When rhizomes are planted late they have no chance to get roots down deep enough to hold them. I use hair pin markers, so I just place the hair pin over the rhizome and push down to anchor them till spring when they can get enough roots to hold them. As long as the rhizome is firm (not mushy) they should be fine. Some folks use a weight like a rock or brick.

What a great idea - thanks! So, even if the top "half" is completely exposed to the air (i.e. not covered by any soil), do you recommend covering it in any way? (It is totally firm, not soft.)

The top half should always be exposed to the air on the east coast. I set a small rock on mine when newly planted.

The top of the rhizome should be exposed regardless of where you live, unless in a very summer hot climate.