What is stratification? How do I give seeds a warm or a cold stratification period?
Stratifying seeds that exhibit delayed germination is an old and generally successful method of dealing with them that closely imitates natural procedures. In the past it was the practice to spread such seeds in flats in layers alternated with layers of slightly damp sand, and to store them over winter or longer, where they would remain moist in a cool cellar, cold frame, or similar place, until they were judged ready to germinate. Then they were sown in the ordinary way in pots, pans, flats, or cold frames. The availability of refrigerators made possible a simplification of this treatment, which still however is generally referred to as stratification.
Modern practice consists of mixing seeds that exhibit simple dormancy with two to three times their bulk of damp but not wet sand, peat moss, or vermiculite, enclosing them in a tightly closed polyethylene bag, and storing them in a refrigerator at temperature of 40 degrees F. from one to four months (depending upon the kind of seeds) before sowing. to promote germination of seeds that exhibit double dormancy, such as those of dove trees (Davidia), hawthorn (Crataegus), hollies (Ilex), junipers (Juniperus, viburnums, and yews (Taxus), put them in bags along with damp vermiculite or other material and store in a temperature 65 to 85 degrees F. for four to six months, then at 40 degrees F. for three months before they are sown.
Several variables, including species and sometimes the part of the natural range of the species from which the seed parent came, age of the seed, and conditions of storage may influence the length of time for stratification that is most favorable for germination. Because of this, it is often worthwhile to experiment a little. The most practical procedure is to collect the seeds as soon as they are ripe, clean them, and store them in a dry, cool place until such time as they can be stratified, so that the period of stratification ends at the most favorable time for sowing the seeds.
Courtesy of the NYBG Plant Information Service