Hi, 

Nutsedge in a garden can be a huge burden. Unfortunately there isn't an herbicide that you can use safely around ornamentals. If you wanted to very carefully apply or paint on an herbicide that is directed for sedge, so it only targets the foliage, you could try that. You can use a piece of carboard to shield the other plants or use a foam brush or old rag to dab on the liquid directly onto the sedge blades You may have to apply multiple times. There are some general herbicides listed on the link. 

If you are concerned or opposed to using chemicals, you can try to hand dig it out. Inevitably it will come back but if you just keep at it, eventually the roots will run out of food storage and it will die. 

Let us know if you have further questions. 

Emily