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Winter Sowing: Before You Start

January 17, 2023

By Beverley Wagar, Volunteer, Royal Botanical Gardens

Before You Start

What is winter sowing?

plastic jugs placed outdoors for a winter sowing garden
Winter sowing jugs and bottles on the author’s back deck

Winter sowing (or wintersowing or WS) is a way for home gardeners to germinate seeds outdoors in mini greenhouses made from recycled containers and bottles. It’s a somewhat controlled way to mimic what nature does when seeds drop in autumn, spend winter outdoors, and germinate naturally in the spring. The technical term for this process is “cold moist stratification.”

There are many ways to cold-moist stratify seeds. For example, they can be mixed with vermiculite and refrigerated in baggies. Or they can sown in-situ in a protected spot in the garden.

This article will look at the winter sowing method popularized on a long-gone Internet community called GardenWeb. GardenWeb’s winter sowing forum, led by Trudi Greissle-Davidoff in the 1990s and 2000s, brought together WS enthusiasts and long before social media came along. Although Trudi’s seminal web site has been offline for a while, she expects to bring it back.

Today, with increasing interest in making pollinator gardens, growing native plants, and replacing invasives, winter sowing is enjoying renewed popularity. These ecological benefits are what motivated these articles.

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Why winter sow?

It requires far less stuff. There’s no need for extra space, grow lights, trays, shelving, bottom heat, and all of the other paraphernalia that comes with starting seeds indoors. All you need is easy and regular access to an outdoor space—on your patio, deck, or right in your garden.

It takes the guesswork out of sowing in-situ. Have you ever scattered seeds in the fall and then, in the spring, wondered if the emergent seedlings are actually your desired plants or something else that blew in? Winter sowing is done in labelled containers so you know that what popped up is what you sowed. As well, the mini-greenhouses protect your seeds from marauding squirrels and hungry birds.

"Why Winter Sow" infographic

Winter sowing is an easy way to do cold-moist stratification. There’s no need to schlep baggies in and out of the ‘fridge or check on them daily. With conventional methods of cold-moist stratification, germinated seeds need to get planted ASAP, whether or not you’re ready. Winter sown seeds, in contrast, germinate in the spring with rising light levels and temperatures, as they would in your garden.

There’s no need to harden off seedlings. One of the drawbacks of indoor seed starting is the need to gradually acclimatize seedlings to outdoor conditions such as bright sunlight, cool nights, and wind. With winter sowing, seeds germinate when they’re ready. By the time they’re large enough to be planted out, they’re more than ready for garden conditions. Winter sown seedlings won’t be early but they will be robust.

It’s an inexpensive way to get lots of plants. Compared to buying plants (especially native species) winter sown plants are virtually free. Recycled clear or translucent containers (such as plastic jugs, pop bottles and tote bins) are free and can be re-used for many years. Seeds are inexpensive to buy (compared to plants) and, if you save or trade seed, they’re free.

shelving unit with trays of young seedlings
Wintersown native plants on shelves outside, late June 2022. Some are waiting to be pricked out; some are in pots for growing on.

It’s easy and fun! Whether you’re a novice or veteran, it’s always a woo-hoo! moment when the first tiny shoots pop up in the spring. In general, your mini-greenhouses take care of themselves. Up until the time gemination starts, they don’t need any attention from you. You can go on vacation in March (perhaps even April, depending on your climate) and not have to hire a seedling-sitter.

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Principles of winter sowing

Winter sowing has few hard-and-fast rules. Veterans, who’ve learned from experience what works for them, will do things their own way. Beginners may still be experimenting. We have our favourite containers, labels, soil mix, and record-keeping systems. Our outdoor spaces are all different. Our preferred supplies and materials may not be available or affordable in the place we live. There are many roads to success and many choices in terms of supplies and protocol.

Choices should be informed, though. They’re important because, if disaster strikes you need to wait a whole year before trying again. Sometimes you can rescue a failure if you have backup seeds and are willing to try again using fast cycling in and out of the ‘fridge. But, for a complete re-do, it’s a whole year of waiting. So it’s important to learn the basic principles.

Choose the right soil mix. You’ll want a finely screened medium for most seeds. For tiny seeds, use soil-less mix made especially for seed germination. The package will say “plug”, “germination” or “starter” mix. If you only have basic potting mix you can put it through a 1/4″ screen to get rid of the lumpy bits. To economize, try using the basic coarse mix for the bottom few inches and the super-fine mix on top, where the seeds rest.

For larger seeds, basic pro-mix, potting mix, or triple mix is usually acceptable. Commercial products vary widely in quality, so don’t shop on price. Triple mix can get heavy and it tends to become compacted easily so try cutting it with perlite or vermiculite. For details about these materials, here’s a comprehensive article on Tom Clothier’s web site.

