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 to allow pectin to activate.
03 - Place the pot over medium-high heat and bring the mixture to a boil, stirring frequently to prevent sticking.
04 - Add sugar all at once and stir to dissolve completely. Return to a full, rolling boil and cook hard for 1-2 minutes to reach setting point.
05 - Remove from heat and skim off any foam with a spoon. Prepare sterilized jars while jam rests briefly.
06 - Carefully ladle hot jam into sterilized jars, leaving ¼-inch headspace. Wipe rims clean and seal with lids tightly.
07 - Process in boiling water bath for 10 minutes for long-term storage. Allow jars to cool completely. Refrigerate after opening and use within 3 weeks.