Ive made rushes using bristles from a cheap paint brush . I just laid some clear plastic film on my workbench, dribbled a blob of hot-melt glue on it then cut a small tuft of bristles off the brush and pressed it into the blob of hot-melt glue . Once I had made a sheet of them up I just cut them off the plastic sheet and glued them around my layout.
For reeds you would just cut longer tufts and dye them green or dip them in paint for summer foilage