Still nobody recognizes this?

It does not spread by seed. All the seed seems to be sterile. It clumps very slowly. About 5-6 feet tall. narrow blade of grass. White seed plume.

Best I can do is an educated guess, I'm thinking that it's probably Miscanthus sachariflorus. This will get up to 7ft tall but could well be less depending upon the ferlility of the site, the ability of the soil to hold moisture and the amount of moisture it gets. Banner grass,as this is called, does have a red tint to the leaves in fall and by late autumn the color is identical to M. purpurascens. The biggest clue though, is the size of the patch from whence it came. A 100 x 30ft stand of grass most likely means that some spreading was going on and this is what sachiflorus is famous for.

Thanks for your comments. I will do more research along this line.

The size of the patch would indeed indicate that it is Miscanthus sacchariflorus. Commonly known as Silver Banner Grass. This is one of the only Miscanthus species that is an aggressive rhizomatis spreader, especially on moist fertile soil.
However, based on the rest of your description and the photo, it could well be Miscanthus purpurescens. Height, width of leaf blade and fall colour would be right for M. purpurescens. So .... if it continues to be a slow clumper you can bet on the M. purpurescens
BUT
Aggressive = M. sacchariflorus
later,
Dale

I transplanted some of this grass a few years ago to my old house. It took at least 3 years before I got a reasonable looking clump of grass with a diameter of 1 meter after starting with 15 plants. I would think it was rather slow spreading.

The original plot of grass had been there for decades, hence it was considerably large, and dense.