Seeds stratifying in the fridge

I can now sit back and enjoy a cup of coffee, if not something a little stronger. (Although, the way Michael makes the double expresso, it is difficult to think of a stronger beverage.) I have sorted my seeds into those needing stratification and those not, and further sorted them by date that they need something done to them. All the seeds needing three months moist cold are now in their zip-top baggies in the fridge. The two native lilies that are hypogeal* germinators are in pots of soil, and the “challenging” species that need warm stratification before they need cold stratification are in baggies on top of the fridge.

Seed packets - mostly native wildflowers.
Seed packets – mostly native wildflowers.

One species defies attempts to tame it: whorled milkweed.  The sources I could track down on-line all implied it has to experience winter and this wussy fridge-time fakery won’t cut it. The whorled milkweed seeds are in a pot of medium in the fridge now. Next time we go to the farm, I will find a sheltered corner for it outside and cover the pot with snow and hope for the best.

Gardeners who do vegetables and annuals and perhaps a few of the easy-to-germinate perennials don’t do any of this. Planning the vegetables for a succession of crops and efficient use of garden space is its own art. I know because I do that as well.

Extraordinarily diverse, native perennials have seeds that will not germinate until they have been convinced they have been eaten by birds, planted by squirrels, abraded by waves, flung about by goldfinches,  pooped out by racoons, and/or experienced winter. Providing cold, moist conditions to mimic winter is the single most important technique for germinating seeds of native plants.

It might seem daunting if you have never done it before. Once you have done it a few times, and found that it works like a charm, you will consider cold-moist just another pretreatment that you take in your stride. Nasturtiums germinate better and faster with an over-night soaking? Fine. Good. Done. Chances are, you still think nasturtiums are easy to grow. An easy cold-moist-treatment seed to start with is the lovely butterfly milkweed (Asclepias tuberosa). I got an extra packet of butterfly milkweed seeds from the Ontario Rock Garden Society Seed Exchange, as a bonus for contributing to the exchange. This year, lots of organization are handing out seeds of this species, to raise awareness for monarch butterfly conservation. You might have a packet yourself.

This is what you do:

  1. Label a zip-top baggie with a waterproof marker:
  • Butterfly milkweed
  • C(90)
  • In cold: Feb 15 , out cold: May 15
  1. Place a folder paper towel inside the baggie.
  2. Put seeds into the fold of the paper towel.
  3. Spritz or sprinkle some water onto the paper towel so that it is damp but not sodden. There should not be any liquid water in the bottom of the bag.
  4. Close zip top. Place in fridge.
  5. Take out on May 15th and sow the milkweed seeds.

Easy. (Sorry about the two Step1’s. I cannot make WordPress put a bulleted list inside a numbered list.)

Lots of folks prefer a little damp vermiculite to damp paper towel. If you have vermiculite on hand, go ahead and use it. The C(90) code comes from Prairie Moon Nursery and it is the most sensible code I have found. In cold for 90 days – C(90). Right.

Butterfly milkweed seed is large enough to be able to handle individually. When seed is very, very small or when it has fluffy bits, as a lot of, say, aster seed does, it clings to the paper towel and is a pain to remove when the time comes to sow it. Use the vermiculite method for these.

Butterfly milkweed is a warm season grower and it will not sprout prematurely in the fridge. With other species, this is a definite risk. Each species has its own chill requirements and quirks. There is always much to learn.

* Delayed double hypogeal germination: To be discussed, but not today.

Baggies of seeds ready to go into the refrigerator.
Baggies of seeds ready to go into the refrigerator.

Published by

Trish Murphy

Artist: botanical, still life, and natural history illustration. Garden designer: native plants and naturalistic gardens