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Sure, we just buttoned up the garden for the winter, but in just a few months, you’ll be starting to grow seeds again. Some of those seeds require cold stratification, which is exposing the seeds to a period of cold temperatures. Usually people fake this cycle of cold temperatures by leaving the seeds in the refrigerator, but you could free up the fridge space by simply leaving the seeds outside, since the cold fridge is meant to simulate the winter anyway. I’ve seen various methods of doing this using empty milk containers, buckets, and ice cube trays—but I recently stumbled across a method from an Instagram gardener that I’m convinced is far better.
If you consider how plants grow in the wild, where plants drop seeds in fall, and then grow again in spring, you understand that those seeds are exposed to the rain, snow, and cold temperatures through the winter. It isn’t just that they survive that cold; some seeds quite literally need that cold blast to germinate in spring. Temperatures as high as 39-40°F are sufficient for this process, so few environments won’t reach that range in winter. While there are no hard and fast rules around what kinds of seeds require this cold treatment, most flowers and herbs do, although some perennials like asters and nasturtiums, and some annual species like zinnias, do not. While most vegetables don’t, a few—like peas and artichokes—do. While many lists of herbs and flowers requiring stratification exist, none are exhaustive. You can often find this information on the seed package, though, or simply search for the name of the plant and “cold stratification.” It’s safest to assume your perennial flowers and herbs do need it.
In some cases, the seeds you purchase in spring might have already been cold stratified, but that’s not usually the case. So for years, gardeners have faked that cold by using the refrigerator, and it is, for sure, a controlled experience—the fridge stays consistently one temperature. On the other hand, you could simply throw your seeds on the ground in fall and let nature take its course—but that exposes the seeds to other threats like the wind, birds, and other wildlife.
My favorite way to cold-stratify seeds outside
But this method, which is featured on the appropriately named Instagram account Wintersowing, by a gardener named Elisabeth, takes the best of both worlds by creating a semi-controlled environment outside for the seeds. What I like about this method is that it requires little in terms of equipment, and the materials are all reusable. At a basic level, you’re seeding a handful of seeds in potting soil in a Ziploc bag, with drainage, that is propped up off the ground and held upright. The seeds experience everything the winter has to offer and will begin to sprout on their own when the weather is appropriate to do so. You can then carefully separate the sprouts and replant them. This method means that the seeds have been exposed to—and survived—the actual environment and weather they’ll grow in, meaning the seeds that grow are more resilient to your growing situation. They don’t require any hardening off, since they were grown outside; you could simply transplant the seedlings into the ground in spring.
For each variety of seed you’ll grow, you’ll need one bag: Using gallon-sized zipper bags with a slider, cut a few small slits in the bottom of the bag as seen in this video. This is for drainage. Label the bag (most zipper bags have a window for this) with a permanent marker. Add a few cups of potting soil and wet the soil down so it is moist, not sopping wet. You can squeeze the bag to get rid of excess water. Holding the bag upright, add your seeds to the top of the soil. The seed package will tell you how deep the seeds should be, and you should still follow those directions. For instance, if they say the seeds should be an inch deep, sprinkle in the seeds and then add another inch of soil to the top to cover them.
You’ll need to find a place to prop up the bags—the space between two raised beds is perfect for this, but you can also build a spot using some pavers or bricks. You need two walls about a foot high from which you’ll suspend the dowels. In the Instagram videos, Elisabeth is using marshmallow sticks instead of dowels, which you can buy in a set.
The bags need to remain open a little so rain water and snow can get into the bag, but they shouldn’t be left completely open. Elizabeth zips the bags mostly shut, leaving a few inches open at one end. That end gets a clothespin clipped to the end of the bag. She then pushes the first dowel through the zippered end of the bag, near the top. A second dowel is laid parallel to the first, near the other end of the bag, and the clothespin is clipped to the dowel and then the bag, effectively pinning the bag to the dowel. You can see the finished result in this post.
Leave the bags out to experience weather
Whether they get covered in snow or frozen, the seeds should be fine. What is meant to germinate will sprout in spring. Elisabeth shows her seeds experiencing the weather in her garden through all kinds of conditions. The only thing you want to check is that the bags remain upright, with exposure to the air via the opening in the bag.
A zipper bag acts like a tiny greenhouse, trapping moisture and temperature. In that way, it’s quite effective a place to grow your sprouts, although obviously they will outgrow their temporary digs. For flower seedlings, most should survive being directly transplanted from the bags to the soil outside. Remove the entire clump of soil from the bag, which will be held together by the roots of the seedlings, and then carefully break apart the clump into individual plants and get them in the ground. The bags, dowels and clothespins can be saved to use again next year.
