The rush used for weaving these chair seats (Scirpus Lacustris, or Common Club Rush) grows wild along the shores of small lakes and ponds, as well as quiet streams and canals. We (it is - or used to be - a family affair) harvest it in the late summer, before it becomes spotted with some kind of rust fungus, which weakens the stem.
Harvesting involves wading chest deep in the water, and using a long sharp knife to cut the stems about a foot or more below the surface. Somehow this was usually my job. A couple of assistants (children) follow along in a rowboat, and gather up the cut stalks.
These are then spread out in a shady, breezy spot to dry, before being bundled and stored away in a cool, dry and well-ventilated spot until it is needed.
When the time comes to weave a seat, a bundle is opened up and spread out on the damp grass, sprinkled with water and then covered with a damp blanket - usually overnight. To weave the seat two stems of rush (each about six or seven feet long) are flattened between the thumb and fingers, and the brittle tips snapped off. They are held butt-to-tip to maintain an even thickness, and twisted as they are woven. Note that "real rush" can be distinguished from the twisted kraft paper substitute by looking under the seat: The true rush will be untwisted (a sensible saving of labour), whereas the paper rush will look the same above or below the seat.
The picture above is of Cayt McGuire, whom I taught to do rushing and chair-caning. She's holding a bundle of dried rush, and has just completed the new seats for two antique spindle-backs. R.I.P Cayt.