What Is Stratification in Plants?

Winter Stratification

• Most often, stratification in botanical terms centers around the exposure of seeds to the naturally colder conditions of winter. The level of winter chill or duration of the cold varies widely among plant species, particularly those native to the temperate zones. The need for winter stratification prevents seeds from sprouting in fall when cold temperatures would kill the emerging, tender sprout. The cold temperatures, often coupled with soil moisture, softens the seed coat and makes it more receptive to sprouting once warmth or longer days return in spring.

Other Methods

• Some plant species' seeds need a warm stratification to germinate. This stratification may involve a hot, dry summer period followed by the return of more rain in the autumn or winter, or simply a long exposure to a warm soil in order to finally sprout. Do not confuse the term scarify with stratify. Many seeds need to be scarified to germinate, such as with a wildfire or exposure to the chemicals in smoke. Scarification usually involves nicking a seed or soaking it in water to soften the seed coat, making it easier for the embryo to sprout. Some seeds may require both stratification as well as scarification.

Usage

• Rather than having to wait for seeds to naturally stratify outdoors during a long winter season, the cold stratification can be mimicked in the refrigerator. Once seeds ripen on the plant, they are placed in a moist, porous medium such as peat or sand and sealed in a plastic bag to prevent premature dehydration. The seeds are then placed in a refrigerator to meet the plant species' threshold for effective stratification. Properly stratified seeds may be planted in a greenhouse at any time of year.

Cold Duration

• Stratification duration varies among plant species. Some plants may only need 30 days of temperatures below 40 F to then germinate, while others need three or more months. Some plants, such as Japanese maples, need multiple stratifications before seeds sprout. The seed must sit dormant atop the garden soil outdoors to experience two winters before sprouting. If a seed does not receive adequate cold stratification, it remains dormant until the threshold is met. The seed may still sprout later as long as the seed is not destroyed by mold or fungus or does not die after many years.