Nut Sedge is a sedge type of
plant. It is close to the grass family, but it is actually a little tiny bulblet. Unfortunately that means that most over the top of lawn
weed killers will not work on it. There are some selective herbicides out there that do target sedges. "Manage" is one of them. The problem is that they tend to be very expensive, and it would probably be cheaper to hire onto a lawn care company to come out and spray them, then it would be to purchase the chemical.
Unless the ground is totally soaking wet and soggy, DO NOT try to pull these
weeds up from the roots. They will reproduce from root stock so if you tear the roots, where there was one
plant this year, next year there will be many!
Round Up will work on this
weed, as well as many other non selective herbicides, but it will also kill the good grass
growing under it, so be careful. Always use the recommended amounts on the label. Just don't "dump" it on them as this can possibly give you many more headaches then the original problem.