Winter Sowing – Part 2

jugs with inner tube as strapping

A step-by-step guide to success

This article (part two in a three-part series) presents how-to information in detail and in sequence, based on my experience and knowledge. If you want to experiment or try some of the many ideas found on YouTube, go ahead but be sure to keep some seeds in reserve for a second attempt. And if you’re trying something really unconventional, I encourage you to do a backup container of the same species, just in case.

If this is your first encounter with winter sowing, please read Part One in the series first.

Checklist for all-in-one type containers
Checklist for all-in-one type containers
Checklist for big-top type containers
Checklist for big-top type containers

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Prepare containers

If you don’t already have your containers on hand, skip ahead to part three of this series “Container Choices” and learn about the many options for pots and mini-greenhouses. But if you’re ready to go with your jugs and bottles, read on.

Whether you are using an all-in-one container (such as a large plastic pop bottle) or a ‘big top’ jug that holds several smaller pots or tubs, you will be sowing only one species per pot. Here’s why:

Each species germinates at a different time. If you have more than one in a single pot, you likely will be pricking out seedlings from one section while other sections have not germinated. Even if you manage to remove the ‘ready’ species without disturbing the other(s) you must now devote the original space to half the plants. As well, pricked-out sections will have less mix (since it gets taken along with the seedlings) leading to to unsupported ‘walls’ and messy collapses.

photo of large container will with germination mix, divided into sections with cardboard
Dividing large containers can causes probems. Dividers get soggy if they’re cardboard, like these. Sections germinate at different times, requiring awkward pricking out and empty sections that can cause others to collapse. Plus– species markers usually don’t stay put.

Another drawback to sowing multiple species per pot is managing the markers or section dividers. No matter what system I tried, the marker stakes, labels, and dividers always failed. They got mixed up, lost, dislodged, attacked by squirrels. Once I was seed-sitting for a vacationing friend who had taped the species names to the side of the greenhouse tray that held a full-size plug tray. I removed the plug tray in order to bottom-water—and I replaced it backwards. Another time I used card stock to make custom section dividers in a big tray of mix. Everything looked good until April when they promptly turned to cardboard mush.

If all you’ve got are big tubs or rubbermaid-type totes, don’t be tempted to fill them with mix and sow multiple species in them. Instead, take the time to sow into individual smaller tubs or pots, and nestle those inside the bigger container. To learn the advantages of this type of setup read the third article in this series Container Choices.

a big-top setup in a very large jug
How to use a really big jug. The hinge can go on the side (as shown) or in a corner.

Cutting plastic is an art. No matter how often I do it, I just can’t cut straight without a guide line. They’re easy to make: lay a marker across a pot (of the desired height), move it so the tip contacts the container, and rotate the container.

Then I melt a pilot hole anywhere on the guide line, insert the tip of my heavy-duty scissors, and carefully cut around. I find it impossible to cut a straight line with a knife, even with a guideline.

hourglass-shaped coke bottle showing cutting lines
Hourglass-shaped coke bottle showing how to mark cutting lines. Turn the bottle, not the pen.

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Assemble supplies

Have your supplies ready (see list below). Clean and prepare all your containers now, before you start futzing with seeds. If you’re doing a lot of species you need to be systematic about this, believe me. Getting all the prep work done in advance is a must, especially if you’ve commandeered the dining room table.

supplies on the dining room table
dining room table with winter sowing supplies. Note the waterproof surface covers.

