscape & Maintenance, Inc.