How does seed stratification work in the nature, in the wild?

Seed stratification in nature is the process where seeds are out in the open and subjected to low and high temperatures, which, in turn, breaks dormancy. It has been said that the seeds of perennials, evergreen trees, shrubs, and herbs will not germinate after being harvested, unless their seeds were pre-treated, which is known as the stratification process. Seeds like these are often times incomplete and would require a bit more time in order to develop completely. Some of them would even have what we call a built in dormancy mechanism that would prevent them from germinating

What types of seed stratification are there in nature?
Basically, there are two types of seed stratification in the wild: cold stratification and warm stratification. The process of cold stratification means exposing the seed to cold temperatures, i.e. in the winter season, when the seeds lie on or in the ground. The process of warm stratification takes place during – you guessed it – the warmer season, when the seeds have fallen off the plant or flower, usually in the autumn. Regardless of it is a cold or warm stratification process, the overall purpose is to awaken the seed from its sleep so that it can start to grow.

As nature can be seen to be unpredictable, humans intervened by making use of modern methods of seed stratification, which is to mix the seeds in a moist like material such as perlite, peat moss, sawdust, or other special potting soil that would help stimulate the type of condition the seed would normally experience out in nature. For the stratification process, polyethylene bags are used to store the seed mixture in. Depending on the seed, the bags would either be stored at room temperature to coincide with the warm stratification process in nature, or it would be placed in a refrigerator where cold and moist conditions are required for seeds that would normally do better under the cold stratification process.

If the natural way of cold stratifying is preferred, then the seeds can always be sown directly into a nursery bed during the Autumn season, or placed into a pot during Spring season as it would satisfy the requirements of the particular seed. These methods would mean that you can skip the pre-treatment process as the natural method would accomplish the same end goal. The only difference being, that the natural stratification process takes a little longer than what would be the case if humans intervened.

To let mother nature takes is course with regards to the stratification process, it is suggested that gardeners or hobbyists just plant the seed in a natural seedbed during fall, and at a time when the first frost is expected. It is best to cover the planted seed with some mulch to help protect it against the elements of nature such as snow, rain, or the wind. Once this has been done, it is a case of displaying the needed patience and let the natural cold stratifying procedure run its course. All these seeds need are time to overcome its dormancy and germinate when the time is right for it. Also take care not to overwater the seeds that needs germination to take place.

From a scientific point of view, the seed stratification process starts when certain levels of endogenous inhibitors have been lowered. These inhibitors keep the seeds asleep, and the stratification process stops that by lowering these levels. For a discussion about this, see this excellent 2008 article by Bentsink and Koornneef (click on image):

Cold stratification, seed stratification, warm stratification, methods of seed stratification, seed stratification in nature

Bentsink and Koornneef (2008)

To conclude, note that some seeds need to be exposed to warm temperatures, e.g. humid, moist conditions, and some need to be exposed to cold conditions, in order to be able to start growing.