What Is Cold Stratification of Seeds?
Cold stratification is a period of moist, cold conditions that breaks dormancy in seeds adapted to winter. Many native perennials, shrubs, trees, and some herbs germinate more evenly after this treatment.
In nature, stratification occurs when seeds spend winter in damp soil or leaf litter and germinate in spring. The treatment is not simply refrigeration: the seed needs moisture along with cold. A typical home method uses a barely moist medium, such as clean sand, vermiculite, or a folded paper towel, held in a sealed but labeled container.
Most cold-stratification directions call for temperatures of about 33°F to 41°F. A refrigerator works well, but avoid a freezer because freezing is not the same as steady chilling and may damage some seeds. Keep packets away from ripening fruit, which releases ethylene that can affect stored plant material.
Required time varies widely by species. Some seeds need about 30 days, many need 60 to 90 days, and others need a warm period followed by cold treatment. Check a reliable species-specific source, ideally a native plant society, botanic garden, seed supplier, or Cooperative Extension publication, rather than applying one schedule to every seed.
To stratify in a paper towel, moisten the towel until it is damp but not dripping, spread seeds so they are not crowded, fold it, and place it in a labeled plastic bag or container. Check every one to two weeks for mold, drying, or early sprouting. If seeds begin to germinate, plant them promptly and handle the emerging root carefully.
Outdoor winter sowing can provide natural stratification in regions with consistent winter chill. Sow seed in a protected, labeled container with drainage holes and a suitable seed-starting mix, then place it outdoors where it receives precipitation but is protected from disturbance. This approach is less precise than refrigerator stratification but often works well for hardy native species.
Do not cold-stratify seeds that do not require it just because they are difficult to germinate. Some seeds need warmth, light, scarification, or a combination of treatments. Start a record with species, treatment dates, temperature, and germination results so you can refine the process for your climate and seed source.
Frequently asked questions
Can I cold stratify seeds in the freezer?
Usually no. Use refrigerator temperatures around 33°F to 41°F unless a species-specific source explicitly recommends a freezing treatment.
Must seeds be wet during cold stratification?
They need moisture, but not standing water. Use a medium that is evenly damp and check it regularly to prevent rot or mold.
What if seeds sprout in the refrigerator?
Plant them immediately in a suitable moist growing medium and provide the light and temperature conditions recommended for that species.
General guidance for US home gardens. Conditions vary by region and season; check with your local Cooperative Extension office for site-specific advice. Page updated Aug 4, 2026.