Which Seeds Need Cold Stratification
Cold stratification is weeks of cold, moist conditions that break seed dormancy. Which flower and herb seeds need it, how long, and how to do it outdoors...
Published by ThriveThyme
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Which Seeds Need Cold Stratification
Cold stratification is a stretch of cold, moist conditions, usually about 4 to 12 weeks near 33 to 41F, that tells a seed winter has passed. Many perennials and native wildflowers need it. Most vegetables do not.
Winter sowing in milk jugs stratifies seed for you outdoors. The Winter Sowing Desk puts these seeds first, 20 to 10 weeks before last frost.
Seeds That Need or Benefit From Cold
- Milkweed (common and swamp): about 30 days
- Columbine: about 3 to 4 weeks
- Coneflower: germinates more evenly after about 4 weeks of cold
- Black-eyed Susan: cold raises germination
- Lupine: nick or soak the hard seed coat too
- Lavender: cold helps slow, uneven seed
- Many native prairie and woodland perennials
Seed packets often list a code. On native seed, "C30" or "C60" means 30 or 60 days of cold, moist stratification. South Dakota State University Extension uses the same code.
Seeds That Do Not Need It
- Most vegetables: tomato, pepper, lettuce, kale, broccoli, beans, squash
- Most annual flowers: zinnia, marigold, cosmos
Hardy annuals such as poppy and larkspur do not strictly need cold, but they sprout best in cool soil, which is why they winter sow well.
Two Ways to Stratify
Outdoors in a jug
Sow in a milk jug in early winter and set it outside. Rain, snow, and freezing nights do the work. See how to winter sow in milk jugs. This needs real cold, so it works best in zones 3 to 8.
In the fridge
- Mix seed with a little damp sand, vermiculite, or a damp paper towel.
- Seal it in a labeled plastic bag.
- Keep it in the fridge, not the freezer, for the days the packet lists. Check that it stays damp, not wet.
- Sow after the time is up, or as soon as you see sprouts.
Use the fridge in zone 9 and warmer, or if you missed the early winter window.
Common Mistakes
- Dry cold does not count. Seed must be moist.
- Sowing stratify-first seed in late spring. It may sit until next year.
- Freezer storage instead of the fridge for moist seed.
Plan jugs by your frost date on the Winter Sowing Desk. Check old packets on the Seed Viability Check.
Sources: South Dakota State University Extension, Winter Sowing in the Northern Great Plains; Penn State Extension, Starting Seeds in Winter.