Place the chopped rhubarb, maple syrup, lemon juice, powdered pectin, and salt in a medium-sized pot. Bring everything to a boil over high heat then continue to allow the rhubarb to boil for 1 to 2 minutes.
Reduce the heat to medium-low. Simmer the rhubarb with lively bubbles for 20 minutes, stirring occasionally.
Remove the pot from the heat and stir in ¼ teaspoon of cardamom. Cool the jam completely in the pot then adjust cardamom to suit your taste, adding the remaining ¼ teaspoonif you like. Spoon the cooled jam into a jar with a tight fitting lid and keep it in the fridge for up to one week.