Add rhubarb, lemon zest, sugar, and 1 cup water to a small pot and simmer until the rhubarb falls apart, 8-10 min.
Pour the mixture through a sieve saving the liquid and discarding the rhubarb. Add the 1/2 cup lemon juice and stir.
Fill a pitcher with the ice and pour the hot liquid over. Stir to cool the lemonade. Add as much water as you would like to the desired strength. Serve immediately.