Combine ingredients and let stand about 15 minutes.
Line a 9-inch pie plate with pastry rolled 1/8-inch thick, allowing pastry to extend 1-inch beyond edge. Fold back to form standing rim.
Fill with rhubarb mixture.
Moisten edge of pastry with cold water, arrange lattice of pastry strips across top. Flute rim with fingers.
Bake in hot oven (450 degrees F) 15 minutes; then decrease heat to moderate (350 degrees F) and bake 30 minutes longer.