Scrub and finely slice the burdock and dandelion roots.
Boil the roots with about one gallon of water (and the carragheen if using) for half an hour. (Experience the aroma of an unpromising vegetable stew.)
Take off the heat, add another gallon of cold water, sugar, molasses, and lemon.
Stir until the sugar has dissolved. Strain the liquid into a clean fermenting or bottling bucket and cover.
When the brew reaches room temperature, add the yeast and keep covered for up to a week.
Bottle in strong swing top bottles. Another week and it will be ready to drink, though it is well worth easing the top off of a bottle every now and then to check for potentially explosive levels of fizziness.
Once ready it is a good idea to keep the bottles in the fridge to prevent further fermentation.