Winter Sowing 201

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Winter sowing is a tradition that reminds me of spring’s inevitable arrival. Although my own garden is full, I grow (mostly native) plants for my garden club’s plant sale, a number of public gardens, and the occasional garden design commission I do. Truth be told, I enjoy the growing more than the planting. The whole seed-starting regime, especially winter sowing, was becoming a chore. This year, after procrastinating for an entire month, I got the bulk of it done in mid-January. Documenting and photographing the process helped re-kindle my enthusiasm and has certainly helped improve my documentation and organization.

The following is not a “how to” post and it’s not really for beginners. It’s intended for gardeners who’ve perhaps tried it once or at least done some reading. Maybe your results didn’t meet expectations and you wonder what you could do differently. This post should help. I’ll focus on the details and some of the hacks and techniques that have helped me fix problems and get consistent results.

If you’re a beginner, check out the links and resources here. http://monarchawardshamilton.org/winter-sowing-resources/

A special note to Monarch Award participants: winter sowing is a great way to increase your supply of native plants for a low cost.

Planning & Tracking

Deciding what you want to grow this year starts with an enjoyable hour (afternoon? day??) sorting through your seed stash. Make note of old seed so you can either buy or save seed for these species for next season. Also note which species you want to try but don’t have. Then check out the North American Native Plant Society‘s seed exchange and sales (NANPS has a table at the annual Guelph Organic Conference trade show), local seed suppliers, swap groups on Facebook, and your local Seedy Saturday event.

Be sure to have a system for keeping track of what’s in the queu to be sown and what’s been completed. It’s frustrating to get mixed up and sow something ON TOP of something else because you didn’t have a set process for keeping track of what’s done. Tiny seeds can be invisible on the sowing surface, so don’t trust your eyes. Write it down.

It’s important to keep track of what’s been sowed and what’s in the queue.

I have two methods. This year I removed the packets from my stash, set them in a “to be sown” box, and wrote the names down as I finished them. Other years I’ve written down all the species and ticked them off when complete. Make a note of any seeds that are more than two years old. For me, it was Asclepias incarnata from 2017. Then, if there’s no germination on certain tubs, you don’t have to blame the squirrels.

Speaking of old seed, all saved seeds should include the year and the source/location it was collected from. Even with your retail seed purchases, be sure to print the year on the front of the packet.

If you’re trying a species for the first time you’ll want to double check that it’s a good candidate for winter sowing. If you’re unsure, reserve some seed for a second try.

Resources

The Gardener’s A-Z Guide to Growing Flowers from Seed to Bloom , by Eileen Powell.
There are many other good resources to use.

Finding information on the germination requirements for each of the species on your list is not difficult. If you’re using commercial seed, check the seed packet. I always compare the seed packet information to my trusted online resources. Have your book(s) and laptop handy and your web sites bookmarked. I’ve listed a few good ones below.

Ontario Rock Garden Society germination guide: http://www.onrockgarden.com/germination-guide/plants
Tom Clothier’s site. An oldie but a goodie: http://tomclothier.hort.net
The ultimate rabbit-hole for plant nerds only. Dr. Deno’s guide: http://files.srgc.net/general/Deno-1st-supplement.pdf

Sowing

A good resource (book or web site) will tell you how deep to sow seeds as well as details about germination temperatures.

There a two key bits of information you need: the planting depth and cycling requirements. For planting depth most resources specify one of three: “surface” (which means the seed requires light to germinate); “just cover” or “cover lightly” (which means the seed is prone to being dislodged, so you need to cover it just enough to hold it in place); or a specific depth. If the resource does not specify a depth, the general rule is to just cover it lightly.

small wire mesh strainer

A small wire-mesh strainer is helpful to “just cover” tiny seeds.

Here’s a tip for getting the “just cover” just right. Put some dry seeding mix in a small sieve (such as a teabag strainer with wire mesh) and press the mix through the mesh over your seeds. With a dust-like sprinkle of just the smallest bits of mix, you’ll avoid burying seeds too deep.

“Cycling” refers to time periods at a certain range of temperatures. These mimic the conditions a seed would experience if it ripened and dropped naturally in the garden. For example, Gillenia (Porteranthus) trifoliata should be sown at 18-22ºC (64-71ºF) for 2-4 wks, then moved to -4 to +4ºC (24-39ºF) for 4-6 wks, then moved to 5-12ºC (41-53ºF) for germination (from http://tomclothier.hort.net). This pattern is basically warm, cold, cool. So, your Gillenia containers should be kept indoors at room temperature for up to a month before being placed outside with the rest. Other species will have different patterns, some covering more than a year, to break seed dormancy.

Sow thick – thin quick. It’s true for carrots–and wintersown seedlings. Don’t worry too much about seed spacing. Just make sure to start checking on your containers in April. As soon as the seedlings germinate you must remove the cover and provide supplemental water as necessary. When the seedlings have their first set of true leaves you can start pricking them out. Don’t want the fiddly work? Try Trudy Griselle’s “hunk ‘o’ seedlings” method.

