[zcaveman]

If you go around and give each donut a healthy shot of Round Up once a month on a day after the sprinklers run, it will keep down the growth around the sprinkler heads. At least once a year remove the donut and clean out the area around the sprinkler head and reseat the donuts. After a period of time they start to sink into the ground and make the sprinkler heads more vulnerable to damage from the lawn mowing machinery.