You should not have cut it back so hard. It's a cool season grass, and should only be pruned by 2/3 of its height.

Division is mid to late spring should be ok, if they survive the excessive pruning. Wait until you see signs of new growth, which may not be uniform. Divide to actively growing sections.

Unless they are in containers, they need no fertilizer at all. If you gave them a heavy dose, directly on the plant, you may see some damage.

Thanks, donn. I see some grains of the fertilizer still stitting on the plants, among the stems. Shall I take out the plants and shake off these grains, and then keep them in a container until I see new shoots and divide them?

I wouldn't disturb them any more than necessary. Try to rinse off, or brush off as much of the ferts as possible, and then just keep an eye on them. Chances are, the severe pruning will do more damage than the ferts.

I don't know what zone you're in, but in my zone 7, I divide cool season grasses right into May, after they have a good bit of growth. Wait to see how they do in their initial growth. Then, if there are sections with no new growth, divide those sections out.

Fortunately, I see new shoots growing and also many of the cut stems have grown up. These stems have blunt tips, though, and I wonder whether their tips will stay like that throughout their growth or become nice and pointed like the new shoots.

My location is Oxford, UK, and I don't know what climate zone it is. How many parts do you divide a carex into? If you cut it in half, each half will be a half circle whereas the trunk should be nice and round (full circle) to look good. How do you get a round trunk for the divided stems?

The clipped shoots will remain squared off. The object of cutting a cool season grass early and not too far down is to avoid trimming off new growth.

A half-round division will become round in a couple of growing seasons, and will look fine anyway. Quarter-rounds will do the same.

FYI, Oxford is in zone 8, probably 8b. Your Carex probably started new growth a couple of months ago.