Crabgrass is a grassy weed that can easily take over lawns when control methods are not used. Pre-emergent herbicide prevents crabgrass seeds from germinating. Gardeners must use pre-emergent herbicide when temperatures favor crabgrass germination. Applying the herbicide before germination prevents crabgrass taking over the lawn in spring.
Crabgrass needs soil temperatures of 60 degrees to germinate, according to the University of Rhode Island Horticulture Program. Soil temperatures must remain around 60 degrees for a week. If soil temperatures dip or if there is a sudden frost, crabgrass germination will be delayed. As soon as the last threat of frost has passed, gardeners should check their soil temperatures. Do not wait until soil temperatures are optimal for crabgrass germination before applying pre-emergent herbicide. Apply it right before it is within crabgrass' optimal range or when soil temperatures hit 55 degrees, according to Danny Lipford. Crabgrass germinates at a higher rate in the spring than in the summer months.
Testing Soil Temperatures
Use a soil thermometer for testing soil temperatures. Pressing a screwdriver 6 inches in the ground creates a hole for the thermometer. Place the soil thermometer in the hole for the designated amount of time as indicated on the directions. Test in the morning, afternoon and late in the day for an overall reading of the soil temperatures.
Pre-emergent herbicide is a weed kill used within the yard to prevent weed seed germination. Because pre-emergent herbicide has no effect on existing weeds, it must be used before the weeds sprout. Use a pre-emergent herbicide that contains siduron prevents harm to your preferred grass seed, according to the University of Rhode Island Horticulture Program. Check the label of the pre-emergent herbicide before use. Certain types of pre-emergent herbicide prevent other types of grass seed from germinating for several months, according to Grounds-Mag.
How to Apply
Apply pre-emergent herbicide to the entire yard, even if you commonly get crabgrass weeds in only one location. As soon as you apply the herbicide, water the grass. Pre-emergent herbicide needs to be watered into the soil or it loses its effectiveness, according to Grounds-Mag. Pre-emergent herbicide generally creates a barrier from the surface to the first few inches of topsoil to prevent weed seeds from germinating.