Prepare your canning jars and lids: Wash jars and lids in hot soapy water. Sterilize jars by submerging them in boiling water for at least 10 minutes. Keep jars in hot water until ready to fill. Heat lids in hot (not boiling) water according to manufacturer's instructions.

In a large, non-reactive pot (stainless steel or enameled cast iron), combine the cut rhubarb and 1/2 cup of water. Bring to a boil over medium-high heat, then reduce heat, cover, and simmer for 15-20 minutes, or until the rhubarb is very tender and broken down.

Place a fine-mesh sieve lined with cheesecloth over a large bowl. Carefully pour the cooked rhubarb mixture into the sieve. Allow the liquid to drain through without pressing the solids. This can take several hours, or even overnight, to extract all the juice. You should aim for about 4-5 cups of rhubarb juice.

Measure exactly 4 cups of rhubarb juice into your large, non-reactive pot. Stir in the powdered fruit pectin until completely dissolved. Place the pot over high heat and bring the mixture to a rolling boil that cannot be stirred down.

Once a rolling boil is achieved, immediately add the granulated sugar, stirring constantly until the sugar is dissolved. Return to a rolling boil and boil hard for exactly 1 minute, stirring constantly. Remove from heat.

Carefully skim off any foam from the surface of the jelly using a metal spoon. Ladle the hot jelly into the prepared sterile jars, leaving 1/4 inch headspace. Wipe jar rims clean. Center lids on jars and screw on bands until fingertip tight.

Process jars in a boiling water canner for 10 minutes (adjusting for altitude if necessary). Carefully remove jars from the canner and place them on a towel-lined counter to cool completely. Check seals after 12-24 hours. Any unsealed jars should be refrigerated and consumed within 3 weeks. Store sealed jars in a cool, dark place for up to 1 year.