Measure out 9 lb (4 kg) of well-ripened plums. Wash them well. Remove their pits and cut them into quarters.
Put them in a deep pot and pour all of the sugar over them. Cover and let sit as is for at least 18-20 hours.
Then put them on the stove and boil, while stirring often with a wooden spoon. Pour a drop of it onto a plate and if it doesn't run it means your jam is ready.
Add the vinegar and geranium, finely chopped, boil another 5 min.
Remove from the stove and distribute into washed and dried jars. Close well with caps and boil for 10 min.