Stratification of tree seed occurs naturally outdoors through the winter. Sowing tree seeds outdoors in the fall takes advantage of this natural effect. To stratify indoors, mix the seed with a moistened, sterile, peat based growing medium in a container, wrap in a ventilated bag, and place it in a refrigerator for 12 to 20 weeks. In the spring plant the seed in a sheltered spot outside to grow into seedlings. Transplant to permanent site when well rooted.