The best time to control lawn weeds is when they’re actively growing, unless you’re trying to prevent weeds from appearing. In that case, you must apply a pre-emergence weed killer before the weeds begin to grow. Post-emergent weed killers control weeds that are already visible in the lawn. Wear protective clothing, including long pants, a long-sleeved shirt, gloves and safety goggles, when applying weed killers.
Controlling Lawn Weeds with a Pre-Emergent Herbicide
Pre-emergent weed killers control annual lawn weeds while they sprout, which means the herbicide must be on the lawn before the seeds begin to germinate. Annual plants grow during one season, and then set seed and die. Summer annual weeds like crabgrass (Digitaria Haller) and goosegrass (Eleusine indica) germinate in the spring and winter. Annual weeds like common chickweed (Stellaria media) and henbit (Lamium amplexicaule) sprout in early fall. Consequently, to control summer annual weeds, apply a pre-emergent weed killer in early spring, and to control winter annual weeds, apply a pre-emergent herbicide in early fall. If you can already see annual weeds in your lawn, it’s too late to apply a pre-emergent herbicide.
The Best Time to Apply Post-Emergent Weed Killer to a Lawn
Post-emergent weed killers are most effective when the weeds are actively growing. Selective post-emergent herbicides target certain types of plants, like perennial broad leaf weeds like dandelion (Taraxacum officinale) and plantain (Plantago major) or annual weeds, but don’t harm turf grass. Perennial weeds return every year. The best seasons for applying a post-emergent weed killer to control perennial broad leaf weeds are early fall and spring. Apply the weed killer on a still day when the air temperature is between 60 and 80 degrees Fahrenheit, and no rain is forecast for 48 hours.
Don’t apply a post-emergent weed killer to a drought-stressed lawn or when air temperatures are higher than 90 degrees F, or the herbicide could damage the grass.
Applying Weed Killer to a New Lawn
New lawns are more sensitive to herbicides than established lawns. Don’t apply pre-emergent weed killer to a newly seeded lawn. When the grass has an established root system, you can safely apply a pre-emergent herbicide product that contains dinitroaniline. Additionally, in spring, you can apply a product that contains pendimethalin to a lawn that was seeded in fall, after the lawn has been mowed four times.
When applying a post-emergent herbicide to a new lawn, again, wait until the lawn has been mowed four times. If the lawn is newly sprigged, wait until the sprigs are rooted and growing. Apply the weed killer at half the rate recommended on the label, and reapply the product seven to 10 days later to treat weeds that may have survived the first application.
Granular and Liquid Weed Killers For Lawns
Weed killers are most effective when applied evenly over the entire lawn, unless the weed killer isn’t selective. Read the label carefully and follow the directions when applying weed killers. Non-selective weed killers are harmful to most plants, including turfgrass. Apply a non-selective weed killer as a spot treatment, spraying it on individual weeds or patches of weeds. Selective weed killers include granules, wettable powders and liquids. To achieve an even spread of weed killer granules, apply a granular weed killer with a rotary or drop spreader. Mix a weed killer powder with water according to the dilution rate displayed on the label, and spray the lawn evenly with the solution. Agitate the spray tank regularly to prevent the suspended particles from sinking to the bottom of the tank. Apply a liquid weed killer with a compressed air sprayer or hose end sprayer to achieve an even coverage.