My observations so far are that, donuts being very inexpensive are a very good investment to add to your irrigation heads that are flush with the grass. The reason being that grass by nature is very invasive and has the strength to actually keep a sprinkler head from popping up, rendering it useless. They also protect the plastic head itself from lawnmower blades decapitating them. I suspect that the lawnmower guy knowing you have concrete donuts will keep his blades a tad above the donuts. Even with donuts, it sometimes is easy to forget or even locate your sprinkler heads. This is not to say one needs to remove the grass within the donuts from time to time. I occasionally turn on the system manually and do a check to make sure the coverage is complete. It is cheap insurance to do a walk around occasionally than to lose your landscaping to drought. Don't hesitate to make a drawing of where your heads are located for someone else in the future.
While I'm on the subject of irrigation, I had a "fertigator" system installed this summer and I'll keep you informed of the results. Its too soon to see the results just yet.
Handie :joke:
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Upstate NY and The Villages-If your not living on the edge, your taking up way too much room."