Most tree and shrub seed are dormant when first collected. To break seed dormancy the seeds need a period of cold moist conditions. Native tree and shrub seeds sown in the fall will naturally receive this conditioning period through the winter and spring.
Conifer seed sown in the spring needs artificial stratification. These seeds are mixed in with Perlite, which is inert, heat treated pumice that retains moisture. The mixture is placed in the cooler for 30, 60 or 90 days depending on the species.
A few species are more difficult germinate and need extended cold periods to break dormancy. Some of these species are soaked in gibberellic acid, which is a plant hormone that reduces the amount of cold treatment needed.