Soil Mix

Tiny seeds require super-fine, well-screened seeding mix. If you’ve had trouble in the past, try using a quality soil-less mix made especially for seed starting. Even the regular Pro-Mix can be too coarse for some seeds. One solution is to buy Pro-Mix PGX plug and germination mix. William Dam Seeds sells it under their own brand at a reasonable price.

Here’s a low cost tip for using mix you already have on hand– even triple mix. Put the coarse mix (or triple mix with added perlite) in the bottom of the container. Add some super-fine germination mix only on the top half-inch or so. By the time the seedling roots descend to the coarser material they will be strong enough to handle it.

It’s time consuming but you can create your own super-fine mix by putting regular mix through a metal mesh kitchen sieve.

A note of caution when using two materials of different textures in your containers: avoid creating distinct layers. Try to blur the “line” between the two materials. The reasons for this have to do with the way water moves in soils of different textures. It’s quite fascinating and I’m sure the nerdiest gardeners are googling that right now.

But back to soil mixes… Commercial soil mixes are all peat-based. Most have added vermiculite and some have added perlite, lime, and wetting agent. Wetting agent is used because dry peat is naturally hydrophobic. When it’s dry, it resists getting wet. The difficulty in getting peat-based mixes to take up water makes it a bit finicky to work with. One solution is to use a simple spray bottle. A fine mist will infiltrate dry peat much better than poured water, even if added drop by drop, from a watering can.

Another tip: since you need to moisten your mix before you fill the containers, do it the night before. An eight-litre bag of dry mix could absorb three or more litres of water. Set a litre or so of the dry mix aside. Then, in a big tub (I use a Rubbermade tote) use a big mixing spoon or your hands to incorporate water into the seeding mix. In the morning your mix will be evenly moist. You want it to be wet enough that a handful, when squeezed, will hold its shape without dripping. If it’s too wet, add more dry mix from the amount you reserved. If it’s too dry, add more water.
(Photo credit: Dorothy Stainbrook. https://farmtojar.com/5-tips-to-help-with-tomato-pepper-seed-germination/)

So I’ve been discussing peat-based mixes. Peat should be used judiciously–it is impossible to harvest peat bogs in a sustainable manner. Using coconut coir as a substitute brings another set of issues (slave labour, trans-atlantic transport etc). Probably the only truly nature-friendly material is well-made, screened compost from a local, known feedstock. I will leave it to you to research your options. If you use peat, be aware that it can and should be re-used from one year to the next. The only caveat is to keep it from freezing during storage. I save the “dregs” from my seedling containers for re-use.

Containers

There are dozens of ideas and options for containers. Don’t go out and buy tupperware or milk jugs–just take a walk in the neighbourhood on the evening before recycling pickup day. Blue boxes are a treasure trove of jugs, clear containers, plastic tubs, and pop bottles.

Here are the guiding principles for container selection.
1. height. must be room for about 4″ of seeding mix. Any deeper and you’ll have difficulty pricking out the seedlings’ roots. Any shallower and the container will dry out too fast or not have enough room for healthy root development. The height of the “dome” is also important. The seedlings need room to grow before hitting the roof. There must be enough space between the soil surface and the rooftop/lid for air circulation to prevent the contents from overheating.
2. drainage. There MUST be drainage holes cut (or burned) in the bottom. If you’re using mushroom tubs (a really convenient size I find) or pop bottles you’ll need to poke at least four holes.
3. rigid sides. Okay, I’ve seen the “baggie method” but don’t try moving those flimsy bags. You really need sturdy sides because at some point you’ll need to re-locate, lift, water, or otherwise handle the container.
4. air circulation. The clear or translucent “greenhouse cover” must have more than one hole for venting. If you’re using a jug or pop bottle you’ll need to poke several more holes near the top in to allow for circulation.
5. precipitation management. Rain and snow must now be allowed to deluge the container. A little bit is okay and even necessary but anticipate and plan for where the rainwater may pool and drip.
6. light. The “greenhouse” cover must be clear or translucent. Make sure that the level of the soil is close to the top of the container. Otherwide the container walls will cast too much shade on the soil surface. I fill, tamp, and level my containers with a straightedge. After a few weeks outside the soil/mix level will drop up to a centimetre or so. This is okay. But avoid starting out with large gaps.
7. size. Choose a container that is the right size for the number of seeds you plan to sow. Lots of seeds = bigger container. Big seeds = bigger container.
8. one species per container. Just trust me on this. Don’t mix them. Don’t do half-and-half. Just use in separate containers.

This type of pop bottle with a curved “hourglass” shape makes an excellent container with a snug-fitting top.
Two cuts are needed. One at about 4″ (first dotted line in the photo) and one just below the label, at the full circumfrence.

If you don’t have the “hourglass” pop bottles, straight ones work too, but without the snug “nesting” fit. Here I’ve used a bottle top on a cut-down yogurt tub, which fits fairly well. Tape (water-resistant Gorilla tape or sheathing tape) them together so rain doesn’t sluice into the tub.

