Bamboo fly rods are made from bamboo native only to the Tonkin region of China. Bamboo is by definition a grass - hence sweetgrass.
An eight foot rod is composed of two 48” sections, a butt and a tip. Each section is a set of six triangular strips glued together. The completed rod narrows from a .289” diameter butt to a .0625” tip. Think size of a pencil at the butt end and thickness of the pencil lead at the tip.
Standing bamboo is cut into six foot “culms” then split into full length strips about 1/4” wide. Split the culm with a knife or chisel. Split first in half then in half again, etc. I usually get 16 strips out of one culm unless a couple strips don't behave.
The four strips in the photo below are: (1.)The 1/4" square strip freshly split from the culm in the photo above. (2.) A square strip run through the power planer changing the profile to a triangular shape. This step also reduces hand planing time drastically. (3.) and (4.) are finished and hand planed butt and tip strips. The reduction in strip size is testiment to the considerable time it takes to plane strips.
Culms are graded into butts and tips. The thicker culm has more fibers for use in the heavier butt section and vice-versa.