Stratification is a cool damp storage period which allows the seed to prepare for germination and is a simulation of natural winter conditions. If the flesh begins to rot this will not harm the seeds.
Under natural conditions, very few seeds would germinate as soon as they fall from the parent tree or shrub. Most will spend the winter in a dormant state and this period is actually necessary to prepare the seed for germination and growth. Berries are eaten by birds and mammals and the seeds are voided in droppings - this provides very effective assistance to germination.
When seeds are stratified under controlled conditions, germination is more even and reliable. It should be noted, however, that up to 40% of acorns fail to germinate even after appropriate stratification.
To grow trees or shrubs in controlled conditions, it is necessary to provide this preparatory period via stratification.
- Containers such as small plastic drums, buckets, or shallow calf feeder type buckets may be used. These need to have holes drilled in their base and a layer of broken crocks or large stones for drainage.
- Seeds should be mixed with washed sand plus leaf mould, ground bark, or a peat substitute – a free draining mix, about one part seed to three parts sand. The best sand to use is sharp in texture which can be obtained from a builder’s supplier. It is free draining and the sharpness deters mice from digging out and eating your seeds. Never use sand from the seashore, which contains salt.
- The containers are filled with the seed/washed sand mix and a layer of washed sand put on top. They should stand out of doors in natural conditions, preferably in a shaded area and it is necessary to make sure they never dry out. Against a north wall is ideal.
Stratification is needed by most seeds, each species having its own requirements. For berries, it stimulates natural decomposition of the flesh, the presence of which actually inhibits germination.
Many seeds need a cold period before they can germinate. As sowing time draws near in February, you must check the stratified seeds for signs of germination. This is especially important if there has been a mild spell of weather (over 10° C). The seeds will look swollen and the tip of the radicle (primary root) will begin to show. Once germination begins in the containers it cannot be stopped. Seed development is rapid, so it is a matter of urgency to get the seeds out of stratification and into the seed bed in a day or two - you cannot leave them for longer, because the growing radicle is fragile and it must not be damaged when planting out. If in doubt, sow early rather than waiting too long!