Peel one piece of ginger root and slice it into ½ " pieces. Place the ginger, one cup sugar, and one cup water into a small saucepan. Reserve the second piece of ginger.
Bring the sugar, ginger water mixture to a boil over medium high heat being careful not to let it burn. Reduce the heat and simmer on low until thick and syrupy. Store in a covered bowl for at least two hours, preferable overnight.
Remove the ginger slices from the syrup and discard, or use for something else calling for candied ginger.
Pour the ginger syrup into a large, deep, heavy bottomed pot. Add the rhubarb, sugar, and lemon juice to the pot. Peel the second piece of ginger and great it into the rhubarb mixture. You can use as little or as much as you like. I really wanted to taste the ginger.
Place a small plate in the freezer to use verifying that the jam has set.
Bring the rhubarb mixture to a full rolling boil then reduce the heat. Simmer for 20-25 minutes or until the rhubarb has broken down.
After 20 minutes take a small amount of the jam and place it on the chilled plate. After one minute push the cool jam with your finger. If the surface of the jam wrinkles it’s ready. If not, continue cooking until the jam passes the chilled plate test.
When ready, transfer the jam into three sterilized canning jars, seal and process the jars in a water bath following canning protocols strictly. You can also freeze the jam in plastic containers or simply store in the refrigerator. Do not store unprocessed jam without refrigeration!