Weeds in the Yard

Pre-Emergent Herbicide

Pre-emergent herbicide is applied before weed seeds have a chance to germinate. Timing is critical. For example, crabgrass germinates when soil temperatures reach 60 degrees F, according to the University of Rhode Island. Gardeners must apply their pre-emergent herbicide before soil temperatures favor weed seed germination for the herbicide to be effective. Apply the herbicide evenly over the yard area and water. If you don't water after applying the pre-emergent herbicide, the chemicals will not form a protective barrier over the surface of the soil.

Post-emergent spray is applied after weeds start growing. There are a variety of post-emergent sprays available to treat sedges, grassy weeds and broad leaf weeds. If you use a selective herbicide, it's important to choose one that won't harm your own grass. Check the label to see if it can be sprayed on your grass type. Non-selective sprays kills any vegetation on contact. These sprays can be applied on a large areas of weeds or in locations that do not contain your turf, such as driveways.

While gardeners can spray their lawns with a host of herbicides, weed problems will only continue if underlining problems -- such as a thick thatch layer -- still exist. A thatch layer over 1/2 an inch promotes weed growth. Thick thatch prevents fertilizer from being absorbed into the soil, reduces air circulation and creates a moist environment for weed growth. To find out your thatch layer, cut a 2-inch wedge of turf out of your yard. Measure the brown spongy material between the soil and grass. Use a dethatcher or rake to remove a thick thatch layer.

Improper mowing encourages weed growth. Low mowing or mowing the grass lower than its recommended height puts stress on grass root systems. This stress can cause thinning grass or bare areas where weed seeds can take advantage. Gardeners can prevent causing grass stress by allowing their grass to grow to at least 3 inches in height, as recommended by Cornell University. Also, never take off more than 1/3 of the grass blade when mowing.