Blurry photo. Sorry.
Shows the nice snug overlap with the “hourglass” pop bottle fitting over the base. Rain does not wash into the tub. A few pieces of tape are still needed to hold the top on.
There are five additional holes around the top for air circulation.

A translucent jug is good for shade-adapted species. This one contained windshield-wiper fluid–I washed it thoroughly and cut six additional vent holes.

Large yogurt tubs can be cut down to size. Tabs are folded into a pinwheel shape and taped with water-resistant Gorilla tape or sheathing tape. Check for drainage by filling with water before you fill with mix.

Making Holes for Vents and Drainage

Before you haul out the electric drill or ruin your good knife trying to cut through rigid plastic, try melting your vent holes. A hot metal rod or narrow blade (I use a steel barbecue skewer) will easily pierce any plastic container. Hold the end of the rod over a lit candle or a flame from a gas stove. It doesn’t have to get white-hot– just hot enough. Then poke away. Wipe off the plastic residue before re-heating the rod. And don’t use it for anything else. It’s now your forever winter-sower-poker tool.

Labelling

Over the years I’ve tried many tools to keep labels from fading and peeling off as they endure sun, rain, snow and wind in their months outdoors. Here’s the rundown.
China Markers (a.k.a. grease pencils) are very reliable for weather resistance. They work on pretty much any surface. But they are clunky and slow as writing instruments. Not suitable for small labels. Don’t get them wet (!)
Paint Markers. Silver and Gold are easy to find at office supply stores. Silver works great on black or dark green plastic or almost any surface. Unfazed by rain, snow or sunlight. Not so legible on clear plastic unless you can find black paint versions. Use them on black electrical tape if your container doesn’t offer enough contrast.
Industrial Sharpies. Special fade-resistant Sharpie-brand felt markers. Will last until spring for sure but not indefinitely. Fast and comfortable to use. Try them on white coloured electrical tape if your container is dark-coloured. Works on most surfaces.
Regular Sharpies. Don’t even bother trying these unless you are marking the bottom of your container. Regular Sharpies will fade with exposure to sunshine.

Label each container in at least two places– the lid and the base. For absolute no-fail reliability, label the BOTTOM of the container where the sun never shines.

Filled yogurt tub.
Labelling supplies: black and white electrical tape, black and white china markers.

This jug is huge. I filled the bottom with a few inches of mix. The added weight keeps it safe from wind and critters. Smaller containers fit inside, kept stable and moist by the soil base.

I’ve tried several methods to use clear plastic freezer bags (heavier 4 or 6 mil types) as greenhouse covers but they always collapsed with the weight of water, snow, and ice.
Here’s one idea that worked. I rescued some poinsettia supports from a bin at a garden centre. The bottom fits nicely over a mushroom tub or a shallow 8″ pot. Adding two lengths of tape in an X shape over the top keeps the rain/snow from pooling and weighing the bag down.

Containers outside. The rubber band will get brittle but will last until April. Bottles sit in large tray just to keep them together. The tray has drainage holes– don’t allow containers to sit in water.
On the left is a mushroom tub (my favourite container) within a poinsettia support with a heavy plastic bag used as a “greenhouse” cover. I save and re-use these each year. Don’t forget to cut vent holes in the bags.

More containers outside. The cardboard orange crate will disintegrate by spring but until then it’ll keep the tubs safe from squirrels and windstorms.
The back porch receives morning sun only, so the containers don’t overheat.

Watering

As winter wanes and the sun arcs higher in the sky, overheating becomes a risk. Check your containers weekly and water as necessary– if the soil/mix is thawed of course. You can bottom-water by sitting the containers in tray of water for a few hours. You can spritz the soil surface without un-taping the bottle or jug by aiming the nozzle of a spray bottle through the top hole. You may need to move the containers to a shadier spot.
For several years I used a glass-topped patio table as a rain shelter for my containers. The glass diffused the light, reducing the risk of heat buildup. The system worked well enough although it was a chore to walk downstairs to the patio to check on the containers. Keeping them in a convenient spot makes it more likely that you’ll check on them regularly.

Pricking Out

You don’t need to do it all at once. Taking out a “slice” of seedlings will not hurt the ones remaining. For really tiny seedlings, plant them three or four to a 4″ pot– they won’t dry out as fast and the process will go much faster. Use potting mix with some compost (or even some healthy garden soil) mixed in. You want the seedlings to grow up in healthy, alive soil.

The most tedious part of the potting process is making plant markers. You need a lot of them. I use cut-up venetian blinds and either a china marker or industrial sharpie, whatever is handy. Here’s what does NOT work so well:
– popsicle sticks or craft sticks. Ink just bleeds and runs. Pencil doesn’t show well enough. China markers break on the wood.
– cut-up yogurt containers. They’re too brittle.
Venetian blind slats can be re-used for many years simply by taping over last year’s text with white electrical tape.

Share your experience

Do you have tried-and-true tips and tricks for winter sowing? I’d love to hear them and share them here. Use the contact form on the right sidebar. If you want to send photos, let me know in your message text. I’ll respond by email so you reply with attachments.