Combine mulberries, sugar, lemon juice, water, salt, and pectin (if using) in a large saucepan.

Bring mixture to a rolling boil over medium-high heat, stirring constantly.

Reduce heat to medium-low and continue to boil for 10-15 minutes, or until the jam reaches setting point (a small amount dropped onto a chilled plate wrinkles when pushed with a finger).

Remove from heat and skim off any foam.

Ladle the hot jam into sterilized jars, leaving about 1/4 inch headspace.

Wipe the jar rims clean, seal with lids, and process in a boiling water bath for 10 minutes.

Remove jars from the water bath and let cool completely. Check seals by pressing down on the center of each lid – if it doesn't flex, it's sealed.