Trim the leaves and woody bottom ends from the rhubarb. Chop stalks into 1-inch pieces. Compost the leaves; rhubarb leaves are toxic and should never be used.

Place chopped rhubarb in a heavy-bottomed pot with the water. Bring to a boil over medium-high heat, then reduce to a simmer for about 20 minutes, until the rhubarb has completely fallen apart and the liquid has turned bright pink.

Transfer the cooked mixture to a dampened jelly bag or a strainer lined with several layers of dampened cheesecloth set over a deep bowl. Let it drip undisturbed for at least 2 hours, or up to 4 to 6 hours for more juice. Don't squeeze the bag; squeezing clouds the finished jelly. Measure 4 cups of strained juice for the next stage.

Prepare half-pint canning jars by washing in hot soapy water, rinsing, and keeping warm. Have lids and rings ready.

Pour the rhubarb juice into a heavy-bottomed jam pot. Whisk in the powdered pectin until fully dissolved.

Bring to a hard rolling boil over high heat, stirring frequently. Add the sugar all at once and stir constantly until fully dissolved. Return to a hard rolling boil that can't be stirred down, and boil hard for exactly 1 minute.

Remove from heat and quickly skim off any foam.

Ladle the hot jelly into prepared half-pint jars, leaving 1/4 inch headspace. Wipe rims clean and apply two-piece canning lids fingertip tight.

Process the filled jars in a boiling water bath canner for 10 minutes (adjust for altitude — see notes).

When the timer goes off, turn off the heat and let the jars sit in the canner for another 5 minutes before lifting them out. This helps prevent siphoning.

Cool on a towel for 12 to 24 hours, check seals, and refrigerate any unsealed jars. Sealed jars keep on the pantry shelf for 12 to 18 months.