You can substitute rhubarb for knotweed, but use the knotweed if you can find it – you’ll be richly rewarded.
Preheat oven to 375 degrees F. In a medium bowl, toss together strawberries, knotweed, sugar, and cornstarch. Prepare pastry for the bottom of the pie; arrange in pie plate. Pour filling into shell and dot with butter. Slice remaining pastry into ¾-inch strips and lay over pie in a lattice pattern. Brush crust with egg wash and sprinkle with sugar. Bake 45 to 55 minutes, until filling is soft and bubbling and crust is nicely browned (if necessary, cover pie edges with aluminum foil for the final 20 minutes to prevent over-browning). Serve warm with ice cream.