The propagation of Haworthias presents no problem. This can be achieved from offsets, seed, leaf cuttings, root cuttings (species with fleshy roots) and by cutting short the flower stems. (Cut them off at about 10 cm height when the first 1 - 2 flowers only have opened. As a rule the uppermost bract will form a new rosette which can then be removed and treated as an offset.) Cultural conditions for propagation are similar to those for other leaf succulents. Seed sowing, which can be done at any time of the year, gives best results on a north-facing windowsill. The temperature should be maintained between 15 - 20°C. It is important this temperature is not exceeded too much as this inhibits germination. Only when all have germinated, and seedlings have become acclimatised to the fresh air, can higher temperatures be tolerated for short periods; not before. Seeds older than one year generally do not germinate.
(Excerpt from Haworthia.info)