Supplies list for container prep:

– clean and dry containers of your choice (do you have enough?)
– drill, knife, soldering iron or other cutting method to make vent and drainage holes
– heavy-duty scissors
– rags or paper towels
– water spray bottle with an adjustable nozzle for a mist / fine spray
– something to write with: an ‘industrial’ Sharpie, paint pen, china marker,etc
– white or yellow electrical tape for labeling containers
– sheathing tape (‘Tuck Tape’)
– waterproof, transparent, UV-resistant tape (greenhouse repair tape or ‘T-Rex’ type)
– straight-edge for leveling mix. I use the handle of wooden spoon
– something to use as a dibble. I use a bamboo BBQ skewer or a chopstick
– a small metal mesh sieve / tea strainer for covering seeds with a fine dusting of mix
– tub or large bowl to hold seeding mix
– something to hold a supply of water for moistening the seeding mix

Drills, knives, soldering irons

If you’re not using nursery pots, cell packs, or plug trays, you’re probably facing the challenge of making holes in plastic. Here are the pros and cons of a few methods.
knife:
– They can be dangerous. The points slip and skid on smooth plastic.
– You need a really sharp, narrow blade. It will become dull very quickly.
– Knives tend to create slits, not holes. Try cutting a ‘V’ shape and pulling out the point to create an actual vent.
– Using a knife is slow.
electric drill:
– Unless you make a dimple in the plastic where you want the hole, drills will skid and slide just like knives do.
– Drills often make jagged, messy holes and it’s hard to get one the right size with a normal set of bits.
– A drill is faster than a knife.
heat (soldering iron, woodburning tool, hot metal rod):
– Whether you use a soldering iron, a metal rod heated over a stove element, or the tip of a heavy-duty hot glue gun, or a woodburning tool, melting plastic creates fumes that require being outdoors, on a breezy day, wearing a mask. Smoke from hot plastic is a serious issue. Do not melt plastic indoors, even in a well-ventilated room.
– Soldering irons are inexpensive (less than $20) and they’re the fastest way of making holes if you have a lot to do.
– I clean my soldering iron after each container by rubbing it on a stainless steel pot scrubber.
– Small craft-type glue guns don’t get hot enough to melt pop-bottle plastic or translucent jug plastic. They do, however, work on ‘clamshell’ plastic and they probably work on mushroom tubs, although I haven’t tried it.

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Prepare seeds

Getting your seeds ready involves cleaning them (removing chaff and the ‘non-seed’ parts), packaging them so they can be properly stored, labeling them with the collection date, source, and species name, and tracking them on a spreadsheet or list.
Here are some reliable resources on seed collecting, handling, and cleaning for the home gardener:

https://www.wildplantsfromseed.com/html/guidelines.php

www.cleannorth.org: native plant seed collection & cleaning

https://blog.cwf-fcf.org/index.php/en/10-tips-for-collecting-native-seeds/

Seeds of Diversity: https://seeds.ca/d/?t=bb17f80f00003738

Box of seed packets filed alphabetically
Seed storage and filing. Packages marked with species name, year collected, and source/site

From your collection, choose which species you will sow in the time you have available (I do mine in batches of ten) and list the names, either on paper or directly into your spreadsheet. Then research (and record) the germination requirements for each species—they’re all different. Here is the key information to look for:
– Is pre-treatment necessary? Some seeds have hard outer shells that require scarification or soaking. Some require moist storage.
– How deep should they go? Should they be gently pressed into the surface of the mix or should they be covered?
– Are multiple or complex cold-warm cycles required to break dormancy?
– How many weeks (months?) of cold are recommended?

Although reliable resources will state ‘surface sowing’ or ‘requires light to germinate’, germination protocols are often silent on planting depth. You may be left wondering exactly how much cover, if any, to provide your seeds. In general, the smaller the seed, the less cover it needs. You simply need to find out (and write down) the specific cover/depth requirements for each species.

A quick note on record keeping: If you’re not a paper-and-pen person, have your smartphone or laptop handy to keep a spreadsheet of the species you’re sowing and any notes about unusual germination protocols.

