Inspect 7 half-pint jars for cracks and rings for rust; discard any defective ones. Immerse in simmering water until jam is ready. Wash new, unused lids and rings in warm soapy water.

Place berries in a large pot and bring to a boil over medium heat until they release their juice. Press all or half of the berries through fine mesh strainer to remove the seeds, if desired. Measure exactly 5 cups raspberry pulp and return to the empty pot.

Add lemongrass and pectin to the pot with the raspberries. Slowly bring mixture to a full rolling boil. Add sugar and stir to dissolve. Return to a full rolling boil on high heat and boil for exactly 1 minute, stirring constantly. Skim off any foam with a ladle or a large spoon. Remove the lemongrass.

Pack raspberry jam into the prepared jars, filling to within 1/4 inch of the top. Run a clean knife or thin spatula around the insides of the jars to remove any air bubbles. Wipe rims with a moist paper towel to remove any spills. Top with lids and tightly screw on rings.

Place a rack in the bottom of a large stockpot and fill halfway with water. Bring to a boil and lower jars 2 inches apart into the boiling water using a holder. Pour in more boiling water to cover jars by at least 1 inch. Bring to a rolling boil, cover, and process for 10 minutes.

Remove the jars from the stockpot and place onto a cloth-covered or wood surface, several inches apart. Let rest for 24 hours without moving the jars. Gently press the center of each lid with a finger to ensure the lid does not move up or down. Remove the rings for storage and store in a cool, dark area.