Cold stratification: why and how to germinate seeds?

Introduction

Some seeds refuse to germinate at room temperature. They need a natural signal to come out of dormancy. This signal is cold. Cold stratification reproduces the passage of winter to trigger the germination of so-called "dormant" seeds.

Used in permaculture, forest gardening, or to grow certain perennials, this simple technique allows you to germinate picky seeds while respecting their natural rhythm.

1. What is cold stratification?

Cold stratification is a method that involves exposing seeds to a period of moist cold (simulating winter) to break their dormancy. It mimics natural conditions: seeds fall to the ground in autumn, undergo winter, then germinate in spring.

Why do some seeds need cold?

Seeds of many perennial plants, trees, shrubs, or medicinal plants have a natural protection against early germination. Cold gradually breaks down chemical germination inhibitors and softens hard seed coats.

2. Which seeds to stratify cold?

• Tree seeds: hazel, oak, maple, apple, cherry…
• Perennial plants: lavender, St. John's wort, valerian, borage, echinacea…
• Medicinal plants: yarrow, arnica, common vervain…
• Flowers: poppy, columbine, foxglove, bellflower…

Seed packets often indicate "stratification required" or "sow after cold".

3. Method 1: fridge stratification (indoors)

Necessary materials

• A zip bag or a small airtight jar
• Sand, vermiculite, or a damp paper towel
• A refrigerator (about 4 °C)

Steps

1. Mix the seeds with slightly moistened sand or paper towel.
2. Place everything in a bag or airtight container.
3. Note the date and the name of the seeds.
4. Put in the refrigerator for 3 to 8 weeks depending on the species.
5. Monitor germination: some seeds will even germinate inside the bag.
6. Replant as soon as a root appears or at the end of the cold period.

4. Method 2: natural stratification outdoors

Principle

This is the most natural method: you sow the seeds directly in pots or in the ground in autumn, and let the winter cold do its work.

Tips

• Use deep pots with good drainage.
• Cover the seedlings with a light mulch to limit evaporation.
• Place the pots outside sheltered from heavy rain.
• Germination will naturally occur in spring.

5. How long does stratification last?

The duration varies depending on the species:

• Medicinal plants: 3 to 6 weeks
• Tree seeds: 8 to 12 weeks
• Flowers: often 4 to 6 weeks

Always refer to the seed packet instructions or a reliable guide.

6. Mistakes to avoid

• Do not over-moisten the substrate (risk of mold)
• Forget to check germination during the cold period
• Use a non-aerated substrate or without drainage

Conclusion

Cold stratification is a simple, economical, and effective method to awaken dormant seeds. It respects the natural cycle of plants and significantly increases germination rates of sensitive species. Try this technique on your medicinal or perennial seeds: guaranteed success in spring!