My big Sunflowers grow even bigger stalks.
They have a pithy center that is fairly easy to remove. The stalks are porous. The big leaves attach at alternating points and sometimes leave little holes at their points of attachment that must be filled, too. I use a combination of Shellac and thin cyanoacrylate (CA) mixed with sawdust. Then I use a wirebrush to scrub the interior befor I pour the “superglue” down the bore. This reveals any remaining open cracks, because it does flow out, the exit points will then can be filled with Shellac. Superglue hardens very quickly, so it is not long before I can blow a little air through it, and stop the end to determine it is fully sealed. If the tube is not completely sealed to begin with, it will not resonate through the entire length. Probably you will not get a note out of it until then.
Next, the first notch is cut on the end and tested, blown again until the correct angle and fipple area is near perfect. I then treat the notch again with thin CA and let the pores absorb it and harden. This is exatly what is currently being done to “stabilize” some exotic wood burls, too.
The tuning comes later, and I will refer you to a previou post about tuning Shakuhachi.
