Seed dormancy is the state in which the seed will not germinate even in ideal conditions, before protective barriers are broken down. Many seeds have developed these protective barriers to endure unfavorable conditions and prevent them from being killed off by hazardous weather. Seed dormancy can aid plant survival in the wild, but can sometimes be a hindrance to those who want to propagate plants from seed at home.
Dormancy is caused by physical impediments such as the seed coat being too hard or the waxy layer keeping water from penetrating. In nature, this can be remedied by animal digestion, microorganisms, or freezing and thawing. Additionally, dormancy can also be caused by chemical inhibitors in the seed that can be broken down by temperature, moisture or light. The means in which these obstacles to germination can be broken down are known as scarification and stratification. Stratification uses temperature to break dormancy, while scarification uses mechanical, chemical, or thermal means to break down a hard seed coat that is impervious to water.
Stratification is the process in which a seed is exposed to moist, cold, or warm conditions in order to break dormancy. Many native wildflowers, especially those that produce seeds in the fall, undergo stratification naturally when they are exposed to the cold and wet of winter. They will then germinate in the spring when conditions are ideal. When planting seeds that require stratification at home, this natural process can be replicated by sowing the seeds directly in the ground in fall, or planting the seeds into pots and setting them outside through winter months before transplanting in the spring once they have germinated. What if you have seeds that you weren’t able to plant in the fall and it is now spring? Don’t worry! This process can be easily mimicked artificially. Seeds can be wrapped in a damp paper towel and placed in a plastic bag inside the refrigerator for the recommended period of stratification, usually between 30-90 days. An even better method would be planting the seed in a pot with potting mix at the recommended depth, dampening the soil (if moisture is required), placing a plastic film over the top to keep the soil moist, and then placing the pot in the refrigerator. Some common plants that require stratification to achieve high germination rate include: Milkweed (Asclepias), Perennial Sunflower (helianthus), Lupine (Lupinus), Prairie coneflower (Ratibida), and Black-Eyed Susan (Rudbeckia).
Dormant potted seeds undergoing stratification
Scarification is the process in which a seed’s hard coat is broken down to allow water to penetrate and the seed to germinate. In nature, this happens naturally when seeds are ingested by animals and pass through their digestive tract, effectively breaking down the seeds’ barrier to germination. Some plants need smoke or fire (thermal scarification) in order to germinate. Also, exposure to changing weather patterns such as ice and snow can trigger a seed to germinate.
In practice, to imitate mechanical scarification, we cause damage to the outer seed coat by cutting or nicking with an abrasive or sharp object such as a file, sandpaper or knife. Another method is by battering the seed coat by shaking or striking. This can be achieved by lightly tapping with a hammer or by shaking inside of a container. Some seeds can be scarified simply by soaking in warm water for 6-12 hours, or overnight. Commercial growers often scarify seeds by soaking them in a sulfuric acid solution for minutes or hours, depending on the species (chemical scarification). Finally, thermal scarification may take the form of boiling in water, smoking or utilizing fire to compromise the seed coat. Some examples of seeds that need scarification to germinate successfully and quickly are Nasturtium (Tropaeolum), Lupine (Lupinus), Morning Glories (Ipomoea), and Sweet Peas (Lathyrus).
Seeds, like many living organisms, have evolved and adapted to climate and environment in order to survive. They have developed protective mechanisms against harsh conditions so they can thrive during ideal conditions. To get the best results when planting our own seeds, we can replicate these natural processes in order for our plants to flourish. I and the rest of the Horticulture staff at Wellfield currently have several native plant seeds that are potted up and being kept cold to undergo stratification. This is in hopes that come spring, we will have many germinating into healthy, happy plants worthy of a botanic garden.