As winter gives way to spring, gardeners get that familiar itch to get outside and begin preparing for the coming growing season. With so many tasks to do, it is easy for well-intentioned gardeners to succumb to the marketing of fertilizer products that contain pre-emergent herbicides. Why not combine the two jobs of fertilizing the lawn and applying pre-emergent herbicide to control those pesky summer weeds in one fell swoop?! As is often said, ‘If something sounds too good to be true, it probably is.’ Such is the case for pre-emergent herbicide and fertilizer combination products.
The problem with using these products is timing. The time for applying pre-emergent herbicide does not match the schedule for applying fertilizers containing nitrogen for most of the turfgrasses used for lawns in South Carolina. When using pre-emergent herbicides for the lawn in spring, the air temperatures need to be 65 to 70 °F. Temperatures generally reach this range around March 1 for the coast and midlands, and March 15 to 30 in the Upstate and mountains. An easy way to know when the timing is correct to apply pre-emergent herbicides is to look for forsythia (Forsythia x intermedia) or redbud (Cercis canadensis) in bloom. Perform a second application nine weeks after the first application to provide season-long control.
There is an exception to pre-emergent herbicide and fertilizer combination products. Often pre-emergent herbicide is combined with 0-0-7 fertilizer. The essential difference between 0-0-7 fertilizer with pre-emergent herbicide and other products is there is no nitrogen fertilizer in the 0-0-7 formulation, only a small percentage of potassium fertilizer. While nitrogen fertilizer can be applied to tall fescue lawns in the Upstate and mountains as late as March 15, warm-season turfgrasses grown in South Carolina should not receive any nitrogen fertilizer until early to mid-May.