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How to Cold Stratify Seeds for Your Butterfly Garden

Milkweed seeds for cold stratification

Cold Stratification Yields the Coolest Results

Cold stratification. It’s a scientific term for a simple procedure that improves your seed germination rate by 300-400%. Anything with that kind of success rate is worth doing, and this “spill and chill” operation is as easy as they come!

In essence, cold stratification mimics the winter season’s chilly, moist weather that triggers a seed’s sprouting from a dormant state. Most perennial plant seeds (such as native wildflowers) require this combo of cold and damp to germinate. In nature, this occurs–well, naturally, of course while the butterflies snooze or cruise for the winter! But you can accomplish the process yourself. The cold temperatures are as close as your refrigerator, and the moisture of the equation is supplied by water. By doing the cold stratifying yourself, you keep the young seeds safer from any animals that might eat them and make it less likely that they’ll succumb to rot or mildew from excess water. Germination is also accelerated.

There are possibly as many methods of stratifying the seeds as there are gardeners, but the key ingredients are moisture and cold. Why not experiment with two or three to see which works best for you? (For those with school-age children–is this not a perfect science fair project?)

The Paper Towel Stratification Method, or Viva the Bounty of Baby Plants! (best for small number of seeds)

  1. Spread the seeds in a single layer across two layers of damp paper towel. Not a sopping wet towel–a towel you’ve thoroughly doused with water (we use bottled drinking water), wrung out and spread out should be about right.
  2. Make sure each seed has good contact with the damp towel. Fold this seeded, moist paper towel up and place it in a Ziploc bag, covered plastic tray or even a glass jar.
  3. Label with seed variety and date, and place in the back or bottom of your refrigerator (no freezer).
  4. After the required amount of time (see plant description), your seeds will be ready for transfer to their new home–in the soil of a container or garden bed.

The Sand (or Peat Moss, or Vermiculite) Method of Stratifying Seeds (best for large number of seeds)

  1. Mix your chosen medium (sand, peat moss, or vermiculite) with just enough water (we use bottled drinking water) to moisten so that a ball can be formed. Here at Joyful Butterfly, our preferred medium is a coarse, washed, all-purpose sand. Very fine sand, such as is used for crafts, has caused us problems in the past.
  2. Mix your seeds into the medium. A ratio of about one part seed to three parts sand (medium) is a good guide.
  3. Place into a labeled, dated Ziploc bag. Place the bag into the refrigerator–in the back or at the bottom where it’s coldest, but not in the freezer! Remove after the recommended time (see each seed description for the time required to break dormancy).
  4. Plant indoors or outdoors within a day or so after removing them because as soon as they warm up they will be ready to grow.  If you leave them sitting around in the bag at room temperature then they will start to sprout!

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