The dried and dead culms of Miscanthus Cosmopolitan are heavier than many other miscanthus. They really neeed to be raked out and cleaned when cut back to be at their best and prolong having to divide.

Sounds like they have the telltale donut shape that says time to divide me.

Zone 5 is also borderline for many varieties of miscanthus. Cosmo is actually a zone 6 miscanthus, which can survive lighter winters in zone 5. It may be possible they suffered some winter loss.

Grass guy, thank you for responding!

I think you're right on all three counts. I've noticed weeds
seeding right into the centers.

Divide now? They are so sparse there are less than a dozen culms
around the perimeters.

The older one, on a hill, has actually done better, even though
it got chewed out by voles two seasons ago. I think the drainage
is better there, which may be the reason, though it's much more
exposed to winter winds.

It didn't seem like a really cold winter, but I lost three out of
four of my Caryopteris, so maybe it was cold/wet in the wrong sequences?

Thanks again for your expert advice!

A lot of folks had trouble with the warming, cold, warming, cold. Hard winter/deep sleep is ok, but to constantly get fooled...not so good.

I'd divide the sections out that show signs of energy, where some new growth is emerging. Give those sections a chance to establish into clumps on there own. I'd divide now before we hit late June and it gets too late.

Hi Grass guy,
The culms are already almost two feet. The centers are full of weeds
that I can't pull out, just keep down.
The oldest one has an eighteen inch dead center and vigorous new growth around it.
Should I perhaps wait til fall and dig them all up then and divide?
Or is it really safe to do it now? They are used as anchor plants
in mixed borders (love the look!)

Either way, how do I best take care of the new divisions to avoid this happening again?
Both areas are reasonably well drained, other perennials and bulbs
that don't like wet feet are doing fine.
Are these Miscanthus particularly heavy feeders?
Any crucial item on the menu?
Anxious to do right by them, thanks for helping me out here!

I still think you can get away with dividing them now. Just don't take them down too hard. Leave large clumps.

Dividing perennials is a fact of gardening, all you can do is divide or relace as they get older. You can prolong a perennial's longevity with good care. Cut the grasses back in early spring when new basal growth appears and really rake through them well to clean all debris. It will keep the centers from dying out so fast.

Established Miscanthus don't really need supplemenetal feed.

I was about to do it, Grass guy, and then the heat wave came.
I'll wait til it cools off and see how they're doing.
Maybe it's not too late.