3cupspitted and chopped plumsabout 1-1/2 pounds/680 g whole plums
4cupspitted and chopped sweet cherriesabout 2 pounds/910 g whole cherries
3-1/2cupsgranulated sugar700 g
1Zest and juice of 1 lemon
23-ounce packetsliquid pectin85 ml
Prepare a boiling water bath and 4 regular-mouth 1-pint/500 ml jars as per our canning basics post. Place the lids in a small saucepan, cover them with water, and simmer over very low heat.
Combine the plums, cherries and sugar in a large pot. Bring to a boil over high heat and cook for approximately 15 to 20 minutes, until the jam looks molten and syrupy.
Add the lemon zest and juice and the pectin. Bring to a rolling boil and boil for 5 minutes.
Remove the pot from the heat and ladle the jam into the prepared jars. Wipe the rims, apply the lids and rings, and process in a boiling water bath for 10 minutes.
Turn off canner, then carefully lift jars out and set on a towel on the counter to cool. Let sit 24 hours, then check that all have sealed. Label and store in a cool place.
Because of the variables in the fruit, pot size and moisture in the air, cooking times can vary a lot. I only cooked my jam for 15 minutes and it was on the soft side. For firmer jam, be sure to cook the full suggested time, and perhaps even a little more, depending on how thick you want it to be.