As you gain experience with winter sowing you’ll develop your own system to get all your seeds sown without mistakes. Over the years I’ve made lots of them. I’ve accidentally sowed two species in the same tub, forgotten to actually sow the seed, and sown the same species twice. My current system is pretty simple. First I remove the ten packets I intend to sow right now, listing them by hand along with notes on any special protocols based on my research. Then, one at a time, I choose a container, fill, sow, tape, and label. Before setting it outside I check the species off my list. If there are leftover seeds I put a check mark beside the year on the packet and re-file it. When all ten packets are finished I transfer the information to my digital inventory spreadsheet, noting the date completed.

Here are some reliable resources to help you learn more about seed germination:
– If you’re using commercial seed, the packet (or the vendor web site) will have sowing instructions.
– Tom Clothier’s siteat www.tomclothier.hort.net is filled with detailed information and really good articles like this one on the connection between soil temperature and germination .
– The ORGHPS at https://onrockgarden.com has an excellent database.
– Prairie Moon ( https://www.prairiemoon.com/plants/ ) is a US-based retailer with good database of plant information that includes detailed germination protocol
– Wild Seed Project is based in Maine (USA) but the germination guide is useful for most of eastern Canada and USA https://shop.wildseedproject.net/#seedtable
– Wild Plants from Seed (wildplantsfromseed.com) is a fantastic site, not super comprehensive on Carolinian species, but the one of the few resources that pays attention to species’ native ranges, ecosystems, and communities.
– Dr. Deno’s work: https://naldc.nal.usda.gov/ (search term “Deno”) is for incurable plant 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 for help finding seeds native to your area.

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Prepare soil mix

Soil mix is covered in more depth in the first article. Here we’ll cover the process of getting your soil mix ready to use.

You’ll need a tub or a big bowl that will hold a few litres of moistened, de-clumped mix. I also have a second, smaller bowl to hold dry mix and my metal mesh sieves, for lightly sprinkling soil on top. More on this later.

First, moisten the mix by pouring water (tap water is fine) over it and smooshing it with your hands for a few minutes. I usually allow it to sit and absorb the water while I do something else, but you can keep smooshing until all the water is absorbed and the mix is uniformly moist.

Determining the amount of water to add takes a bit of practice. You will be surprised at how much water a peat-based mix can hold, so don’t try to use a spray bottle. You’ll probably need at least a litre.

The mix should be wet enough to form and hold a clump when you squeeze and release it in your fist but not so wet that it drips.

It’s easier to massage out any clumps when the mix is moist.

Germination mix is moistened before it goes into the container.
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.

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Fill containers

I fill my containers over the tub so any excess simply falls back into the pile.

Fill containers one at a time. Trying to do an assembly line, in my experience, leads to mistakes. Pots get missed. They get seeded twice. Seed packets and labels get mixed up. Your brain may be more methodical than mine, but I find that the slightest distraction can completely mess up my plan for a streamlined process.

Fill gradually, tamping the mix evenly and gently as you go. This will reduce the amount of settling that happens when the pots encounter snow or rain. Even with tamping you can expect up to 1/2″ of settling over the pot’s time outside.

Overfill the containers, give a final tamping, then use a straightedge to flatten the mix at the level of the container rim. I use a wooden spoon handle but you can use anything that lets you level the soil. Give the surface a misting of water from your spray bottle, and you’re ready for seeding.

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Plant seeds

Choose a seed packet and check your notes on seeding depth and germination requirements. Open it carefully. Decide how many seeds are appropriate for the size of the container, the number of plants you want, and whether you want to hold some in reserve for a second try or to share with friends.

Scatter, spread, or push the seeds according to your notes. I try to spread them evenly but I don’t fuss too much over this part. It’s okay to sow thickly, especially if the seed is a few years old and you expect reduced germination rates. Also, you’ll be pricking out the seedlings when they’re very small, so they don’t need a lot of room.

If your book or resource says to cover with a “fine dusting of soil”, use your metal mesh tea strainer as a sieve to lay an even, light layer of dry mix on top. The purpose of this dusting is not to cover the seeds—it’s simply to keep them in place. Your resource may also suggest sand or perlite and you can use your sieve for these as well.

