Crabgrass is usually found along the edges of the lawns. Most lawn edges abut some sort of hard surface- concrete, interlock, asphalt.
It is important to recognize now crabgrass grows and functions. It is an annual weed- it will die in the fall with a heavy frost. However, the seeds are scattered in the lawn and will grow next year. Removing the current crop of crabgrass plants will reduce the supply of crabgrass seeds that will be in the soil. This is labour intensive and may not be worthwhile.
The seeds germinate next year when soil temperatures are warm enough for them.
This means that crabgrass does not germinate until late June in most cases.
Crabgrass need warm soil to germinate. So it tends to grow where the soil is warmer. Places where the soil may be warmer include lawn areas that are not thick. A thin lawn may have areas of bare soil. Bare soil warms up easier than soil that is protected by a thick stand of lawn.
Crabgrass rarely grows in shady lawns for this reason. Crabgrass growing in the middle of a lawn is most often in lawns that have suffered insect damage the previous year. This causes the lawn to be thinner than normal and allows crabgrass to germinate and grow.
Lawns along the edges of hard surfaces will warm up more than other lawns. The asphalt and concrete absorbs the sunlight and gets warm. This warmth is transferred to the lawns along the edges and provides conditions that crabgrass will thrive in.
We do recommend that lawns be mowed at a height of at least 3 inches. This allows for a denser canopy of grass blades. The longer blades provide more shade on the ground than shorter blades. The more shade, the cooler the soil will be – and crabgrass will be less likely to grow.
The way the edges are trimmed can also be a factor in how well crabgrass grows. Edges along paved areas should NOT be trimmed lower with a string trimmer. This trimming will shorten the mowing height along the edge and reduce shade and increase soil temperatures. This can lead to an increased risk of crabgrass infestation.
Another factor that relates to mowing height is when the lawn is slightly raised above the pavement or walk. If the lawn for instance is 1 inch higher than the pavement – when the mower is run along the edge of the lawn, the lawn along the edge will be mown at a height lower than the 3” mowing height of the lawn that is farther away from the edge.
Lowering the lawns along the pavement edges, can help to ensure that lawn edges are not inadvertently mowed lower than is desirable. Doing this work should only have to be done once but will likely pay dividends in reducing crabgrass over the long run.
If the lawn along a pavement edge is not as dense as it can be, if it is thin or bare, then it is doubly susceptible to crabgrass infestation.
There are a number of things that can make lawn edges bare. These include
-being driven on by vehicular traffic
-burn or scorching from trash bags and recycling boxes
-salt damage from winter ice melting
-damage from snow clearing
To reduce bare areas- first reduce the cause that may cause damage and second repair the damage to increase lawn density and reduce crabgrass germination.
If salt is used regularly on the driveway or walkway- treatments can be used to reduce damage from salt. A granular applications of Turf King LOGI-CAL will help salt accumulations wash down the soil profile to below the root zone of the lawn. This can be applied in the spring after damage is noticed or applied as a preventative measure in the late fall.