In horticulture, stratification is the process of pretreating seeds to simulate natural conditions that a seed must endure before germination. Many seed species have what is called an embryonic dormancy and generally speaking will not sprout until this dormancy is broken.
For seeds of trees and shrubs from temperate climates, stratification involves soaking and chilling seeds prior to sowing. This simulates natural conditions where the seeds would remain through a winter on cold, wet ground. Seeds will usually germinate promptly and uniformly after stratification. Unstratified seeds may take up to two years to germinate, if they do so at all.
In the wild, seed dormancy is usually overcome by the seed spending time in the ground through a winter period and having their hard seed coat soften 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.
In its most basic form, when the cold stratification process is controlled, the pretreatment amounts to nothing more than subjecting the seeds to storage in a cool (ideally +1° to +3°C; not freezing) 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 you merely place the seeds in a sealed plastic bag with moistened vermiculite (or sand) and refrigerate it. Use three times the amount of vermiculite as seeds. It is important to only slightly dampen the vermiculite, as excessive moisture can cause the seeds to grow mouldy in the bag. As such, err on the side of drier rather than wetter. To give an idea, it should not be possible to squeeze any dripping water out of the vermiculite.
After undergoing the recommended period of cold stratification, the seeds are ready to be removed and sown in the nursery bed for germination.
Many sources recommend using peat when cold stratifying seeds in the belief that peat is highly sterile and pathogen free. This has not proven reliable, and peat is best avoided. Its use often leads to fungus growing on the seeds, particularly if too much moisture and/or no fungicide has been applied. The fungus can either kill the seeds or attack the emerging root. Current advice is to use vermiculite or sand which is sterile, inexpensive, and readily available.
Preparing a stratifying medium
Use of a fungicide to moisten your stratifying vermiculite will help prevent fungal diseases. This should be used as stipulated by the fungicide manufacturer. (Note: a convenient and safe fungicide is Chinosol. It is primarily a disinfectant, non-toxic, and can be used in solution from 0.1% for prevention, to 0.5% for treating an existing infection. It is often recommended for growing succulents from seed, which are generally prone to mold, so it should work well also for any other seed germination. It is not usually sold as a pesticide, but is available from vintner suppliers, as it is used for wine making in small quantities.)
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 mould 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.
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.
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 in stratification.
The time taken to stratify seeds depends on species and conditions; in many cases two months is sufficient to break the seed dormancy. After undergoing the cool moist treatment the seeds are ready to plant and will usually sprout in a few days to weeks.
*Some plants requiring cold stratification are also parasitic. Betony (Pedicularis procera) and Indian paintbrush (both sold on this site) will germinate and grow, but until they find a host plant that they can latch their roots to and draw nourishment from, they'll never become mature and bloom. Most horticulturists recommend slow spreading grasses as host plants, such as Phalaris aquatica for this.