If you wonder where the crabgrass in your lawn comes from each year the answer is that it germinates from seed every spring. Crabgrass is a summer annual plant which means it germinates from seed each spring, grows bigger as the weather warms up, produces seed in late summer and is killed by a hard frost in fall. The crabgrass seed that germinates in your lawn this spring was most likely produced late last summer. If you had crabgrass in your lawn last year, it is important that you take steps to control it this year.
The most effective way to control crabgrass is to prevent the crabgrass seed from becoming established in your lawn by applying a preemergent herbicide, commonly referred to as crabgrass preventer. A preemergent herbicide forms a herbicide barrier at the surface of the soil. As a crabgrass seed germinates, the emerging root contacts the herbicide barrier. The preemergent herbicide disrupts normal root growth and the crabgrass seedling soon dies without ever being noticed.
For most preemergent herbicides to be effective they must be applied prior to crabgrass seed germinating. The preemergent herbicide must be present in the soil as the crabgrass seed germinates so the new root will contact the herbicide. Crabgrass seed begins germinating as the soil temperature at a two-inch depth reaches 50° to 55° F for several consecutive days. The date when crabgrass seed germination begins varies depending on geographic location - with crabgrass seed germination beginning in southern locations in late winter or early spring and in northern locations in late spring. Some Crabgrass control formulations, like Green Mark's Formula Spring Fertilizer with Crabgrass preventers, also provide post-emergent control of crabgrass. It will control crabgrass seed as it germinates like other preemergent herbicides plus it will control small crabgrass plants that have germinated and have a leaf or two present at the time it is applied (typically up to four weeks after germination).
The advantage of the Green Mark's Formula Spring Fertilizer with Crabgrass Preventer formulation is that you have greater flexibility on the date when you make the product application. If you miss the date of crabgrass seed germination by up to four weeks, it will still control the tiny crabgrass plants that have already started to grow plus any crabgrass seed that has yet to germinate. The fertilizer component of the product will help the lawn green up, grow and most importantly, will encourage the existing turfgrass plants to fill in thin areas resulting in a thicker, denser stand of turf. The increased turf density will help the turf grass plants crowd out crabgrass.