Wash and remove the pits from the plums.
Mash them carefully.
Pour the mash into a large pot, add the sugar and water and put to boil on moderate heat. Stir with a wooden spoon from time to time.
Once thickened, add the citric acid, walnuts and geranium.
Distribute into dry jars and seal them.
Sterilize the jars for 15 min.