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The P&PDL Picture of the Week
for 23 December 2002

Bare Spots in the Lawn Left Behind by Summer Annuals

Glenn Hardebeck, Turfgrass Research Agronomist, Agronomy Department, Purdue University

With the first few frosts of the fall, summer annuals such as crabgrass, knotweed, black medic, and spotted spurge came to the end of their life cycle. By now these weeds have long since desiccated and are crumbling away leaving behind bare spots in the lawn. Properly dealing with the bare spots is the first step to controlling next year’s weed problem.

Many times we look at lawn weed problems from the wrong prospective. Instead of seeing the weeds as the problem, we should be looking for the reasons the weeds are there to begin with. For example, low mowing heights generally reduce turfgrass vigor and therefore increase weed pressure. The weeds don’t necessarily choke out the lawn, but the weeds can take advantage of a weak, thin turfgrass stand. A thick, healthy turf is the best form of weed control. With this in mind, it is important to access a lawn in the fall and determine what to do in order to promote thick turf during the cool growing seasons of fall and spring. While fall fertility is the primary means to revitalize most cool season turfgrass stands, lawns with baseball to softball-sized holes should be overseeded to promote density.

The best method of overseeding is to utilize a power overseeder or power rake capable of cutting slices through the turf and into the soil to ensure good seed to soil contact. Small areas can be hand raked to loosen the soil. Early fall is the best time to seed followed by dormant seeding just prior to the onset of winter with a spring seeding being the last choice. It is also possible to drop seed onto bare soil during the winter allowing the freeze/thaw cycles to work the seed into the soil. Finally, it is important to remember that dormant seeded turf can’t be treated with crabgrass preventer next spring since it will also control the turf seedlings.
For more information check AY-10: Control of Crabgrass in Homelawns and AY-13: Lawn Improvement Programs.

Click on the small image to view a larger image.

Perennial Ryegrass mowed at two different heights July 8
Same area Nov 19 after crabgrass has desiccated and crumbled away

Photo courtesy of: Glenn Hardebeck, Purdue University

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Last updated: 23 December 2002/amd
The Plant & Pest Diagnostic Laboratory at Purdue University