What Is Seed Cold Stratification

Cold stratification is a technique that mimics the real-world cold and moist conditions a seed goes through; this is an undergoing natural form of "cold stratification" or pretreatment.  

Seeds of many trees, shrubs and perennials require these conditions before the germination time.

The seeds should additionally be stratified in a warm area first, followed by the cold period in a refrigerator later.

Warm stratification requires temperatures of 15-20°C (59-68°F). In many instances, warm stratification followed by cold stratification requirements can also be met by planting the seeds in summer in a mulched bed for expected germination the following spring.

Some seeds may not germinate until the second spring.

The seeds need to be storaged in a cool 33°- 37°F (ideally +1°C to +3°C; not freezing) and moist environment for a period of time that may vary from one to three months.

Larger seeds tend to do well in a bit of moistened peat or sand, placed into a plastic bag.  Smaller seeds can be distributed onto moist paper towels. 

To accomplish this, seeds should be placed in a sealed plastic bag with moistened vermiculite, or sand, or even a moistened paper towel, which is refrigerated.

Soaking the seeds in cold water for 6–12 hours immediately before placing them in cold stratification can cut down on the amount of time needed for stratification, as the seed needs to absorb some moisture to enable the chemical changes that take place. Do not keep seeds longer than 6-12 hours soaked.

Label the variety and date clearly on the bag.

Alternatively, the seed may be sown in small pots filled with moist soil and then the whole tray enclosed inside a plastic bag before placing inside a common refrigerator.

If seedlings start to sprout in the bag in the refrigerator, remove immediately and either plant in the ground or in pots until it’s time to plant outdoors.

The time you need to keep your seeds in the refrigerator depends on the variety, but 4-5 weeks should be a sufficient amount of time for most seed varieties.

As a general rule, the hardier the plant, the longer they need to cold stratify. 

Once there’s no more chance of frost in your area, take your seeds out of the fridge and sow as indicated.

The process of cold stratification helps the seed germinate and grow in your garden bed.

If you need to store your seeds for the next season kepp them in a dry place.

Humidity and warmth shorten a seed's shelf life, so the refrigerator is generally the best place to store seeds, but keep them far away from the freezer.