Some of the green and white variegated varieties are hardy to zone 4. If I had a greenhouse, I'd leave some out, mulched, and put some in the greenhouse for protection.

Mine survives in Zone 6. I love the stuff too!

Ellen

I've found it tough as nails for me here in zone 3/4 with no protection.

I have two forms of variegated Hakone grass, and neither have white as a predominant color. It is usually a VERY slow grower and I'm surprised that you had to do so much dividing.

Kate

If it's Hakone grass, it should not be a problem to overwinter it in zone 5. I question that it is already knee high, as mine never gets knee high, and it is a very slow grower. Takes years before the clump is ready to divide.

It isn't ribbon grass, Phalaris arundinacea, is it? That grass seems to be taller, a more rapid grower, and a spreader. It is also hardy in zone 5.

Here is a link that might be useful: Ribbon Grass info

I think John is on to something with his suggestion that you are dealing with Phalaris and not Hakone. If it's Phalaris or "Ribbon Grass," do the best thing possible and plant it with an underground barrier all around it somewhere where you can see early or late light through its foliage. I just sheared my ribbon grass to the ground about 2 weeks ago and there is already 10 inches of fresh new foliage standing. What fun as long as you keep the stuff from spreading throughout the garden! I love that it can (and really should) be shorn mid-summer for a fresh push of fine foliage.