Stratification is a method of carrying seeds through the period from the time they ripen until they are ready to germinate, with minimum trouble and occupying minimum space. Some seeds, especially those of certain trees and shrubs, will not germinate soon after they ripen, yet perish if they are permitted to dry. They need a storage period of several weeks or even months at low temperatures and in a moist, dark place before they are ready to grow. In nature these conditions are provided for by the seeds’ falling to the ground in autumn or early winter and soon being covered by forest debris, which assures darkness and prevents drying. Winter provides the necessary low temperatures.
Gardeners often find it convenient to stratify such seeds rather than to sow them immediately. Stratification provides the conditions the seeds must have without their taking up as much space or requiring as much attention as would be the case if they were sown immediately after they were gathered.
Stratification consists of mixing the seeds with sand or sand and peat moss that is just moist and storing them where they will remain moist and cool. A favorite way is to place layers of sand, or sand and peat moss, alternately with layers of seeds in wooden boxes, then bury the boxes 6 in. deep outdoors in a shaded place where the soil is well drained. It is a good plan to surround the boxes, before burial, with wire netting or screening to foil the attempts of rodents to dig up the seeds. If fine seeds are being stratified, they may be spread between layers of cheesecloth as they are buried in the sand, or sand and peat moss. The cheesecloth makes it easy to find the seeds when they are needed for sowing.
Yet another method of stratifying seeds is to mix them with slightly moist sand, or sand and peat moss, place the mixture in jars, and store these in a refrigerator.
Whichever means of stratification is employed, the seeds should be picked or sifted from the sand or sand and peat moss mixture and should be sown in the regular way in pots, flats, cold frames or outdoors when it is time for them to germinate.
The majority of seeds needing stratification germinate the spring following the summer or autumn in which they ripen, but some kinds will not germinate until one year later than that.
All seeds that are handled in this manner can be germinated by sowing in a cold frame or outdoors as soon as they are ripe, provided the seedbed is kept evenly moist and always favorable to the needs of the seeds.