To repeat some of the fine print in the above-linked article: you do not need to add anything to seed-starting mixes. They don’t need to have bio-stimulants, moisture pellets, mycorrhiza, or fertilizers—the seedlings will not be in there long enough to make use of these additives. One thing to look for, though, is a wetting agent, which overcomes sphagnum peat’s natural aversion to getting wet. If you’re an organic grower and want to adhere strictly to the standards, be aware that the wetting agent found in many commercial mixes may not be on the ‘allowable’ list. OMRI-listed mixes are available but you may have to look harder to find them.

If you’re really averse to a peat-based mix (and most contain peat) or if germination mix is unavailable, it is possible to use home-made compost if it’s screened. You will have to put it through a 1/4″ screen (or finer) to make it appropriate for native plants seeds or any other tiny seeds.

The question of sterility is contentious and a bit problematic. Most experts recommend not using soil or compost that comes from your garden. The late Larry Hodgson explains the conventional wisdom:

Let’s look at the main drawbacks of using garden soil for wintersowing:

  • You’ll get unwanted plants from whatever seeds are in the soil you dig up. Healthy garden garden soil has a healthy seed bank.
  • Garden soil is alive. Healthy soil contains between 100 million and one billion bacteria. This soil biome is crucial to plant health—not to mention life on earth. Soil organisms function in a complex network of relationships with other things, both living and dead. Without the full network of checks and balances, it’s likely that the ‘bad’ organisms (pathogens) will come out on top. A wintersowing mini-greenhouse is inherently artificial; it cannot replicate the complexity of a garden ecosystem. So garden soil offers little benefit for wintersown seeds.
  • Garden soil is often prone to compaction. Soils that are mostly clay lack the pore spaces that make air and moisture available to plant roots. Commercial seed-starting mixes are primarily organic matter, which provides the appropriate texture for seedling root development
  • Garden soil may contain eggs of invasive jumping worms. You may have them and not know it. Unless you plan to use every plant you grow in your own garden, do not risk transferring jumping worm cocoons (eggs). They are tiny—the size of poppy seeds—and virtually undetectable in soil.

On the other other hand, garden soil is basically free. If money is extremely tight you can try it—some gardeners do so with good success. For small quantities an Instant Pot can be used to sterilize compost or soil.

The depth of soil in the container should be about four inches. Anything more than five inches is a waste of seeding mix. The shorter the soil height, the faster the container will dry out. As well, if you don’t prick your seedlings out promptly, you may also end up with a dense, tangled mess of roots duking it out in a confined space.

Seeds need moisture. Moisten the mix before you fill the containers. Add tap water to the point where a squeezed handful holds its shape but does not drip. Once the seeds are planted, don’t allow the mix to dry out. The purpose of the ‘greenhouse’ cover is not to conserve heat or protect seedlings but rather to conserve moisture—it keeps condensation from evaporating. You’ll see your containers ‘fog up’ even in the winter. This is normal—it’s simply the soil moisture condensing.

During hot spells, even in winter, containers may thaw out for long periods and the soil surface may start to dry out A spray bottle is handy to moisten the soil surface but not dislodge tiny seeds—just point the nozzle through the largest vent (or bottle neck / jug spout) and spray a fine mist. If you’re using something that won’t fit a spray nozzle, you’ll need to open the lid in order to spray. You can bottom-water or drip water down the side of the container but don’t keep the soil sopping wet—plant roots need air and saturated soil has fewer pore spaces for air.

Handful of soil mixture
Germination mix is moistened before it goes into the container. Squeeze a handful. If it holds its shape and doesn’t drip, it’s wet enough.

Seedlings need air / venting / room to grow. Air must be allowed to escape from the mini-greenhouse or else the heat buildup, even after a few minutes of sunshine, will cause the soil and seeds to cook. Remember that hot air rises. If you’re using clear plastic as a greenhouse cover, cut several slits in the top. If you’re using a pop bottle or similar, leave the cap off and cut, melt, or drill several more holes in the top. If you’re using a grow-dome, cut holes in the clear plastic lid. The seedlings will need room to grow, so ensure there are several inches of “head space” between the soil surface and the greenhouse roof.

Containers need drainage. Containers must have holes or slits cut in the bottom so excess rain and water can drain out. If you’re using a large container to hold several smaller pots be sure to cut drainage holes in both the interior pots and the large exterior jug or tote.

If you’re placing your containers on garden soil or grass and you’re worried about slugs getting in through drainage holes, try lining the container with a coffee filter. Or use a clear tote bin bottom with drainage.

