Preheat oven to 400 degrees F.

Rhubarb Pie Filling: In a large bowl combine the sugar, tapioca flour, and zest. Add rhubarb and toss to coat.

Assemble: Add one rolled-out pie crust to pie plate. Sprinkle 1 tablespoon coarse sugar over the bottom of the pie crust, to prevent it from getting soggy. Add rhubarb pie filling. top with small pieces of butter dotted over the top of the rhubarb filling. Roll out second pie crust and place on top. Seal the bottom and top crust together. Cut a few slits or a small "X" on the top crust for steam to escape.

Bake at 400 degrees F for about 1 hour (tent the top of the pie loosely with aluminum foil part way through cooking to keep the crust from browning too much).

Cool: Remove from oven and allow to cool for at least 4 hours before cutting. After it cools, refrigerate for up to 4 days.