Wash and trim the rhubarb, discarding all leaves. Cut the stalks into roughly 1/2-inch pieces so they soften evenly during simmering.

Place the chopped rhubarb and 4 cups of water in a saucepan. Bring the mixture to a gentle boil, then reduce the heat and simmer for 15–20 minutes, until the rhubarb is very soft and the liquid is strongly flavored.

Strain the cooked mixture through a fine-mesh sieve into a bowl or pitcher. Avoid pressing the solids if you prefer a clearer lemonade; pressing releases more pulp and creates a cloudier drink.

While the strained rhubarb liquid is still warm, stir in the sugar. Continue stirring until the sugar has completely dissolved.

Cool the rhubarb syrup to room temperature, then refrigerate it until cold. Cooling before adding the lemon juice helps preserve its fresh flavor.

Stir the cold rhubarb syrup together with 1 cup of freshly squeezed lemon juice and the remaining 4 cups of cold water. Mix thoroughly so the flavor is evenly distributed.

Taste the lemonade and adjust it if needed. Add a little more sugar for a sweeter drink or more lemon juice for extra tartness.

Serve the lemonade over ice and garnish with lemon slices or extra rhubarb. Stir the pitcher before each pour, especially after it has been refrigerated.