Stratified (botany)

In horticulture, stratification is the process of pretreating seeds to simulate natural conditions that a seed must endure before germination. Many seed species have an embryonic dormancy phase, and generally will not sprout until this dormancy is broken.

The term stratification can be traced back to at least 1664 in Sylva, or A Discourse of Forest-Trees and the Propagation of Timber, 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 conditions to encourage subsequent germination.

Cold stratification is the process of subjecting seeds to both cold and moist conditions. Seed of many trees, shrubs and perennials require these conditions before germination will ensue.

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 "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 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 seeds are placed in a sealed plastic bag with moistened vermiculite (or sand or even a moistened paper towel), which is refrigerated. Three times as much vermiculite as seeds is used. It is important to only slightly dampen the vermiculite, as excessive moisture can cause the seeds to mold in the bag.

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.

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Wikipedia