Place all ingredients in a heavy-bottomed sauce pan and bring to a boil. Turn down the heat and simmer for about 25 minutes, until the rhubarb has completely broken down.
Using a fine mesh strainer placed over a bowl, remove all the solids and save the juice. The mixture is going to be quite thick so be patient while it drains, it will take about 20 minutes. You will have approximately 4 cups of juice.
If your juice has cooled while straining, return it to he pan and bring it up to a boil.
Pour into pint or half-pint jars, leaving ½ inch headspace. Process either size jar in a boiling water bath for 10 minutes - find water bath canning instructions here : http://www.enneacollective.com/?p=3762
To drink, pour 1-2 ounces of shrub into a tall glass filled with ice, and top with sparkling water.
Notes
Makes 2 pints
Recipe by Ennea Collective - crafting a fibre life at http://www.enneacollective.com/spiced-rhubarb-shrub/