Small mesh strainer for sifting (dry) seeding mix over surface-sown seeds
Small mesh strainer for sifting (dry) seeding mix over surface-sown seeds for a very light dusting of cover.

After sowing and spritzing, press gently with your fingers to ensure the seeds have good contact with the mix. Then give it a final spritz with water and set it aside while you do the admin work. Don’t skip the record keeping—just trust me on this.

Put a mark on the seed packet to indicate that it’s been sowed (I put a check mark beside the year). Re-seal the packet and re-file it right away, to reduce the risk of it getting wet, dropped, or misplaced. Then, on your list, check off the species you just sowed and add a note if you like. I always indicate if a packet is finished, so I can update my seed inventory spreadsheet. Remember that if you’re not typing directly into your spreadshet you’ll be transferring your handwritten notes, so be neat and legible.

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Label and tape

Losing a label (whether it detaches or fades to oblivion) can be distressing. The extra time you spend now will save you the heartbreak of mystery jugs in April. So don’t scrimp or rush this part.

Labeling is slightly different for ‘all-in-one’ containers and ‘big top’ setups. These are names I’ve coined to describe the two ways of creating a mini-greenhouse. For details, see part three of this series: Container Choices.

Regardless of your setup, make sure to label both top and bottom. Although labels will be facing the elements for several months, they only need to last one season. Whatever you currently use for marker stakes in your garden (china marker, garden marker, ‘industrial’ sharpie, paint pen, etc) will most likely work for labeling containers. If you’re trying a marker for the first time, be sure to check for fading after a few weeks outdoors.

Instead of writing directly on the plastic, I now use white or yellow electrical tape for labeling. Combined with black lettering, it provides high contrast and legibility at a distance (from eye level looking to ground level). When you have dozens of containers outside, you’ll appreciate the ability to quickly find a species without having to handle and squint at each jug. And because you’ll likely be re-using these jugs next year, removing tape is much neater than crossing out permanent marker.

To minimize fading and keep the electrical tape from peeling off, cover labels with clear, waterproof, UV-resistant tape. Greenhouse repair tape (available at Lee Valley) or clear ‘Gorilla” or ‘T-Rex’ brands (available at home improvement stores) will work, but check the product label for resistance to ultraviolet light.

I always use the scientific names (botanical latin) for labels. It helps me remember both the species name and the spelling. And it prevents confusion between species of the same genus—Monarda fistulosa and Monarda punctata, for example.

If you’re using a big-top system, make a label for each species in its relative position. This will help greatly in the spring when you notice that one of your tubs has germinated. The labels will tell you which it is without having to remove the top and check the tub label.

Checklist for big-top type containers
Checklist for big-top type containers
'Big top' setup with four tubs inside
A really big ‘big top’ setup with four smaller tubs inside a large ‘greenhouse’ container.
translucent jugs and nursery pot showing labels
A ‘big-top’ setup. Both the outer jug and inner pots are labelled. Note the black marker on light-coloured electrical tape for legibility from a distance. Cover with clear, waterproof, UV-resistant tape so it stays put and doesn’t fade. The nursery pot is labelled as well– this is silver coloured paint pen.
mushroom tub being used for seeding pot.
Mushroom tub used for seeding container. White electrical tape makes label visible from the top means less need for handling / disturbance.
photo of pop bottle winter sowing container, with labels top and bottom
Waterproof tapes
left: sheathing / drywall tape. Extremely waterproof. Opaque. Very sticky. Does not stretch. Difficult to handle (thin)
right: greenhouse repair tape. Transparent. Very sticky to plastics but not great on dirty or wet surfaces, Resistant to ultraviolet light. Similar to clear ‘Gorilla” or “T-Rex” brands.
a 'big top' setup ready to be labelled. I use a single tub inside a jug because the sides of the plastic bottom are often thin and prone to twisting and bending. This can open open up gaps and cracks in the soil.
diagram- how to make a nesting jug
How to make a ‘nesting’ jug On the jug bottom cut a 1″ vertical slit in each corner. Overlap the edges by about 1/4″ and tape together. The circumfrence is now about 1″ smaller. allowing the top to slip snugly over.
big-top jug with six pots inside
A big-top setup with six pots inside. Outside labels correspond to position of pots
looking through the spout of the jug. Six pots inside
Looking through the large spout of a jug. Six pots inside, each individually labelled.

