Place pitted and halved plums into a large bowl and drizzle over the sugar. Stir plums until all coated. Let them sit for one hour.
Transfer coated plums into a large cooking pot and boil until the mixture is bubbling. Simmer for 10 mins then turn off the heat and allow to cool to room temperature.
Repeat step 2 - four times. On the last time boil at lowest temperature and stir frequently to avoid burning.
Transfer your boiling hot jam to the jars using a glass measuring cup and funnel. Screw the lids on enough to seal tight but don’t over tighten them. Place cans into the canning pot and cover with 1-2 inches of water. Bring to the boil and process 15 mins. Remove from the pot and leave at room temp for 12-24 hours. Check that the seal has formed.
Recipe Note
Use good quality, freshly picked fruit. You do not need to peel the plums as they contain high amounts of pectin which aids the jam to set.