I'd go out at least 6" from every culm and down to about 12" deep. If you do that you should have enough of the roots that it will do OK. You can divide it if you need to, but that may slow it down and you risk cutting off new shoots that haven't emerged yet.

Thank you. It never did send up any new shoots last year, but the other one I have nearby did. Several canes have died too, so I wonder if it wouldn't shoot this year either with it's declining health. I've worked some leaf mold into the soil where it will go, so hopefully it will recover and start growing again next year.

Hardiness varies, some plants pretty tender. Have seen I believe a nearly 20 degrees F. range of variation indicated. One I planted near Seattle froze to the ground in 1990, before getting much size so I figured it wouldn't be any good and turfed the area over. Another, planted a few years ago on Camano Island grew rapidly to make an arching clump in a sheltered position - and get partly burnt this past winter. So, maybe your dead culms are frost injury.