Heat the water and sugar in a saucepan and stir until the sugar has dissolved. Turn off the stove, add the lemongrass and let it steep for about 2 hours, longer if you like.
Bring everything to the boil, add the citric acid and simmer for about 5 minutes.
Pour the syrup through a sieve into a clean bottle. Close the bottle tightly, let it stand upside down for a while, then turn it upside down and let the syrup cool completely.
The syrup is suitable for flavoring fruit salads and drinks such as sparkling wine, mineral water and iced tea.