What is Seed Stratification?
In order to successfully grow certain plants from seeds, an additional step is often required: seed stratification. This process involves mimicking the natural conditions necessary for germination, such as periods of cold or warmth, to break the seeds’ dormancy and allow them to sprout.
Understanding the Purpose of Stratification
The natural dormancy state of seeds occurs when the conditions for germination are not met. For some plants, this dormancy period ensures that the seeds do not germinate prematurely and risk being affected by harsh winter temperatures. This dormant state is essential for successful germination, and seed stratification is the process used to artificially break this dormancy and prepare the seeds for planting.
The Stratification Process
Seed stratification involves recreating the natural conditions necessary for germi nation. During stratification, seeds are placed in containers with layers of suitable substrates in order to create a stratified environment. The specific methods and timeframes for stratification vary depending on the plant species.
Using Refrigeration for Cold Stratification
Cold stratification is commonly carried out using a refrigerator, as it provides the ideal conditions for simulating winter in temperate climates. Here’s how to carry out cold stratification using a refrigerator:
- Rehydrate the seeds by soaking them in water for 24 hours. Optionally, coat them with wood ash to prevent rotting.
- Prepare an airtight container, such as a terrine with rubber seals, or resealable plastic bags.
- Fill the container with approximately 3 cm of fine sand, sifted soil, or seed starting mix (finer than regular potting soil). Alternatively, you can place a layer of moss, vermiculite, or moistened paper towel at the bottom of the container.
- Moisten the substrate without saturating it.
- Arrange the seeds in a way that they are not touching each other.
- Cover the seeds with another layer of substrate.
- If you have a larger quantity of seeds, repeat the process to create additional layers of substrate and seeds.
- Seal the container tightly and place it in the refrigerator.
- Regularly check the seeds for signs of germination and remove any seeds affected by mold or rot.
- In early spring, take the seeds out of the refrigerator and the container. You can use a sieve to separate the seeds from the substrate.
- Proceed with the planting process by sowing the germinated seeds.
Outdoor Cold Stratification
For certain larger seeds, cold stratification can be carried out directly outdoors in pots. This method is particularly suitable for trees with large seeds, such as stone fruits or pome fruits. Here’s how to carry out outdoor cold stratification:
- Choose a container, preferably made of terracotta, as it allows better moisture retention and prevents excessive rotting compared to plastic pots.
- Fill the container with about 3 cm of fine sand, sifted soil, or seed starting mix.
- Moisten the substrate without saturating it.
- Place the seeds in the container, ensuring that they are not touching each other.
- Cover the seeds with another layer of substrate.
- If needed, repeat the process by adding more layers of substrate and seeds.
- To prevent damage from birds, rodents, or cats scratching the surface, you can place a piece of wire mesh on top of the pot. Alternatively, you can elevate the pot to reduce the risk of pest damage.
- Place the container against a north-facing wall for the duration of winter.
- Regularly check the moisture levels of the substrate to ensure it stays moist.
- In early spring, check if the seeds have germinated and proceed with sowing them.
Hot Stratification
Just like cold stratification, hot stratification follows the same principle, but the seeds require a consistently warm environment of around 20-25°C (68-77°F) to break their dormancy. Here’s how to carry out hot stratification:
- Follow the same steps as for cold stratification, but instead of placing the container in the refrigerator, keep it in a warm room with a constant temperature.
Seeds That Require Stratification
Not all seeds require stratification. Typically, it is necessary for seeds of trees and plants that originate from colder regions and are accustomed to flowering in spring or summer, followed by seed production in the autumn. Some examples of plants that require stratification include fruit trees like cherry, almond, plum, peach, mulberry, apple, apricot, pear, walnut, quince, hazelnut, and chestnut, as well as climbing plants like kiwi and hops. Perennial plants like gentians, astrantias, columbines, aconites, echinaceas, eryngiums, primroses, and scabious also require stratification. Additionally, some trees like maple, ash, alder, and rowan benefit from stratification.
Alternative Methods for Breaking Seed Dormancy
In nature, there are various strategies for breaking seed dormancy. Some plants require exposure to extreme heat, such as from a wildfire, to trigger germination. These plants, known as pyrophytic plants, have seeds protected by thick and tough tissues that only open under extreme heat conditions. Examples of pyrophytic plants include cistus, bottlebrush (Callistemon), and Aleppo pine.
For other plants, damaging or softening the seed coat is necessary to break dormancy. This can be achieved by soaking the seeds in warm water for approximately 24 hours, as is the case with pea seeds. Alternatively, a more aggressive technique known as scarification involves manually scratching or sanding the seed coat. This method is used for seeds of acacia, carob, crown vetch, albizia, tetragonia, and baobab, among others.
Lastly, some seeds require exposure to an acidic environment to break dormancy. These seeds are typically consumed by birds, and the acidic conditions of the bird’s digestive system soften and break down the seed coat. You can replicate this condition by soaking the seeds in diluted vinegar for a few hours. Seeds of plants like cranberries, bilberries, amelanchiers, strawberry trees, currants, and sea buckthorns fall into this category.
