Step 1: Prepare Your Mulberries. Gently rinse your mulberries under cool running water. Carefully pick through them, removing any leaves, debris, or visibly damaged berries. Small, soft stems can be left on. Measure out 5 cups.
Step 2: Combine Ingredients. In a large, heavy-bottomed pot or Dutch oven, combine the cleaned mulberries, granulated sugar, and fresh lemon juice. Stir gently to coat the berries.
Step 3: Macerate (Optional, but Recommended). For a better flavor extraction and to help the fruit release its juices, cover the pot and let the mixture sit at room temperature for 1-2 hours, or even overnight in the refrigerator.
Step 4: Start Cooking. Place the pot over medium-high heat. Stir constantly until the sugar dissolves. Once the sugar is fully dissolved, bring the mixture to a rolling boil, stirring frequently to prevent sticking.
Step 5: Cook to Setting Point. Continue to boil, stirring occasionally to prevent scorching, for 20-30 minutes, or until the jam reaches its setting point. Test for set using the cold plate test (a small spoon of jam wrinkles when pushed on a chilled plate), a candy thermometer (220°F/104°C), or the sheet test (jam runs off a cold spoon in a sheet).
Step 6: Prepare Jars and Can (or Store). While the jam is cooking, sterilize your canning jars and lids according to standard canning procedures. Keep the jars hot. Once the jam has reached its setting point, remove it from the heat. Carefully skim off any foam from the surface with a spoon.
Step 7: Fill and Process. Ladle the hot jam into the hot, sterilized jars, leaving about 1/4 inch of headspace. Wipe jar rims clean with a damp cloth, place lids on top, and screw on the bands until finger-tight. Process in a boiling water canner for 10 minutes (adjust for altitude). Alternatively, if not canning, let the jam cool completely, then store it in clean containers in the refrigerator for up to 3 weeks or freeze for up to 6 months.