Baby It’s Cold Outside: Cold Stratification

A clean milk jug can be converted into a mini greenhouse for starting seeds.

While there are many ways to propagate plants one of the most common is by starting seeds. There are four factors necessary for seed germination. Germination is accomplished when a seed soaks up water and the living embryo inside begins to grow. The other factors are light, temperature and oxygen. Not all seeds are equal in these requirements. Seeds have evolved different strategies, and some need a cold resting period before the ideal conditions arrive. To be successful in growing from seeds it is important to mimic each plant’s “ideal conditions”. The seed package or grower may give you this information, and some seeds just aren’t that fussy.

Horticulture is the study of the art and science of growing, designing, and enjoyment of plants. Gardeners are known to manipulate the growing environment to suit his or her purposes. We use all kinds of tricks to extend the season of our tomatoes or create a “microclimate” so that we can enjoy citrus or other tropical fruits and flowers. Some of the machinations to improve germination success include refrigeration (cold stratification), warming mats, covering seeds with peat moss or vermiculite, and/or using grow lights.

Since it’s winter and spring is around the bend, cold stratification has been on my mind. So far, I’ve had some good “beginner’s luck.” I have a hardy hibiscus that I’ve never been able to grow from seed. A friend visiting my house last fall told me about a technique that she uses to grow native seeds that she’s collected-legally and responsibly (more on that later.) Using a clean gallon plastic milk jug, you cut the top third horizontally, leaving enough for a hinge below the handle, making a mini greenhouse. Drill 4-6 holes in the bottom for drainage and fill 5-6” with moist potting media. Sow the seeds, cover, and tape the jug closed. I removed the cap so that there was air exchange. Check occasionally to make sure the media doesn’t dry out and keep the containers in a shady place. Ideally, the seeds will germinate when the temperature is right, but you do need to check on that too.

Even though every child alive planted sunflower or zinnia seeds in kindergarten to bring home for Mother’s Day, there are some important things to keep in mind. Always use fresh clean seeds. Some may store for centuries in an Egyptian burial urn, but that is probably the exception. Use sterile potting media, if possible, clean tools, surfaces, equipment and hands. Ethical seed collection is a thing. You should collect seeds with respect for natural plant communities, make sure that the seeds are mature, no more than 30% of a population, and only with permission from property owners. See more at https://livetoplant.com/best-practices-for-collecting-seeds-through-ecosourcing/

Timing is important, and I found a list of perennials that benefit from cold stratification. I have not verified all of these, but keeping a garden journal is helpful if you happen to be of that personality.

Rudbeckia spp. Black eyed Susan CS* 4-6 weeks before last frost and plant when average temperature is 65˚F
Nepeta spp. Catnip and catmint, refrigerate for 1-2 days and soak overnight
Echinacea spp. Coneflower, CS 4 weeks
Oenothera spp. Evening primrose CS 3-4 days
Heliopsis spp. False sunflower , 4 weeks CS
Solidago spp. Goldenrod 8 weeks CS needs light, don’t cover seeds
Hibiscus moscheutos Rose mallow, 2-3 months CS.
Agastache foeniculum Anise hyssop, 4 weeks CS.
Vernonia novaborensis Ironweed, CS 6-8 weeks
Eupatorium spp. Joe Pye weed CS 1-2 months
Delphinium spp. Larkspur, refrigerate 2 weeks