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Taping containers shut

Use waterproof tape to hold the top and the bottom together. You may experiment with using twine or elastic cord, but my experience is that anything other than tape leaves containers at risk. My early experiments with large elastic bands left me with tops that blew off when the elastics rotted. Twine is fiddly to tie and re-tie if you need to check inside. Once I tried using a rock to hold down a greenhouse dome—the squirrels played soccer with it.

For closing up containers my preferred tape is sheathing tape (‘Tuck Tape’). It stays sticky even when re-used and it’s 100% water resistant. Being opaque, it should be used sparingly. I cut four narrow strips crosswise from the roll and use one on each side of the top. For pop bottles I use three small strips spaced evenly.

There is no need to create a seal. If rain or snow get inside, that’s okay. It’s actually beneficial because it reduces the risk of a container drying out.

Have you noticed the nesting or overlap in some of the photos? I like to modify containers this way to have more control over the amount of snow, rain, or insects that can get inside. It also greatly reduces the amount of tape needed. To learn how to make these overlaps, read part three in this series: Container Choices.

taping options
Jugs showing taping methods. Center shows a basic butt joint. Right shows a nested top and bottom, created by cutting slits vertically in each corner, overlapping the cut edges slightly, and taping.

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Place containers outside

This is the ‘set and forget’ part. Once containers go outside you simply wait until spring. The mix and seeds will repeatedly freeze and thaw, just as they’d do if they’d dropped naturally. You don’t need to worry about them or protect them from snow or rain.

Some tips on choosing a site:
– Find a spot in the shade or, ideally, with a few hours of morning sun.
– Look for a flat surface that will not collect water. Don’t put containers in a tray or tub or anything that does not have drainage holes. You don’t want them sitting in rainwater or snow melt.
– If you put them on grass or a garden bed, protect containers from slugs that may find their way in through drainage holes.

There is no need to shovel out after a storm or protect jugs from snowfall. Allow the snow to melt and enter the containers naturally.

Squirrels and other critters may decide to romp in your winter sowing area. If you’re using big jugs or totes in a big-top setup, they’ll likely be okay. Pop bottles or smaller jugs may get knocked over and, if the soil mix is disturbed, there may be trouble with seed germination. In the event of a container mishap, do a second sowing in a new container.

Jugs can be protected from squirrels by placing them in a clear, uncovered tupperware-type bin with low sides (with drainage), or a wire basket, a milk crate, or by strapping them together with a bungee cord. I use a bicycle inner tube, cut twice lengthwise, as an impromptu bungee cord.

containers buried in snow
My wintersowing menagerie completely buried in snow. This is normal. There’s no need to spring into action
Jugs in snow, melting after storm
After the snow storm, jugs reappear. I have them packed in tightly. Keeping them stable will keep squirrels from knocking them over.
jugs with inner tube as strapping
thin lengths of pool liner, or a bicycle inner tube, can be used as straps to hold jugs together and thwart squirrels

In the spring, especially after the mix has started to thaw during the day, start checking on your containers to make sure they’re not drying out. If you notice that the surface of the mix is dry, put the nozzle of your spray bottle through the jug spout (or bottle neck opening) and spray a fine mist. If your containers are really dry (experienced gardeners will be able to tell by the weight) you can set them in a shallow pan of water so they’ll absorb through the drainage holes. If you find you’re routinely spritzing, move the containers to a more shady location.

