Cold Stratification: How to Stratify Seeds in the Refrigerator

Cold stratification, also known as seed stratification, is the process of exposing seeds to cold and moist conditions to encourage germination. In nature, the stratification process takes place when fallen seeds overwinter underground or beneath a layer of snow. In the spring, the temperatures rise, thawing the ground and breaking the seed out of its dormancy period. The seed sheds its hard seed coat, beginning the germination process. You can mimic this process indoors to prepare seeds for propagation.

Depending on the type of seed, cold-moist stratification may be required to break the seed dormancy period. This is because the hard seed coat needs time to break down before the seeds can germinate and sprout. Many types of wildflowers, perennials, shrubs, and trees produce seeds that must undergo a cold period for some amount of time for seed germination to begin. Generally, native plants grown in cold climates will require cold stratification before you plant them in the early spring. Common seeds that require cold stratification include common milkweed, St. John's wort, and many coneflower species.

Follow these steps to cold stratify your seeds in the fridge.

1. 1. Place the seeds in a damp medium. Small seeds can be sprinkled onto a damp paper towel. Larger seeds should be placed in a moist medium such as peat moss or vermiculite. Ensure that the medium is moist but not soaking wet.
2. 2. Store the moist seeds in a plastic bag. Once your seeds are wrapped in a damp paper towel or planted in a moist growing medium, place them in a plastic bag. To prevent excess water from accumulating inside the bag, allow for some airflow by leaving the bag partially open or puncturing the bag with a few small holes.
3. 3. Place the bag in the fridge. Most seeds require about a month of cold stratification to increase germination rates. However, the exact amount of time needed for the stratification process will vary depending on the type of seed you choose. Consult the cold stratification timeframe suggested on the seed packet. Check on your seeds periodically and if they start to sprout, remove them from the bag and plant them.

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