01 - In a large non-reactive pot, combine strawberries, rhubarb, raspberries, and mixed berries. Mash lightly with a potato masher to release juices.
02 - Stir in lemon juice and fruit pectin. Mix well and let stand for 5 minutes.
03 - Place the pot over medium-high heat and bring the mixture to a boil, stirring frequently.
04 - Add sugar all at once and stir to dissolve completely. Bring back to a full, rolling boil, then boil hard for 1-2 minutes.
05 - Remove from heat. Skim off any foam with a spoon. Carefully ladle hot jam into sterilized jars, leaving ¼-inch headspace. Wipe rims, seal with lids, and process in a boiling water bath for 10 minutes, if storing long-term.
06 - Allow jars to cool completely. Refrigerate opened jam and use within 3 weeks.