Rinse the rhubarb. If the stalks are thick, peel off the tough skin at the bottom and cut the rhubarb stalks into 1 cm long pieces. Mix with the preserving sugar and steep covered for 3 hours.
Bring the mixture to a boil, stirring, and bubble for 4 min.
Rinse the lemon balm, pluck the leaves, fold in and pour the jam with the leaves into the prepared jars.