Combine the trimmed rhubarb and water in a large, non-reactive saucepan. Bring to a boil, then reduce heat and simmer gently, covered, until the rhubarb is completely soft, about 20-25 minutes. Stir occasionally.

Strain the cooked rhubarb mixture through a jelly bag or several layers of cheesecloth set over a bowl. Allow the juice to drip through naturally for at least 4 hours, or preferably overnight. Do not squeeze the cloth, as this will make the jelly cloudy.

Measure exactly 4 cups of the strained rhubarb juice into a clean saucepan. Add the powdered pectin and bring the mixture to a rolling boil, stirring constantly. The boil should be rapid and unable to be stirred down.

Quickly stir in the measured sugar. Return the mixture to a full rolling boil. Boil hard for exactly 1 minute, stirring constantly to prevent scorching.

Remove the pan from the heat immediately. Skim off any surface foam with a spoon. Carefully ladle the hot jelly into sterilized canning jars, leaving 1/4-inch headspace.

Wipe the rims clean, place the lids and bands on securely, and process the jars in a boiling water bath canner for 10 minutes (adjusting for altitude). Turn off heat, let rest 5 minutes, then remove jars and let cool completely on a wire rack.