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Watch and wait for germination

Winter sowing is not for the impatient. Some species will germinate in March and some as late as June. Some will need another winter. So watch and, if you’re scientifically inclined, make a note of the germination date.

Once germination starts be extra vigilant with sunlight, heat, and water. Tubs should not be allowed to dry out but neither should they be sopping. Bottom-watering is an option if conditions are right for damping-off problems. I have never experienced damping off with winter sowing—because I use commercial seeding mix and I remove seedlings before they get too crowded. As well, as soon as seeds germinate I no longer spray water on them. I either remove the lid and gently drip water onto the soil mix surface (avoiding the leaves if possible) or drip water down the sides of the container. Bottom watering is also an option.

As spring progresses and the sun rises ever higher in the sky, your containers will likely experience more hours of sunlight. Don’t let them overheat! Either pop off the lid or crack it open slightly to allow better air flow.

On a hot day, shade is vital. Try putting containers under a lawn chair or a table, so long as it’s not glass. I’ve rigged up shade tents with sheets and garden stakes. There are many creative ways to keep your little seedlings from roasting.

After germination the plants will start putting down roots and their water needs will increase, so begin providing supplemental water as necessary at this point. The larger the plant, the more water it will need. You may want to thin out your tiny seedlings or you may just let them grow until they are ready to be pricked out and potted.

After germination you can start opening the lid on sunny days to keep the containers from overheating and allow seedlings to experience outdoor air conditions. Remember to close the lids at night or when you’re not around—squirrels will party in any open containers.

There’s no need to add fertilizer or amendments of any kind. The seedlings will not be in these containers long enough to need fertilizer.

seeds germinating inside wintersowing pop bottles
Germination happening.

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Prick out, pot, and grow on

“Pricking out” and “transplanting” are often used interchangeably but I prefer “pricking out” because the word better describes the fiddly activity of choosing, scooping, teasing, and transferring tiny seedlings out of the germination container. I guess I also like the heritage of the term and the so-very-English plantsman or plantswoman who is likely to pop up on an online search for “pricking out”.

When should you prick out seedlings? How big should they be? The traditional approach is to wait for the appearance of true leaves. The first leaves are seed leaves (cotyledons). All subsequent leaves are true leaves that resemble the leaves on the mature plant.

But many experts prick out seedlings earlier, at the seed-leaf stage, with good success. Here is a short video by the venerable Charles Dowding to show how it’s done: https://youtube.com/shorts/O76UBnczp-s

One advantage of pricking them out while small is the ease of handling the roots. On the other hand, larger plants make it easier to handle the leaves. Whatever you do, hold plants by a leaf, not the stem. Plants can recover from damage to (or loss of) a leaf; damage to the stem may prove fatal.

Pricking out can be a long, tedious process. What if you just can’t get them all done at the appropriate time and you end up with a sprawling tangle of seedlings in your tub? Just slice out a “hunk-o-seedlings” (Trudi Griessle-Davidoff’s term) and plunk the chunk into the new pot. You may lose several around the edges, but most seedlings will survive. Once the ‘hunk’ has recovered from the move you can always thin it down to the strongest three or four seedlings You’ll be surprised by their resilience.

Seedling showing cotyledons and first true leaf.
Seedling showing cotyledons and first true leaf. (CC BY-SA 4.0) attribution: Assafn, Wikimedia commons
Asclepias seedling
Asclepias seedling showing two cotyledons, the first set of true leaves, and the second set just forming.

