Cold-moist stratification is a winter in a bag. Many native seeds stay dormant until they’ve spent a set number of days cold and moist. Winter sowing does that outdoors. The fridge method does it in your refrigerator, so you can sow in spring instead.
Use it when you missed the winter window, when you’d rather sow in spring, or when you want to keep an eye on a small batch of seed.
How many days each number needs
| No. | Species | Prairie Moon code | Fridge days |
|---|---|---|---|
| 1 | Purple Coneflower | A | None required; 30–60 days evens out germination |
| 2 | Ozark Coneflower | C(60) | 60 |
| 3 | Black-eyed Susan | C(30) | 30 |
| 4 | Butterfly Milkweed | C(30) | 30 |
| 5 | Swamp Milkweed | C(30) | 30 |
| 6 | Common Milkweed | C(30) | 30 |
| 7 | Wild Bergamot | A | None needed |
| 8 | Lanceleaf Coreopsis | C(30) | 30 |
| 9 | Rough Blazing Star | C(60) | 60 |
| 10 | New England Aster | C(60) | 60 (or more) |
| 11 | Missouri Evening Primrose | C(60)* | 60 recommended |
| 12 | Golden Alexanders | C(60), G, M | Sow outdoors instead (see below) |
*Sources differ on No. 11. Prairie Moon codes it C(60); the Lady Bird Johnson Wildflower Center says no treatment is necessary.
Short on time? Prairie Moon notes that for seeds needing 60 days, one month may work for many species if time is a constraint. Expect fewer seedlings.
Golden Alexanders (No. 12) is the exception. Prairie Moon codes it “M”: best planted outdoors in fall, because artificial stratification may not work well. It also germinates best in cool soil. Sow it outdoors in fall or winter, or in early spring while the soil is still cool.
What you need
- Your seed
- Clean, fine sand (play sand works)
- A zip-top plastic bag
- A permanent marker
- Water
- A refrigerator that holds 33–40°F
Step by step
- Mix seed and sand. Use equal parts sand and seed, or slightly more sand than seed. For a small packet, a few spoonfuls of sand is plenty.
- Moisten it slowly. Add water a little at a time until the mix is damp but not wet. It should just hold together in a ball, and you shouldn’t be able to squeeze any water out.
- Bag and label. Seal the mix in the bag. Write the species, the start date and the finish date on it.
- Refrigerate at 33–40°F. Don’t freeze it.
- Check weekly. If seeds start sprouting early, sow them right away.
- Sow when the time is up, after the threat of a freeze has passed. Spread the whole seed-and-sand mix on bare, prepared soil or in pots. Press tiny seeds onto the surface; cover others lightly, no deeper than the width of the seed.
Count back from planting day
St. Charles County’s median last spring freeze is around April 10 (National Weather Service, 1991–2020 normals). To sow then:
- 60-day seeds: start them in the fridge by about February 9.
- 30-day seeds: start them by about March 11.
Later is fine too. Native seed can go in the ground through spring; just keep seedlings watered as the weather warms.
Fridge or winter sowing?
Both work. Winter sowing is less fuss and toughens seedlings from the start. The fridge method gives you more control and lets you sow on your schedule. Many gardeners do both: jugs outside in January, a few bags in the fridge for anything that arrives late.
Sources: Prairie Moon Nursery, germination codes and instructions, and “Germination Codes and Seed Starting Basics”; Lady Bird Johnson Wildflower Center (Oenothera macrocarpa, Zizia aurea); National Weather Service St. Louis, frost/freeze normals 1991–2020. Full links in our sources list.