In my humble opinion, the sprinklers do not sink. The sod establishes roots and is higher than when initially installed. You can see this by the driveway and curb. This happened up north without donuts and without heavy mowers on the lawn. Cut-off risers are available at both big box stores for less than 40 cents each. Dig around the head to the depth of the elbow where the sprinkler head is attached. Unscrew the head and insert the riser, cut to the desired length, and re-install the sprinkler body. Recommend removing the sprinkler nozzle to flush out any sand, and to clean the screen. Install the nozzle and adjust as necessary. I raised 60 heads this spring, and they are still at the appropriate level.
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