Gardening: Are the weeds invading your turf?

Eileen Ward
Weeds can get a foothold in your lawn in areas where the grass is thin or weak.

The cooler weather means that it will be safe to have your lawn treated with an herbicide to try and bring those weeds under control.

Our winter season has arrived with cooler temperatures and lower humidity. We could, however, have a return of the hot summer temperatures and humidity so watch the temperatures.

Treating your lawn for weeds with temperatures in the 90s can stress and damage your turf. There are herbicides labeled for use in hot temperatures but my observations show stress to the grass with these products as well. It is best to wait until it is in the mid to low 80s before having your lawn treated for weeds.

Weeds can get a foothold in your lawn in areas where the grass is thin or weak. Weeds do not overtake healthy turf. St. Augustine is a very aggressive and competitive grass allowing it to compete with weeds. If allowed to, St. Augustine grass could overtake and kill most shrubs in your yard.

Most weeds appear because of improper cultural practices such as mowing too closely or infrequently, improper fertilization or irrigation. These are the primary causes for thin or weak turf. In addition, inadequate control of insects, diseases, and nematodes will also damage the turf and allow weeds to invade.

Herbicides may temporarily remove the weeds, however, unless the basic cause of the weakened turf is corrected, weeds will continue to be a major problem. Weed control will only be successful if the turf is returned to a good healthy growing condition. The thick blanket of grass will not allow weeds to germinate at the soil line.

Pre-emergence herbicides provide several weeks of residual control in the soil and will kill the seedling weeds as they emerge. Pre-emergence herbicides should be applied before germination of the weed seeds. The timing of applications of this type of herbicide must coincide with the various germination times of weed species. If applied too late in the growing season the weeds will have germinated and escaped control. Applying the pre-emergence herbicide too early may also result in undesirable control since the control is time limited. Adequate soil moisture is important before and after application so timing for rainfall or irrigation is necessary. Most pre-emergence herbicides will control germinating weed seeds over a six or twelve-week period. The best time for application in South Florida is the first of February.

Once weeds have germinated and are actively growing, they must be controlled with post-emergence herbicides. Weeds are easier to control when in the seedling stage or large and actively growing. Applications made to weeds under stress may reduce the effectiveness. It sounds strange but you want your weeds to be healthy and vigorously growing when you kill them. Post-emergence herbicides are absorbed through the foliage, roots or both and then trans-located throughout the plant. If conditions are dry, irrigate prior to application to ensure active growth and translocation. Fertilization prior to application is also a good idea. Do not water or mow for several days following herbicide application to give the chemicals time to absorb into the weeds.

Atrazine can be used in St Augustine grass to control most broadleaf and annual grass weeds as a pre-emergent if applied before the weed seeds germinate or a post emergent after germination. Apply every six months (spring and fall) for continued effective pre-emergence weed control. Crabgrass control will require a special herbicide. They have taken the best product off the market but there are some replacements appearing. These are not as effective and so patience and a healthy turf will be necessary when trying to kill crab grass. Do not over water as this can encourage crab grass. Do not apply herbicide during extreme hot or cold temperatures to avoid damage to the grass.

Trismic can be used in Bahia grass to control most broadleaf weeds. Use only herbicides labeled for your type of grass as severe injury or death will usually result if the wrong type of herbicide is applied. Herbicides in general should not be applied to freshly mowed turf or to turf under stress.

While herbicides can help with the weed control in your lawn the best way to a weed free lawn is to practice good horticultural practices like proper watering, mowing, insect and disease control to prevent weak areas which will be susceptible to weed invasion. Using biological organic amendments is a great way to kick start the soil ecosystem in your yard to help make your lawn lush and green and discourage weed invasion in the first place.

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Peter and Eileen Ward have sold Greensward of Marco after 40 years in the lawn and landscape business on Marco Island. You can reach Eileen with comments or questions on her columns via email at or call 239-269-0192.