Stratification is a process of treating seeds to simulate natural conditions that the seeds must experience before germination can occur). Many seed species have an embryonic dormancy phase and generally will not sprout until this dormancy is broken). The process of stratification can be traced back to at least 1664, where seeds were layered (stratified) between layers of moist soil and the strata were exposed to winter conditions). Cold stratification is the most common type of stratification and is the process of subjecting seeds to both cold and moist conditions). Seeds of many trees, shrubs, and perennials require these conditions before germination will ensue. Cold stratification simulates the natural process by subjecting seed to a cool (ideally +1° to +3°C ). Seeds are placed in a medium such as vermiculite, peat, or sand and refrigerated in a plastic bag or sealed container). 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.

Dry stratification is another type of stratification and is a seed pre-treatment in which the seed is subjected to cold temperatures of 32˚F or lower, for a period of a month or longer. This simple treatment of keeping the seeds in cold, dry storage helps increase germination rates by imitating a natural winter dormant period. Many native prairie species require only dry stratification. Moist stratification is a process wherein the seed is mixed with moistened inert material (fresh sawdust, builder’s sand, vermiculite, peat moss, or similar material) and stored cold for ten days to three months. Planting the seed outside in the fall (direct seeding) achieves the same results as moist stratification.

The seeds of most annual garden flowers and vegetables require no stratification. However, many trees, shrubs, perennials, and wildflowers will only germinate after their seeds are stratified. If you sow these seeds outdoors in the...