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Cold Stratification.

Milkweed Seeds

If you collected any perennial seeds this past fall, now is a good time to start preparing those seeds for Spring.

As unpleasant as those cold winter nights can be, they provide many seeds with the signals they need to begin the germination process.

Cold stratification is done to imitate the moist, chilly winter weather that triggers a seed to sprout. Most perennial plant seeds require a combination of cold and damp to germinate.

Common varieties that require cold stratification for spring planting:

  • Catmint (nepeta)
  • Coneflowers (some varieties)
  • Heliopsis
  • Ironweed (Vernonia gigantea)
  • Larkspur (Delphinium)
  • Lavender (Agastache foeniculum)
  • Lupine (Lupinus)
  • Marsh Marigold (Caltha palustris)
  • Milkweed (Asclepias)
  • Prairie Coneflower (Ratibida)
  • Prairie Violet (Viola pedatifida)
  • Perennial Sunflowers (Helianthus)
  • Primrose (Oenothera speciosa)
  • Shooting Star (Dodecatheon meadia)
  • St. John’s Wort (Hypericum perforatum)

Paper Towel Stratification

  1. Spread the seeds in a single layer across double layers of damp paper towel. Wring the paper towel out so that it is not dripping wet.
  2. Make sure each seed has good contact with the damp towel.
  3. Insert the damp paper towel into a Ziploc bag or sealed container.
  4. Label the container with the date and seed variety.
  5. Place seeds in the back or bottom of your refrigerator where it is coldest. Do not place your seeds in the freezer.
  6. In most cases, after 30 days your seeds will be ready to be removed from the refrigerator and potted. If seeds start to sprout in the refrigerator, remove them immediately and pot them in a suitable medium until it’s warm enough to plant them outdoors.

Potting Soil Stratification

Instead of the paper towel, you can also dampen a small amount of your favorite potting mix until it can hold together in a ball. Mix the seeds into the mix (approximately 1 part seed to 3 parts soil) and then place them into a ziploc bag at the back of the refrigerator.