Here’s a delightful one-minute clip from Cloudberry Flowers (UK) showing how to prick out larger seedlings.
https://youtube.com/clip/UgkxQHH0H9otw6RwboNQujYqMUB2CfSa_YUx

Potting mix

For germination use germination mix; for potting use potting mix. Buy a good quality potting mix, an ‘all purpose’ growers mix, or make your own from commercial bagged products. Garden soil is not a good idea. Among other drawbacks, garden soil may harbor invasive jumping worm eggs. They are tiny—the size of poppy seeds—and virtually undetectable in soil. Unless you plan to use every plant you grow in your own garden, do not risk transferring jumping worm cocoons (eggs). As well, garden soil is alive. Its biome includes bacteria, fungi, nematodes, springtails and other invertebrates—and seeds. At this point you don’t want any of these hanging out with your seedlings.

I make my own potting mix. It’s fairly inexpensive, the ingredients are easily available where I live, and I can tweak the ratios based on what I’m potting. In general, I use one part leftover germination mix, two parts triple mix (a.k.a. 3-in-1) and as much perlite as I think the batch needs to lighten it up and quicken the drainage. If there’s bagged compost on hand I often use that in place of some of the triple mix.

Don’t use peat moss. There’s already peat in the triple mix and any leftover germination mix you may have added. Not only is peat harvesting unsustainable and destructive to wetlands, it is also likely to lower the pH of your mix. Germination mix includes lime to buffer the acidifying effect of the peat, but basic sphagnum peat does not.

If you’re looking for an alternative to peat, consider coconut coir. There are drawbacks, of course, such as the energy required to process and ship it. Research shows good results growing a variety of plant species in mixes where coir is 35-80% of the total volume.

A well-screened thermophilic compost can also substitute for a peat-based seeding mix, or a portion of it. Here is a detailed look at peat-free options: https://marylandgrows.umd.edu/2022/02/18/peat-free-potting-mixes/

If you choose to use peat-based products, remember that you can re-use and recycle them next year if stored properly.

If you use triple mix, make sure it’s a reputable brand. The compost portion must have undergone a complete and well-managed thermophilic process. Make sure the producer is a reputable company that meets quality standards.

Pots

I mainly use recycled nursery pots. I also like yogurt containers and deep mushroom tubs. If these are in short supply, try cutting and taping litre-sized milk cartons. If you don’t mind the extra work of cutting drainage holes, recycled plastic ‘solo’ type cups are an option.

Now that invasive jumping worms are an issue, everything gets scrubbed with soap and water, or solarized, before re-use.

Peat or coir pots? Give those a miss. They dry out too quickly and, as any experienced gardener will tell you, they just don’t decompose in garden soil as advertised.

The size of pot to use depends, not surprisingly, on the size of the seedlings. If you pricked out early (at the seed-leaf stage) you’ll need ~2″ pots, cell packs, or plug trays. Putting a single tiny seedling into a large mass of potting mix is risky. The roots will not be able to use all the moisture the mix is capable of holding—or at least not use it fast enough. Overly wet soil can literally suffocate plants, as the soil pore space that would normally be occupied by air is instead filled with water.

If you have a lot of seedlings with several sets of true leaves and well developed roots, you can plant several into a pot that would be too large for just one. I often use a four-inch pot to hold four or five fresh-pricked seedlings. By the time they’re ready to go into the garden, the roots will be well developed but can still be separated into individual plants. If you want them bigger before planting out, the next step up would be into a 4″ pot for each seedling.

Plan ahead for your plant labels. Each pot needs a separate label or stick marker. Here is an article to help home gardeners cope with the need to produce a large number of labels—too many to print by hand. https://morethanaprettygarden.com/2022/05/19/eco-friendly-plant-tags/

Protection and acclimatization

Potted seedlings fresh from a wintersowing container face two main challenges. The most serious threat is wind. Not only are displaced root systems not yet fully effective at moving soil moisture up to the leaves, but stems are now required to support the full weight of top growth without help from supportive neighbours. The other main threat is sun. Moving seedlings from the shade of the winter sowing site to a sunny spot on a shelf or a patio results in almost instant leaf scorch.

