Happy, some of what i can remember about how we overwintered perennial grasses at some local nurseries.
We didn't over winter any in less than 3 gal pots, The varieties were all hardy here, not marginal. They were lined up and grouped on top of weedblock cloth over a 3" gravel bed and the unheated hoop house was covered in late Fall with white poly and pretty well sealed until March/April when doors could be open during warm days, or sides rolled up. They usually were not watered or fed until new growth appeared in mid spring and it was time to uncover them. My understanding as to why under cover/hoop house was to keep them dry and avoid root rot. Also, stored that way was a major deterrent to rodent damage and easier to set traps, etc. Before stowing them they were cutback and several varieties were unpotted, about a third of the bottom of the rootball was removed, new soil was added to bottom of pots and clumps reinserted.. Not sure of the rhyme or reason for this, but by mid spring this root removal was done on all grasses. They were fed with a time release fertilizer as new growth started.
One year we did overwinter gal pots of purple fountain grass but that was done in a cool green house but with not very satisfactory results, meaning, that instead of becoming stock plants that could be divided and sold, theses were spindly and generally not sellable as is. We did divide most of our other grasses in spring into gal pots and up potted to 3 gal in July/August pre flowering.
Sorry I can't be of more help. Keep em dry and safe from rodents.
We did Hosta, ferns, and many perennials in the same way in unheated hoop houses.
