Cold Stratification

Cold stratification is a natural process that certain seeds require to break dormancy and germinate. It involves subjecting seeds to a period of cold temperatures, usually around freezing or slightly above, for a specific duration of time. This cold treatment mimics the winter season and helps to break down the seed coat and trigger biochemical changes within the seed that are necessary for germination to occur when conditions become favourable.

It is often necessary because the seeds have evolved mechanisms to prevent germination during warm seasons when conditions may not be suitable for seedling survival, such as during hot summers or dry spells. If you don’t stratify seeds, they will still germinate but the success rate is often much lower, and they can take 2 or 3 times longer to germinate.

There are two ways to stratify seeds. One is dry and the other is wet.

Dry

Seed packs can be placed in the refrigerator for 4-8 weeks. The temperature should be near 0° in theory, but most refrigerators are several degrees above freezing and that works. Buy your seeds, that need stratification, a few months early and toss the seed packs in the refrigerator. Take them out about 5 days before planting and let them feel the warmth of your home.

Wet

Plant your seeds in a moistened sand in a container and place it into the refrigerator exposing the whole set up to a cold period. You can also place the seeds in a bag on a damp paper towel. As with the dry method, take them out about 5 days before planting and let them feel the warmth of your home.

Vegetables that benefit from cold stratification

1. Artichoke
2. Asparagus
3. Broccoli
4. Brussels Sprouts
5. Cabbage
6. Carrot
7. Cauliflower
8. Celeriac
9. Celery
10. Collard Greens
11. Kale
12. Kohlrabi
13. Leek
14. Lettuce
15. Mustard Spinach
16. Onion
17. Parsnip
18. Pea
19. Radish
20. Spinach
21. Silverbeet
22. Turnip

Herbs that benefit from cold stratification

1. Basil
2. Catnip
3. Chamomile
4. Chervil
5. Coriander
6. Dill
7. Echinacea
8. Fennel
9. Hyssop
10. Lavender
11. Lemon Balm
12. Lovage
13. Marjoram
14. Oregano
15. Parsley
16. Rosemary
17. Sage
18. Savory
19. Thyme
20. Valerian

Flowers that benefit from cold stratification

1. Black-eyed Susan
2. Poppy
3. Larkspur
4. Marigold
5. Pansy
6. Sweet Pea
7. Viola
8. Wallflower