At this point in the year, the nutgrass should be slowing down, even in Houston. As soon as cooler temps begin, that process takes place.  I say that because the chemical will move even slower in the plant which slows the effectiveness of it working.  Houston did have a much colder winter last year but I don’t know about this year.  You could go ahead and apply the Sedgehammer now and it will do some good or you can wait until spring and begin in earnest treating your lawn.
Remember, it is best to start spraying the nutgrass when it is shorter and you will have to re-apply a few times to get complete control.  It took me 2 years to eradicate it from my flower beds because there are nuts in the soil that will come up in intervals before the chemical has time to completely kill it.