Nut grass is a sedge rather than a grass and is a huge problem here. Two years ago I declared war on nut grass. It crowds out the vegetables and the literature says that it even gives off a growth inhibitor to inhibit growth of it competition. If you pull one up, cut it off, burn it or freeze it, it just grows more leaves from an underground nut. I had already tried that. So I conducted a few tests to try to kill it.
There are three commercial sedge killers available for home use. They are Bonide Sedge Ender, Image Nutsedge Killer and Sledgehammer sold by several suppliers. The active ingredient is different in each. I bought a bottle of Bonide Sedge Ender.
I picked an area with lots of nut grass and in a spot where I would not plant anything for a couple of years. I isolated three little areas. 1. Treated with Sedge Ender. 2. Treated with Roundup. 3. Untreated control.
Results: Sedge Ender knocked them down in about three days. Roundup did the same in about five days. After 16 days new ones were coming up in the test area although the Roundup looked a little better. A second treatment knocked them down again but they eventually came back. FAILURE.
So I started digging them out with a shovel. By going deep I could get the nuts. I recently completed two years of digging nut grass. During that time I dug up 43,123 nut grasses. My little round garden is about 90 feet across so that is about 6.8 nut grasses per square foot. It worked. The nut grass is well under control. A few come up every week but I try to stay on top of them so they will not get out of hand again. My garden this year looks better than it has ever looked.