Gardening: Are the weeds invading your turf?


Cooler weather means that it will be safe to have your lawn treated with a herbicide to try and bring those pesky weeds under control. 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.

I watched as several spray companies did serious damage to St. Augustine lawns trying to kill weeds this summer. The result was a thicker bed of weeds as the grass was unable to compete.

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 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. 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.

Pre-emergence herbicides provide several weeks of residual control in the soil and will kill the seedling weeds as they emerge. 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.

Atrizine 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.

Trimic 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.

Eileen Ward and her husband Peter have owned and operated Greensward of Marco, Inc., a lawn maintenance and landscaping company since 1981. Watch Eileen’s gardening videos on Eileen is an FNGLA Certified Horticultural Professional, has a Commercial Pesticide license in Natural Areas Management and is a registered Dealer in Agricultural products in Florida.

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