Some seeds, for example Cimicifuga, germinate extremely hard, sometimes it takes one year or longer befor germination starts. To best utilize space and avoid drying out, this seed must be "stratified" (placed in layers of wet sand - alternately a thin layer of seeds and a layer of well-moistened sand, etc.). The stratification boxes have to be kept in the shade to benefit from weather effects - especially winter. A fine wire mesh will protect them from mice and birds. Nurseries have found that concrete boxes are useful for large amounts of seed. In spring frequently check to see if germination has begun. When germination has started, the seeds must be sown immediately in the prepared bed with the moist sand.
It is possible to use a refrigerator with a temperature of about 0°C (32°F) to create an artificial freezing period. Since it is usually not possible to put the seed trays into a refrigerator, you can mix the seed with moist sand. It should be put into a plastic bag and placed into the refrigerator after the required warm period. The sand must be kept constantly moist. After the required cooling period sow the sand with the seed into seed trays and leave at required temperatures.