I know I'm chiming in a little late on this thread, but I think I may finally have enough experience on a subject to pay back a little of what you folks have given me.
Up until just a couple of months ago I was treating phragmite like it was river cane. As it turns out, there is no river cane in New Braunfels, TX.; but phragmites grow all over the place. You have to be very selective in the field when harvesting this stuff or you're gonna have a bad time. If the cane break you are in is yielding one paper thin shaft after another do yourself a favor and move to the next one. The good (and bad) stuff seems to all grow in the same areas. As stated earlier, you'll want to harvest mature shafts. They'll have grey sheaths and, at least in my area, yellow stems that may show a little green at the nodes. Make sure it has green leaves at the top of the shoot, anything standing dead is useless. The best stuff will be in the center of the break along with the rattle snakes, cow spiders, and whatever other creeping nasty your imagination and South Central Texas can conjure up. Seriously, you gotta want it.
When making arrows, I've found the following tips to be most helpful:
1) harvest about 2 or 3 days after a good rain. I don't know why, but it seems like the walls are thicker and stronger.
2)dry slowly and naturally. No ovens, heat guns, or fires. Patience, Grasshopper. Takes about 3 months in A Texas autumn. That's like a summer in Hell. (I shucked mine for drying).
3)use a foreshaft. I use Bodark or Mesquite because thorns are awesome.
4)leave about 2" of culm and run the foreshaft all the way to the node. Wrap behind the support node and at the shaft/foreshaft intersection.
5)do not rush the straightening process. Use a heatgun, not a torch. Once it starts to shine it's ready to bend. Once it starts to darken it's ready to break.
6)read #5 again
7)you can drill out the culm to fit the foreshaft if you have to. Take a couple of wraps of masking tape around the section to be drilled. Make sure you get a good, tight wrap ;because wrinkles in the tape offend my sense of asthetics. (did I spell that right?)
8)use a nock plug. Again, try to pick a hardwood that's thorny and painful to handle.
9)if a shaft has an oversized bulge or a drastic bend close to the node, or if it just won't straighten or stay straight, get rid of it. It's just a stick, not a point of honor.
10)MOST IMPORTANT :IF YOU HAVE BEEN IN THE SHOP FOR 9 HOURS AND YOU'VE BROKEN 67 ARROWS BUT ONLY STRAIGHTENED 3 DO NOT KICK THE DOG. HE LOVES YOU AND IT'S NOT HIS FAULT. It's the cats fault. Just ask the dog.
Most kidding aside, phragmites make a really good arra, if treated properly. I wouldn't do allot of stumping with them, bit they will fly fast and true. Good for targets and game. Hope this helps.