Just wanted to point out the name is Phragmites and I have thought about modeling them too, as they seem to inhabit many drainage ditches, sumps, tidal eras etc in the North East US. I always associate them with purple flowers/leaves at the top (I guess not all species have this), so what if you took some model reeds, and flocked/sifted/glued some purple ground foam (or model grass or whatever) just to the top of the reed?
OK, to be helpful, I checked my old article folders, so lets go back to the future:
RMC, July 1986 - Making Mud - Jim Wild/Dwayne Easterling
Basic concept - hydrocal plaster, then add dents, footprints, etc, then shifted dirt, then Min-Wax Jacobean Stain, and cover w/ Acrylic Resin to thin coat the muddy area (the resin will also gather in any dips, representing puddles - add raw umber to get muddy water).
RMC July 1988 - Modeling A Marsh - Jim Wild/Dwayne Easterling (again - hmmm)
Various concepts, but their recommendation for reeds uses Scrub brushes with NATURAL bristles (good luck w/ that at this late date) and Wisk brooms - basically chopped up to size, and planted in the (future) marshland - do fluff up the bristles a bit.
RMC Jan 1998 - Creating A Swamp
Still no Phragmites, but discusses Bulrushes using raffia (a palm leaf) and some 1" lengths of 22gauge bare wire painted either flat green or flat tan, depending on season modeled (some insulation pieces are retained, placed back on the wire, and painted brown to represent the cattails). ‘Swamp grasses’ clumps are simply bristles from a bottle brush (3/4" length), bundled up 20 or so, tied at the bottom w/ thread, and painted medium & dark green.
MR Mar 2001 - Lily Pads, Cattails, and Pond Scum.
The reeds are bundles of Woodland Scenics green/dark green/gold tall grasses. Catttails are .010 plastic rod (for HO cattails) 1" long, painted coach green - then a blob of brown nail polish coated on