Cook the rhubarb. Combine the chopped rhubarb with the water in a large pot. Bring it to a boil, then reduce the heat and simmer until the stalks fall apart and the liquid turns a deep pink, about 15 minutes. Using rhubarb you froze from your own garden? Go straight from frozen — it just needs a few extra minutes to break down.
Strain it. Pour the cooked rhubarb through a fine-mesh strainer lined with cheesecloth, or a jelly bag if you have one. Let it drip on its own for a clearer juice, or press the pulp through for a little more volume (and a slightly cloudier result). Measure out 6 cups of juice, topping off with a splash of water if you come up short.
Juice the lemons. Squeeze and strain out the seeds and pulp, since leftover pulp can leave a faint bitterness in the finished drink.
Combine and sweeten. Pour the rhubarb juice, lemon juice, and sugar into a clean pot. Stir over medium heat just until the sugar fully dissolves and the mixture is hot through. No need to boil it or hit a specific temperature — that's only necessary if you're canning, and this version isn't.
Cool completely before bottling, freezing, or pouring yourself a glass.
