Pre-emergent herbicides must be applied before weed seed germination occurs, and that can pose a challenge because there are varying environmental conditions even within a single lawn care route.
While application timing is based on an average soil temperature of 55 degrees (or some look to when forsythia bloom in the north, and dogwood in the south,) location gets even more specific than that.
Soil temperatures warm up faster on south-facing slopes. Bare spots in the turf also get warmer because they’re directly exposed to sunlight, whereas lush turf keeps soil cooler for longer.
The strips of lawn abutting sidewalks or driveways pull in heat from pavement surfaces, which ups the soil temperature compared to the rest of the yard.
“Most LCOs take a look at the calendar, but they also need to take look at the application site,” says David Loecke, herbicide product manager at PBI Gordon. “Take a look at the areas in which you need to make your application and make sure you’re timing accordingly.”
That means that properties on a route with south-facing lawns, for example, might get positioned first on the list for receiving a pre-emergent herbicide application. Or, one product might be used to treat properties earlier on in the route, while properties falling toward the end of the service cycle might get another product – perhaps one with a post-emergent mix to catch any weeds that have already started germinating.
If you’re planning on aerating properties in the spring, the pre-emergent application will still be effective. The same goes for seeding, Loecke points out.
“If you are planning on seeding, there are certain pre-emergent products out there you can use,” he says, noting that it’s critical to read the label because in a lot of instances, pre-emergent products can actually prevent the grass seed from germinating.