1. Rinse the rhubarb, trim the stalks, then cut them into pieces 1 inch long. Measure the orange juice and add enough water to make 2 cups. Put it in a wide pan with the orange zest, sugar, and vanilla bean. Bring to a boil, stir to dissolve the sugar, then reduce the heat to a quiet simmer. Add the rhubarb and cook carefully, turning the pieces so that they cook evenly. Often a piece that is cooked on one side is still a bit firm on the other side. It takes only about 10 minutes for them to be done.

2. Use a slotted spoon to remove each piece as it finishes cooking to a wide bowl, alternating with the blackberries.

3. Return the poaching liquid to the stove and boil until only ¾ cup remains, after 8 to 10 minutes. Stir in the rose water, then pour the syrup over the fruit. Refrigerate and serve cold.