In horticulture, stratification is a process of treating seeds to simulate natural conditions that the seeds must experience before germination can occur. Many seedspecies have an embryonic dormancy phase, and generally will not sprout until this dormancy is broken. The term stratification can be traced back to at least 1664 in Sylva, or A Discourse of Forest-Trees and the Propagation of Timber, where seeds were layered between layers of moist soil and exposing these strata to winter conditions. Thus, stratification became the process by which seeds were artificially exposed to conditions to encourage subsequent germination.
Cold stratification
Cold stratification 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.
In the wild, seed dormancy is usually overcome by the seed spending time in the ground through a winter period and having its hard seed coat softened up by frost and weathering action. By doing so the seed is undergoing a natural form of "cold stratification" or pretreatment. This cold moist period triggers the seed's embryo; its growth and subsequent expansion eventually break through the softened seed coat in its search for sun and nutrients.
Process
In its most basic form, when the stratification process is controlled, the pretreatment amounts to nothing more than subjecting the seeds to storage in a cool and moist environment for a period found to be sufficient for the species in question. This period of time may vary from one to three months. To accomplish this seeds are placed in a sealed plastic bag with moistened vermiculite, which is refrigerated. Three times as much vermiculite as seeds is used. It is important to only slightly dampen the vermiculite, as excessive moisture can cause the seeds to mold in the bag. 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. After undergoing the recommended period of stratification, the seeds are ready to be removed and sown in the nurserybed for germination. Alternatively, the seed may be sown in small pots filled with moist soil and then the whole thing enclosed inside a plastic bag before placing inside a common refrigerator.
Preparing a stratifying medium
Many sources recommend using peat, a combination of peat and sand, or vermiculite as the medium for cold stratifying seeds. The medium must be sterile to prevent harm to the seed by pathogens including fungi.
Preparing the seed
The seeds should be cleaned of any additional material, but the shells of nuts should not be removed.
Any seeds that are indicated as needing a period of warm stratification followed by cold stratification should be subjected to the same measures, but 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. 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.
Use of a fungicide to moisten the stratifying vermiculite will help prevent fungal diseases. Chinosol is one such fungicide used to inhibit Botrytis cinerea infections. Different seeds should be placed in different bags rather than putting them all into one bag, and large quantities are also best split into several small bags. That way any fungal outbreak will be restricted to only some seeds. If no fungicide is used, a close check should be kept on the seeds, removing any which show signs of mold or become soft and with a decaying smell. If an outbreak of fungus occurs, remove the seeds and re-apply fungicide, then place them in a new bag with new slightly moistened vermiculite. Always keep the bag sealed. The stratifying seeds should be checked on a regular basis for either fungus or germination. If any seeds germinate while in the refrigerator, they should be removed and sown.
Sowing and seedlings
The medium/soil is not critical as long as the soil is light as well as lightly firmed down but not heavily compacted. Sterilized potting soil will minimize problems with Botrytis or Pythium fungal disease. These problems are much more likely to occur if air circulation is poor. Most seeds need only be planted at a depth equal to their own thickness in order to germinate. Seeds planted outdoors are best planted a little deeper to avoid disturbance caused by heavy rainfall. The soil should be slightly damp but never soaking wet, nor allowed to dry out completely. Most seedlings, whether grown in pots or beds, benefit from good air circulation which discourages fungus growth and promotes sturdy stems.