Plastic jug with a drainage hole
A soldering iron for fast and easy venting / drainage holes in plastic. Melting holes in plastic creates fumes– so do it outdoors and wear a mask.

The soil surface must receive sunlight—but not too much. When choosing a place to put your containers, look for a spot with diffused light, morning sun, or dappled shade. To prevent overheating you may want to put your containers in the shade until the soil mix is thawed and germination is possible. Then move the containers to a sunnier place where the gradually increasing light levels in the spring will promote germination.

Should you always use untinted transparent or translucent containers for your mini-greenhouse? If all you have is tinted plastic, go ahead and try it. Many winter sowers claim that tinted (red, blue, green) lids or containers work just as well as untinted. Anecdotal evidence aside, the science (on the connection between light colour and seed germination) tends to focus on indoor growing under artificial light. And most research on tinted greenhouse plastics is about plant growth, not seed germination. So there is no easy or sound answer about the benefits or drawbacks of tinted plastic on seed germination. Remember, though, that nature has been providing natural light for millenia. When in doubt, mimic nature.

The part of the container holding the soil mix can be opaque if you fill it to the brim and tamp it firmly to minimize settling. Remember that tall sides will cast shadows on the soil surface; seeds planted near the edges may germinate later than the ones in the center.

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What can I grow?

Any species of plant that is native to (or hardy in) your region is a good candidate for winter sowing. Many native species require cold moist stratification in order to germinate, so these ones go at the top of the list. For species that that are native or hardy in your region and do not require cold moist stratification, winter sowing will not harm them. After all, seed is how these plants reproduce. If they are native to a climate where winters are cold, there is a good chance they will do well with winter sowing.

Anything that re-seeds freely in your area is a possible candidate—so long as you know the name of the species and have done some research on its potential (or actual) invasiveness. You can winter sow certain vegetables and other annuals but remember that they won’t give you the head start you’d get with an indoor setup.

Each species is different in terms of the number of weeks (or months) of cold required. Some require more than one cycle of warm/cold (summer/winter) and some will germinate faster if their seed coats are abraded or nicked (scarified) or if they are soaked in water before planting. Some seeds require light (no soil cover) and some require darkness (complete soil cover). Check the seed packet instructions or a reliable online resource for these details.

Stored seeds have a shelf-life that is determined by both the species and the storage method. For information on storing seeds, read “Good and Bad Places to Store Your Seeds” on the Seeds of Diversity web site.

seed packets on table with file box
Seeds stored in glassine or paper envelopes. Packets list the species name, year collected, and site information. Seed storage: filed alphabetically by species name, stored cool and dry (upstairs closet on outside wall)
Examples of seed germination protocols infographic
This is the kind of germination information that a good online resource provides.
  • Below are some good sites to help you decide what to winter sow. Some resources specify soil temperatures and/or sowing depth, as well as requirements for the duration and number of cold cycles:
  • Tom Clothier’s site is an old but excellent resource for seed starting.
  • The Ontario Rock Garden & Hardy Plant Society (ORGHPS) has a comprehensive database of species and their germination protocols.
  • The Wild Seed Project is based in Maine (USA) but the listed species overlap considerably with other regions in eastern Canada and USA.
  • Prairie Moon is a US retailer of native plants and seeds. Many of their offerings are also native to Canada. Check on VASCAN or BONAP to check species’ native range.
  • Check out Kristi Walek’s excellent web site: Wild Plants from Seed
  • Dr. Deno’s work is for serious seed nerds
  • Connect with the North American Native Plant Society, your local Conservation Authority, social media groups for native plants, hort societies, and local seed libraries.

When should I winter sow?

In general, the earliest time to winter sow is just after seeds have naturally ‘hit the ground’ outdoors. Depending on your location, this could be as early as September and as late as December.

How late is too late? There’s no one-size-fits-all answer. Each species has a unique germination protocol. For some, periods of cold are irrelevant; they germinate readily as soon as the soil temperature reaches a certain level. For others, a considerable period (one, two, or three months) of cold is required. Some species require a warm period (summer) followed by a cold period (winter) for germination to occur. You’ll find a huge range of species that will germinate reliably, so don’t daunted if a few of them appear complex.

In Part Two of this series we’ll look at the step-by-step process and present tips for success. Part Three takes a close look at container choices.

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All photos, unless otherwise noted, are © Beverley Wagar under Creative Commons licence CC BY-NC-ND 4.0

About Beverley Wagar
Bev Wagar caught the winter sowing bug in 2005. She has 23 years experience making, tending, and learning about gardens. Since 2012 she’s been a volunteer at the Royal Botanical Gardens where she leads a volunteer gardening crew at the Helen M. Kippax Native Plant Garden.

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