I've never made arrows from Phrag but I have used horse weed, goldenrod, dog fennel and other "weeds" all with good results. It's not necessarily the materials thjat make good arrows but good arrow makers and that is a learned process.
Once you get the hang of it it get pretty simple. Most of the arrows I make are either sourwood shoots or cane and I vary rarely use my spine tester except to find the stiff side, which goes against the bow. Also, I never build arrow sets. I make each one individually. If one doesn't fly well I'll add fluflu fletching and use them for squirrels, etc.