How do I remove nutsedge from a rock and flower garden?

Hi Anna, sorry you are having to deal with nutsedge. I hope this will help you. You can control nutsedge in your lawn by applying Ortho® Nutsedge Killer Ready-To-Spray. It's effective against newly emerged and established sedges. The weed is yellowed in 1-2 days, and complete killoccurs in 2- 3 weeks. It can be used on Northern and Southern turf grasses and is rainproof in 2 hours.

How To Control & Kill Nutsedge or Nutgrass - Ortho

https://www.ortho.com/en-us/library/your-lawn/how-control-kill-nutsedge-or-nutgrass

Did you use the right type of Roundup suggested for this kind of weed?

https://www.canr.msu.edu/news/understanding_roundup_products

Try this method: https://www.craftylittlegnome.com/permanently-kill-grass-weeds/

SedgeHammer. Powder you mix. There are some premixed types as well.