Go for 1-2 year old culms(shoots) b/t 3/8 to 1/2 diameter at the base.
Straighten them with heat after you cut them, then agian after drying them for a few months(about 6 months). an alternate method which is really easy(and I like) is to cut them 1" longer and stick them in ze oven for 1 hour at 250 degrees F. Never go higher than 275, as they can go WHOOOSSSHHHH and combust at a little over 300('boo combusts at 300 F)
then fletch up and you are done! easy peasy!
The tops of bigger culms(shoots) are generally too tapered.
If its hard on the outside and not too brittle, then you can make good arrows from it IMO.