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The Garden Decoder: What Is ‘Cold Stratification’?

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The Garden Decoder: What Is ‘Cold Stratification’?

October 6, 2022

You’ve decided to up your plant game and bought some seeds for plants outside your comfort zone. You flip over the seed packet and find the term cold stratification and wonder if you’re in over your head.

Don’t return the seeds. 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.

So what is cold stratification, exactly?

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.

Echinacea purpurea ‘Magnus’ blooms on the High Line in New York City. Photograph by Elvert Barns via Flickr. The flowers&#8\2\17; seeds need to be cold stratified before planting.
Above: Echinacea purpurea ‘Magnus’ blooms on the High Line in New York City. Photograph by Elvert Barns via Flickr. The flowers’ seeds need to be cold stratified before planting.

Why not just plant the seeds before winter?

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. Cold stratification allows you to start the seeds’ germination process without relying on winter.

How do you cold stratify your seeds?

It’s as easy as flipping over the seed packet.

On the back of the packet, you’ll find the time and temperature needed to cold stratify the seeds. There are two kinds of stratification: dry and moist. Generally speaking at least a month in the fridge will do it for most seeds needing moist cold stratification. Just put the seed packet in the fridge. Best practice is to stow it away from stuff that could drip on it. 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. If you don’t have a garage or it doesn’t get cold enough, in the fridge it goes in a sealed plastic bag. After the correct length of time, you can plant the seeds as specified on the back of the packet.

Above: When sown, common milkweed seeds benefit from cold stratification. Photograph by Marie Viljoen, from Weeds You Can Eat: Milkweed Buds.

What are some common plants that need cold stratification?

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.

If you do want to get in over your head and are looking for a challenge, nothing beats the Himalayan blue poppy (Meconopsis). These poppies are known to be hard to grow. And like most poppies, they don’t like to be transplanted. They require three weeks of cold moist stratification in your fridge. And then a period of time under grow lights with a very specific temperature range. All timed so that they can be very carefully transplanted after the last frost. If you’re able to meet all of this particular poppy’s needs, you will be rewarded with an amazing and delicate sky blue flower that will capture your heart.

For more Garden Decoders, see:

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