I used to spec collars to go around grasses like bamboo that worked out great and kept them from invading into places I did not want them. You can use concrete or even better PVC drain pipe, cut deep enough to keep the grass roots from running.
Now, you may have to dig the plants out and replant them in the burried collars or sleeves. I suppose you might be able to trim your plants and slip them over the top.
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