Greenhouse (cold frame) side walls - footing question
I went though the same issue. Better to use 1 7/8  galvanized pipe  4 feet long set in concrete (3 80 lb bags each hole) as a sleeve. Then 1 5/8 poles slide into the sleeve and are anchored with tek screws. Make sure your sleeve is 8 inches above ground of course so that you have something to fasten too. If you are useing 1 3/8 then you can find emt that will work as a sleeve as well. I have experienced 50mph winds with no problem on a 14x28 greenhouse 12 feet at the center in height.

As to sono tube... the fasten point at the saddle will be the weakest point of the structure as the horizontal wind load will be significant under high wind gusts. It will hold but I felt better with the set up above as the sleeve encased concrete and tek fastened would require the wall pole inserted into it to fail. At that point the structure would be toast regardless.