I put all my newly potted seedlings (regardless of the species’ sun preference at maturity) at the back of the shelf where they are sheltered from wind and sun during their recovery. If you don’t have shelves, consider rigging up some shade cloth, a trellis awning, or some other sunblock. I knew a gardener who set the trays on a garden cart that he wheeled into the shade when the sun got too strong. Of course, the easiest solution to acclimatizing seedlings is to put them in a sheltered spot that receives only morning sun.

Geum triflorum / Prairie smoke just germinated. Note the unsprouted seeds on the surface.
Geum triflorum / Prairie smoke just germinated. Note the unsprouted seeds on the surface

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Fertilizing?

There’s no need to fertilize wintersown seedlings. In fact, I often pinch them back to curb their enthusiasm. Remember that we’re growing perennials, so there’s no need to push growth for a sale date or get an early harvest.

Planting out

When the roots have reached the edge of the pot (but before they start circling or becoming pot bound) it’s time to plant them in the garden. Here are some tips:
– Prepare the planting area months in advance. Relieve compaction if necessary. Add compost / organic matter on top of the soil– do not till it in.
– Choose a cloudy, cool day when the soil is moist from a recent rainfall.
– Prepare the planting area the day before: if soil is dry, water the area deeply; mark or dig the planting holes; and have your stakes or garden markers ready.
– Water the plants deeply or soak the root ball in a bucket of water. Have a tool on hand to tease out pot-bound roots.
– Read about root-washing or bare-root planting. These techniques are especially helpful if you have invasive jumping worms or heavy clay.
– Be prepared to water young plants at least once a week for the first season. With adequate rainfall you can water less, but even drought-tolerant plants will need supplemental water for one year.

Some species (Lobelia cardinalis comes to mind) are simply slow, barely reaching plantable size by September. Seedlings likely won’t survive winter in a pot unprotected, so plunk them in the ground even if they’re small, in a nursery / plunge bed if you have room.

I tried to find a reliable online resource specific to planting out seedlings of native plants, but almost all the comprehensive information is for trees or vegetables. This fact sheet from Washington State University is the best I could find—but if you’re doing a native or naturalized garden ignore the part about 3″ of mulch. It’s okay to lightly mulch a native plant garden for the first year when the plants are small. But eventually they will provide their own mulch (fallen leaves and spent stalks) and cooling shade (leaf canopy).

Speaking as a naturally untidy gardener, I urge you to take care of the mounting pile of pots, tubs, and pop bottles right now, while it’s warm and you can wash them outdoors. Don’t wait until the night before the first blizzard of winter hits to realize you need to get everything clean, organized and packed in the shed or basement. For big-top jugs I mark matching tops and bottoms with a little graphic (an arrow, star, smiley, crescent, star—something quick and easy) to save fumbling and searching next December.

doodle marks help match top to bottom
a big-top setup with doodle mark to help find matching tops and bottoms when re-using these next year.

With your winter sowing season complete, it’s time to look back and make notes about what worked and what didn’t. If results were disappointing, share your experience with other winter sowing enthusiasts; creative solutions are often just a conversation away.

If you’re happy with your experience, share the news of your success and teach others. Plant propagation does not need to be difficult or expensive. Winter sowing removes many barriers to gardening—and it gets easier every year.

Good luck!

In Part 1 of this series we’ll look at the step-by-step process and present tips for success. Part 3 takes a close look at container choices.

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About the author. Bev Wagar caught the winter sowing bug in 2005. She has 23 years experience making, tending, and learning about gardens, mostly in urban settings. She’s an Accredited Practitioner with the Society for Organic Urban Landcare and a Master Gardener with the Halton Region group in Ontario. Since 2012 she’s lived in Hamilton where she founded the Monarch Awards and co-founded the Crown Point Garden Club. Currently she leads a volunteer gardening crew at the Kippax Native Plant Garden at the Royal Botanical Gardens in Burlington and volunteers with the Hamilton Naturalists Club. She enjoys writing, sewing, knitting, designing gardens and (of course) winter sowing.

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