Quote from Entity
what about just using 1 piston on the 2nd reed level (itll cascade and take down the 3rd anyway).
or hm, I guess it messes up the drop in the water... well you could replace the pistons with sticky ones that have blocks attached, that should avoid breaking the pistons.
The latter is the solution. Replace the regular pistons with sticky pistons attached to blocks. Make sure you have a ceiling above the unit or you risk sugar cane popping up on top of the extended block and being dropped on to the soil the sugar is planted on. Also consider adding glass pane/iron bars/fence gates down the central water line to prevent reeds being thrown to the opposing soil blocks.