Wiregrass is another name for Bermudagrass (Cynodon dactylon). Because it is a perennial, it will return each year. As you've discovered, it's persistent. It can spread rapidly as its rhizomes creep along underground. Because digging has been ineffective for you, you're left with chemical control. Glyphosate (RoundUp) works, but because it kills whatever it comes in contact with, it may be risky to use in a bed where ornamentals are growing. Obviously, you shouldn't use it if there's any risk of spraying your desirable plants. You could apply Roundup with a paintbrush so you touch only the grass and not your flowers. Be patient; it might be several weeks before Roundup takes effect. A second or third application often is needed to kill wiregrass. Give the area where you are going to apply it a good watering a couple of days before you apply the herbicide. This will help actively growing weeds to absorb the herbicide quickly.

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 Another option is the herbicide fluazifop, which is sold in garden centers as "selective grass killer" or "grassy weed control." It selectively kills grasses but leaves most ornamentals alone. You'll see a long list of plants that are and aren't harmed by it on the product's label, so be sure to read it carefully and follow all directions. It's available in ready-to-use spray bottles.