Scarification - The hard protective coating on some tree seed is nature's way of protecting the seed. But hard coats on some hard seeded species actually inhibit the germination of the seed because water and air can not penetrate the hard coating.
Interestingly, many tree seeds require two dormant periods (two winters) before the protective coating breaks down enough to germinate. The seeds must lay on the ground completely dormant for one full growing season, and then germinate the following growing season.
Scarification is the artificial way to prepare hard seed coats for germination. There are three methods or treatments that will usually make seed-coats permeable to water: (1) soaking in a solution of sulfuric acid, (2) soaking in hot water or immersing the seed for a short period in boiling water, or (3) mechanical scarification.