Step 1: Prepare your canning jars and lids by washing them in hot soapy water, rinsing well, and sterilizing. Keep them hot until ready to fill.
Step 2: In a large, heavy-bottomed pot or Dutch oven, combine the cleaned mulberries and fresh lemon juice.
Step 3: Place the pot over medium heat and bring to a gentle simmer. Cook for 10-15 minutes, stirring occasionally, until the mulberries soften and release their juices. Gently mash some of the berries if you prefer a less chunky jam.
Step 4: If using powdered pectin, whisk it into the fruit mixture until fully dissolved. Add the granulated sugar to the pot and stir constantly until the sugar has completely dissolved.
Step 5: Increase the heat to high and bring the mixture to a vigorous, rolling boil that cannot be stirred down. Continue to boil for 8-12 minutes, stirring frequently to prevent scorching, until the jam reaches its setting point (220°F/104°C on a candy thermometer, or passes the cold plate test).
Step 6: Remove the pot from the heat. Skim off any foam that rises to the surface using a clean spoon.
Step 7: Carefully ladle the hot jam into your prepared hot, sterilized jars, leaving 1/4 inch of headspace. Wipe the rims of the jars clean with a damp cloth.
Step 8: Place the lids and screw bands on the jars, tightening them to fingertip-tight.
Step 9: Process the filled jars in a boiling water canner for 10 minutes (adjust time for altitude if necessary).
Step 10: Carefully remove the jars from the canner and place them on a towel-lined counter. Allow them to cool completely undisturbed for 12-24 hours. Do not disturb the lids during this time.
Step 11: After cooling, check the seals. The center of the lid should be concave and not flex when pressed. Store properly sealed jars in a cool, dark place for up to 1 year. Refrigerate any unsealed jars and consume within a few weeks.