Lemon balm leaves in a large baking bowl of 4 liters form. Boil water, pour over the leaves. Add sugar and citric acid, stirring. Cover baking bowl with a dish rug, let stand 24 hours, stirring occasionally. Pour liquid through jelly bag or through a strainer lined with a fine crockery rangle into a roasting pan. Bring liquid to a boil while stirring, fill boiling hot to just below rim into clean, preheated bottles, seal on the spot, cool on insulating surface.
Serve: Dilute syrup with water in a 1: 5 ratio.
Shelf life: cooled and dark about 6 months. Once opened, keep the bottle in the refrigerator, consume syrup quickly.