The Garden Decoder: What Is ‘Cold Stratification’?
You’ve decided to up your plant game and bought some seeds for plants outside your comfort zone. Cold stratification sounds complicated, but it’s a really simple (and critical) step in the germination process.
Featured photograph by Michelle Slatalla, from Growing Guide: 135 Rare Heirlooms from Jardin Seed Co.
It’s a process that artificially recreates the trigger for the seed’s natural germination process. The process is used for plants that need a cold period of time to germinate.
So what is cold stratification, exactly?
Echinacea purpurea ‘Magnus’ blooms on the High Line in New York City.
Photograph by Elvert Barns via Flickr.
Well, sometimes it doesn’t get cold enough, or cold enough for long enough—either because winters have trended warmer or you live in a climate where winters are always mild.
Why not just plant the seeds before winter?
How do you cold stratify your seeds?
It’s as easy as flipping over the seed packet. For dry stratification, if your area does get cold enough, you can put the seed packet in a metal (mouse proof) box in a heated garage.
When sown, common milkweed seeds benefit from cold stratification.
Photograph by Marie Viljoen, from Weeds You Can Eat: Milkweed Buds.
Plants that need dry stratificaiton are purple coneflower, bergamot, brown eyed Susan, showy goldenrod, and New England aster. Here are a few common plants that need cold moist stratification: common milkweed, New Jersey tea, rattlesnake master, cardinal flower, and golden Alexanders.
What are some common plants that need cold stratification?