Place rhubarb, water, and ginger in a medium stock pot over high heat. Cook until boiling. Reduce heat and continue to cook until rhubarb softens and is falling apart, about 10-12 minutes.
Line a strainer with cheesecloth and place it over a bowl or put a nut milk bag in a large bowl, then pour in the rhubarb pulp and water. Strain the liquid into the bowl and discard the pulp.
Measure rhubarb liquid and add it and an equal amount of sugar back to the stock pot. Cook until sugar is dissolved and the mixture begins to thicken. Skim the foam off the top and discard.