Seeds are very smart little packages of nutrients, with just a tiny sprout all wrapped up in a protective casing. Although some seeds may only germinate for a few hours (Certain willow species) to years (maples), other trees have devised a strategy to keep seeds alive for decades (Juniperus). To do this, the plant in the seed is in complete sleeping mode, and often near completely dried out. This state of complete sleep is called dormancy. Getting a seed to start growing is referred to as breaking dormancy.
he young seedlings would not have time to establish themselves before winter arrives, and the seedlings would die. So, many treeseeds require a period of moist dormancy, during which temperatures drop below the 10-15 degrees Celsius. Effectively: The seeds need winter in order to germinate.
Because many seeds are collected upon ripening, and then send out to the grower, these seeds will not germinate if just planted in seeding trays. They first need to be placed in a moist medium (e.g., sphagnum moss) under cold conditions for 60 to 120 days. This stimulates winter conditions. If the seeds are then planted in a substrate and exposed at room temperature conditions, germination is rapid. In many cases seeds may break dormancy during stratification. If that happens, they can be taken out of the cold storage and planted in suitable substrate, being careful to not damage the young root tip. Of course, one can also order and sow seeds in the fall, and just wait till spring arrives: The seeds will germinate by itself. The one downside is the presence of rodents and bird that may feast on your carefully selected seeds..
Other seeds are evolved to survive many years. Here it is not the temperature that regulates germination, but the seed skin: These seeds have a thick, water-proof outer skin that only lets water in after several years. Once the skins starts to let water pass, the seeds will germinate the following spring, again often guided by a period of cold. As most people do not want to wait years for their seeds to germinate, we need to assist breaking the outer skin of the seed. This is referred to as scarification. Several methods exist, each for specific species. One is placing the seeds in water of 80 degrees Celcius and letting ti cool down. After some 12 hours of soaks part of the seeds will have clearly increased in size and are ready for sowing. Repeat for the rest. The next step is physically breaking the skin by rubbing on sanding paper, scratching or even bathing the seeds in weak acids. After this, again, soak in water for a night or so to see which seeds were successfully pretreated. Depending on the species you may after this have to stratify the seeds for several weeks to stimulate germination.
Have a look at our guide to breaking dormancy by tree species for more information.