Prepare Jars: Examine all jars for cracks or chips and discard any damaged jars. Wash jars and rings in hot soapy water, rinse thoroughly, and keep jars hot in a 180°F oven or hot water until ready to fill.
Heat Lids: Place new canning lids in a small saucepan, cover with hot water at 180°F, and keep hot until ready to use. Do not boil lids as this may damage the sealing compound.
Prepare Canner: Fill your boiling water bath canner with enough water to cover jars by 1-2 inches when submerged. Begin heating water to a rolling boil while preparing jelly.
Extract Rhubarb Juice: Place chopped rhubarb and water in a large pot and bring to a boil over medium-high heat. Reduce heat and simmer for 10-15 minutes until rhubarb is completely soft and broken down, stirring occasionally to prevent sticking.
Strain Juice: Pour cooked rhubarb into a fine-mesh strainer or jelly bag suspended over a large bowl. Allow juice to drip for 2-4 hours or overnight without pressing solids for clearest jelly. Measure strained juice and calculate sugar needed at 3/4 cup per cup of juice.
Cook Jelly Base: Pour measured rhubarb juice into a large heavy-bottomed pot. If using pectin, stir it into the juice until completely dissolved. Bring juice to a full rolling boil over high heat, stirring constantly.
Add Sugar: Add all measured sugar at once to the boiling juice, stirring constantly to dissolve completely. Return to a full rolling boil that cannot be stirred down and boil hard for 1 minute if using pectin, or until jelly reaches 220°F or passes the plate test if making without pectin.
Test and Skim: Remove jelly from heat and quickly skim off any foam with a metal spoon. Test consistency by dropping a small amount on a cold plate - it should wrinkle when pushed with your finger and hold its shape.
Fill Jars: Using a ladle and wide-mouth funnel, fill hot jars with jelly, leaving exactly 1/4 inch headspace. Remove air bubbles by running a plastic tool around the inside of each jar and wipe jar rims clean with a damp paper towel.
Process in Water Bath: Center hot lids on jars and apply rings finger-tight only. Using jar lifter, place filled jars on canner rack and lower into boiling water, ensuring jars are covered by 1-2 inches of water. Process for exactly 10 minutes, adjusting for altitude: add 5 minutes for 1,001-6,000 feet, add 10 minutes for above 6,000 feet. Remove jars and cool undisturbed for 12-24 hours.
