Preheat your oven to 375°F.

Place one pie crust into the bottom of your pie plate.

In a large mixing bowl, gently combine the mulberries, rhubarb, sugar, and flour.

Pour this mixture over the bottom pie crust.

Cut the second pie crust into strips to create a lattice top, then crimp the edges.

Cut the butter into small pieces and sprinkle it over the filling.

Brush the top crust with milk and sprinkle with coarse sugar.

Cover the edges of the pie with aluminum foil to prevent over-browning.

Bake for about 40 minutes, until the top is golden brown and the filling is bubbly.

Once done, remove the pie from the oven and let it cool on a wire rack.