Growing trees from seed can be a tricky yet rewarding endeavour. The seed of most tree species won’t germinate immediately when planted because they are in a dormant state. Dormancy must be broken before the seed can germinate.
In some tree species, dormancy is the result of a thick, hard seed coat. The seed coat may be broken in a variety of ways — this process is referred to as scarification. Mechanical means, such as a metal file or coarse sandpaper, can be used to break the seed coat. Treatment with boiling water has also been successful for a number of tree species. In nature, the seed coat may be broken by microbial action, passage of the seed through the digestive tract of birds or other animals, exposure to alternate freezing and thawing, or fire.
Our seeds are pre-stratified before we receive them, however, they can go back to a dormant state if they sit for several months. Seeds will not germinate until they have been exposed to cool temperatures and moist conditions for several weeks or months. Winter weather provides the necessary conditions to break dormancy. This is why a fall, outdoor sowing in a cold frame is the preferred method.
Gardeners can simulate these same conditions (stratification) by placing the seed in a moist 50/50 mixture of sand and peat moss, wrapping the container in plastic, and freezing it solid in the refrigerator. Leave it in the fridge for 3-4 weeks, then bring it back indoors under your grow lights. You cannot put the seed packet in the freezer and expect the same results. It is the combination of cold temperature and moisture that induces proper stratification.