Véronique, do you lift your fargesii, and store it each winter? I would think that it should be winter hardy for you. I haven't had any problems germinating many Arisaema species by placing the seed between moist paper towel, and then putting them inside a ziplock plastic bag. I leave them at room temperature to pre-germinate. This has worked for many species, (amurense, candidissimum, ciliatum, consanguineum, costatum, erubescens, fargesii, franchetianum, heterophyllum, serratum, sikokianum, tortuosum). It hasn't worked so well for me on Arisaema griffithii or Arisaema nepenthoides (only if the seed of nepenthoides is very fresh).