Understanding and Successfully Carrying Out Seed Stratification

Many wild and native plants, such as red columbine or common milkweed, require a  cold stratification period in order to germinate in the spring. 

In northern climates, this simply corresponds to winter. Seeds must spend several weeks in cold and humid conditions to break dormancy, just as they would naturally outdoors. This combination of cold, humidity, and time signals the end of their dormancy and allows them to wake up once spring arrives.

 

Why Do Some Seeds Need Stratification?

In nature, many plants have developed mechanisms to avoid germinating at the wrong time. Germinating in the fall or too early in the spring could be fatal, which is why their seeds remain dormant until they have spent enough time exposed to the cold and humidity.

Artificial stratification reproduces this natural process. Depending on the species, this dormancy can be simple or more complex. Some seeds may even require the equivalent of two winters before they can germinate.

 

The Simplest Method: Let Nature Do the Work

If you sow in the fall, nature will take care of everything: cold, humidity, and the required amount of time. The seeds will naturally receive the stratification they need.

→ For more information on fall sowing, consult our article on the topic.

However, because many gardeners plan their sowing in winter for indoor seedlings in spring, these cold conditions must often be reproduced at home.

The Three Types of Stratification

In our catalogue, stratification needs vary from one species to another. Here is an overview of the different types of stratification to reproduce.

 

1. Simple Stratification (Cold Stratification Only)

Principle: A period of cold and humid conditions in the refrigerator for the duration indicated on the plant’s product page.
What this simulates: Winter.

 

2. Double Stratification (Double Dormancy)

Principle:

• An initial period of cold stratification for the duration indicated on the plant’s product page,
• followed by a warm stratification period of about 30 days,
• then a second cold stratification period for the duration indicated on the plant’s product page.

 

What this simulates: Two winters separated by a warm spell.

Important note: For species requiring a double cold stratification, it is also possible to carry out the first cold stratification in the refrigerator, then sow outdoors in the fall so that the second cold period occurs naturally during winter.

 

3. Warm Then Cold Stratification

Principle:

• A first warm stratification period for the duration indicated on the plant’s product page,
• followed by a cold stratification period in the refrigerator for the indicated duration.
   

What this simulates: The end of the warm season followed by winter, usually for species whose seeds mature early in the season and fall to the ground before the cold arrives.

 

How to Carry Out Stratification at Home

Regardless of the type of stratification required, the basic method remains simple and accessible.

 

What You Need

• A substrate that can retain humidity. Sand or vermiculite is ideal, although a clean potting mix can also be used. It is important to avoid mold developing during stratification. At Akène, we use vermiculite.
• Water, ideally filtered or left to sit for a few hours to allow chlorine to evaporate.
• A sealed container or a Ziploc-style bag.
• A pen or marker to label your seeds.

 

Preparation

The exact quantity of substrate is not critical; the goal is simply to keep the mixture humid and to prevent seeds from clumping together. We generally use a ratio of one part seeds to three to five parts substrate.

Spread your substrate in a bowl or shallow dish, then gradually add small quantities of water until it is humid but not soaked. Too much water can encourage mold during stratification. Sprinkle the seeds on the surface and mix well. 

Place the mixture in the sealed container or bag and refrigerate it for the duration indicated on the plant’s product page. Avoid starting stratification too early, as seeds kept in cold conditions for too long may begin to germinate in the refrigerator before the appropriate time.

For species requiring a warm stratification period, simply keep the bag at room temperature, out of direct light, for the duration specified for the species.

Planning the start of stratification is essential. The ideal approach is to choose a moment that will allow the stratification to finish around the time of the last frost in your region.

For example, for a 60-day stratification period in a region where the last frost usually occurs in mid-May, it is best to begin stratification in mid-March.

 

Do not forget to write down the species name and the start and end dates of stratification on your bag!

Monitoring Stratification

Check periodically to ensure no mold is developing and that the mixture has not dried out. If it becomes too dry, lightly mist it.

 

After Stratification

Once stratified, the seeds can be sown directly outdoors or started indoors in small pots. Refer to the growing notes for each species for more detailed guidance.

That’s it!