Winter Sowing: Before You Start

By Beverley Wagar, Volunteer, Royal Botanical Gardens

Before You Start

What is winter sowing?

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.”

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?

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 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: https://laidbackgardener.blog/2020/11/13/garden-myth-you-can-use-sterilized-garden-soil-to-start-seeds/

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.

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.

• 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