I don't have a spine tester, but I can tell you that for the most part they are remarkably stiff for their weight.  There is a a lot of variance from place to place though.  When I was using a lot of this stuff, I would only harvest it in two places.  East St. Louis Il and salt lake city.  This was because the phragmites in both places was near identical in properties.  I have cut some in other places like Sioux city IA  a few places in Michigan and Wisconsin and the material was to spindly.  Since this stuff is essentially a tapered shaft, you will find that it is not that finicky about the weight of the bows shooting it.  I would guess this arrow would spine 55 - 60 lbs.  However, with a heavy foreshaft and point to bring the weight of the arrow up,  the spine would be around 40-45 lbs.  But again, the taper of the shaft makes the arrow quite forgiving in regards to spine.  Josh