When weeds attack: How to restore lawn grass after weeds take control

·3 min read

“I contracted with a landscape firm to prep and seed my front and back yards. The landscapers removed the degraded grass entirely and then prepped and seeded. The front yard is filled with weeds and very little grass. Perhaps the tilling brought up weed seeds because the grass barely had a chance.

I now have crabgrass, purslane, and something else all several inches high that are completely covering the front yard. The yard was seeded just with the soil that was there with no amendment. How can I get rid of the weeds and put down grass seed again with success?”

– Donna Simmons, Grayslake

You will need to seed again, as there is not enough grass to fill the lawn in.

There must have been a large seed bank of weed seeds in your front yard. The removal of existing grass and tilling created optimum conditions for the weeds to take over.

Since hand weeding is not a practical solution, use a broadleaf herbicide for lawns to spray the weeds. A broadleaf herbicide will not kill grassy weeds, so the turfgrass that did start to grow will not be affected. Crabgrass is an annual grassy weed that will not be killed by a broadleaf herbicide. It is too late in the season to use an herbicide that targets crabgrass, so hand pull the crabgrass that remains if that is feasible.

You can apply a preemergent herbicide next spring that prevents weed seeds from germinating to control the crabgrass. The best time to apply the preemergent herbicide is when the soil temperature reaches 55 to 60 degrees Fahrenheit. You can also use the flowering of forsythia to time your application.

Be sure to water in a granular application and do not core aerate or power rake after application, as this herbicide creates a barrier that prevents weeds from germinating. You will not be able to seed grass after applying a preemergent herbicide.

The middle of August to the middle of September is the best time to seed a lawn. In my experience, fewer weeds come in when seeding is done at this time. It is best to wait a couple of weeks after spraying the weeds before seeding. You will probably need to mow down the dead weeds to create a better seed bed for the grass seed. Set your mower at a low height.

You will get the best results if you use a slit seeder to seed the lawn. A slit seeder is a machine that cuts a slit in the soil and drops the grass seed into it. You can rent one or engage a contractor to do the work. Broadcasting grass seed on top of compacted soil will not work.

Another option is to use a power rake to loosen the surface of the soil and then broadcast and rake in the seed. This option may be more difficult in your front yard because of the plant debris. I like using a biodegradable seed starter called PennMulch to top dress the area after raking in the grass seed. Avoid using straw or peat moss.

There are many fewer weeds in the backyard with lots of open ground. Spot spray the weeds here before seeding. A nonselective herbicide that will kill all the weeds would be an option here. The power raking method to prepare the soil before seeding should work well here. If you decide to slit seed the front yard, I would do the same for the backyard. You will not need to top dress the grass seed if you slit seed the lawn.

Watering is very important for the first two to three weeks. You will need to keep the seeded areas moist with frequent light watering until the grass germinates in about two weeks. Plan on watering lightly twice a day during hot or windy weather. Once the grass is up and growing, you can water less frequently and more deeply.

For more plant advice, contact the Plant Information Service at the Chicago Botanic Garden at plantinfo@chicagobotanic.org. Tim Johnson is senior director of horticulture at the Chicago Botanic Garden.