Wash and hull the strawberries, then crush them until they are juicy but still slightly chunky. Wash the rhubarb, discard every leaf, and finely chop the stalks.

Measure the prepared fruit accurately after crushing and chopping. Add the strawberries, rhubarb, bottled lemon juice, and powdered pectin to a large pot.

Bring the fruit mixture to a full rolling boil over medium-high to high heat, stirring frequently so it does not scorch.

Add all 5 cups of sugar at once and stir until dissolved. Return the mixture to a full rolling boil that cannot be stirred down.

Boil hard for 1 minute while stirring constantly. Remove the pot from the heat and skim off foam if needed.

Ladle the hot jam into hot, sterilized jars, following the headspace specified by your pectin or canning instructions. Wipe the rims clean and apply the lids.

For shelf-stable storage, process the filled jars in a boiling-water canner for 10 minutes, adjusting for altitude according to current canning guidance. Otherwise, let the jam cool and refrigerate it.