esshup,
The stems are round like a knitting needle. There's no sections and can't be pulled apart. The second photo shows how they have lined the bank, which is quite steep and slippery. I may get to them once the pond water goes down in July like it has in past years. I'll leave a few for the pumpkin seeds.
Most likey are rush, as you suggested. Does it have another name other than rush?
