Slice 2 stalks of rhubarb into bite size pieces. Put in a small pot with the lemon juice, agave and sugar.
Bring to a simmer over medium heat and continue to cook for another 10 minutes, whisking occasionally.
Remove the mixture from the heat and pour into a large carafe; stir in 5 cups water.
Refrigerate lemonade for at least 3 hours to allow flavors to meld. Cut remaining rhubarb stalk in half lengthwise and use as garnish for glasses before serving.