Put first four ingredients in heavy saucepan and cook on low stirring occasionally. When it begins to bubble add the Cardamom. Continue cooking until the Rhubarb breaks down and can be "mushed" against the side of the pan, you may have to add a little more water to keep it from sticking. Pour into blender or food processor and process until almost smooth. Pour hot into clean jars.
You can water bath this to preserve as with any other jam.