1. Combine rhubarb, plums, sugar, orange juice, water, and orange zest in a saucepan over medium heat; bring to a boil. Reduce heat to medium-low and cook, stirring occasionally, until thick, about 45 minutes.

2. Meanwhile, inspect 6 pint-sized jars for cracks and rings for rust, discarding any defective ones. Immerse in simmering water until jam is ready. Wash new, unused lids and rings in warm soapy water.

3. Place a rack in the bottom of a large stockpot and fill halfway with water. Bring to a boil.

4. Pack jam into hot, sterilized jars, filling to within 1/4 inch of the top. Run a clean knife or thin spatula around the insides of the jars to remove any air bubbles. Wipe rims with a moist paper towel to remove any residue. Top with lids and screw rings on tightly.

5. Lower jars 2 inches apart into the boiling water using a holder. Pour in more boiling water to cover jars by at least 1 inch. Bring to a rolling boil, cover, and process for 5 minutes.

6. Remove the jars from the stockpot and let rest, several inches apart, for 12 to 24 hours. Press the center of each lid with a finger to ensure the lid does not move up or down. Remove the rings for storage and store in a cool, dark area.

Cook's Note:

Use up to 4 cups sugar if you like sweeter jam.