Stratification (botany)

In horticulture, stratification is the process of treating stored or collected seed prior to sowing to simulate natural winter conditions that a seed must endure before germination. Some seed species undergo an embryonic dormancy phase, and generally will not sprout until this dormancy is broken. The time taken to stratify seeds depends on species and conditions; though in many cases two months is sufficient.

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 "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.

Stratification is the process of subjecting seeds to both cold and moist conditions. Typically, temperatures must be between 1°C and 5°C (34°F and 41°F). The term can be traced to at least 1664 in Sylva, or A Discourse of Forest-Trees and the Propagation of Timber, Vol. II. where seeds were layered (stratified) between layers of moist soil and exposing these strata to winter conditions. Thus, stratification became the process by which seeds were artificially exposed to cold-moist conditions between layers of soil or peat to encourage subsequent germination in spring. Seed of many trees, shrubs and perennials require these conditions before germination will ensue.

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 (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 or even a moistened paper towel) 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.

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