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Making Winter for Seeds:
Cold Stratification From Fridge to Snow Sowing

For Germinating Tough Perennial Seeds

Seeds That Need a Winter

Unlike the seeds of annuals, lots of perennial seeds require a period of moist cold (cold stratification) before they will germinate.  They get this naturally in the wild, lying on the ground, being half buried by fallen leaves, the digging of animals, and frost heave. Without this period of cold and wet, certain seeds, usually perennials, won't germinate at all. Instead, they continue to wait inside their shells, asleep.

But natural cold stratification usually results in the death of many seeds (which is why plants that rely mostly on seeds for reproduction make so many of them--it's insurance). Seeds get eaten by critters, end up wet and rotting, land in conditions not good for growing (a driveway), and more. Humans, as caretakers and partners with plants, have developed a number of ways of creating an awakening winter for seeds that need it. In the gardening world, this is called cold moist stratification (usually just called stratification or cold stratification). There are six methods of cold moist stratification to choose from: cold water soaking, refrigeration, fall planting, winter/solstice sowing, outdoor treatment, and snow planting.

Cold Water Soaking

 

I've tried this method on a few seeds--belladonna, henbane, black mandrake, white mandrake, and various monkshoods--and it is promising enough and low-tech enough (no fancy chemicals needed) to make it worth trying with other seeds that need cold stratification, especially if they are not too small. Use a small jar (and old vitamin bottle works well). Put the seeds in and fill with cold water. Store in the fridge, and change the water for fresh daily. This is important--you're trying to imitate snowmelt here, and how snowmelt apparently works is to wash away germination inhibitors in the seed. Also, if you don't change the water, it will get moldy. After two weeks, take the seeds out and plant as usual. I usually plant in jiffy pellets and put under fluorescent lights until it's warm enough to put them outside. This method works well if you are within a few weeks of your last frost date. It's short and sweet. If you try this with other seeds, let me know your success (and failures) so that I can add the info here. Today I just read of another way of getting the cold water soak going - putting seeds in little muslin bags and then suspending them in the toilet tank, lol! Top

Refrigeration

 

Americans usually use the refrigerator to create an awakening winter for seeds. In fact, we've gotten so reliant on this method that we've forgotten that previously everyone simply planted seeds that need cold stratification outside once it got cold in the fall, usually in late October. Top

You need something to hold the moistness: traditionally, sterile soils like sphagnum moss, peat moss (good for acid-loving plants), clean sand (not from the beach or dredging--too salty), or vermiculite are moistened (not slopping wet), but you can also use a paper towel that has been wet and wrung out. Mix the seeds with the sterile soil--it's good not to use too much, because unless the seeds are large, you will be sowing the soil together with the seeds once they start germinating. Soak seeds overnight and mix in with the moistened sterile soil. Or if using a paper towel, fold the wrung-out paper towel in half and sprinkle the seeds on. Fold the paper towel closed and press it gently between your hands to get good contact between the seeds and the towel. With any of these methods, I normally use water that has some liquid kelp added to it, because kelp contains chemicals that help with germination of seeds. You can get liquid kelp at most garden stores in the summer. Top

Put the sterile soil/seeds or paper towel/seeds in a resealable plastic bag and put in the fridge. Three months is a good time, but sometimes you can get away with less, even 3 weeks. This is an area for experimenting. After a month, check the seeds every week or so to see if they are germinating. Some will germinate in the fridge; some won't. Plant them as you normally would when the time is up, whether they have germinated or not. Top

I have had much more luck using the paper towel method than using the ones that involve some kind of sterile soil. With a paper towel, it's a lot easier to open it up and see if the seeds are starting to germinate--or if they are just rotting. If they get brown spots around them or smell musty, they are rotting and should be tossed. (Seeds can rot because they were not stored properly before planting--usually too warm and/or moist. Keeping seeds at room temperature for too long causes many seeds to die, although some are actually conditioned this way to germinate better. Most seeds need cool and dry conditions for max storage life.) Top

Another way to use the fridge for brief cold stratification is to plant the seeds in individual jiffy pellets and put them in either a plastic covered container (such as for home baking little bread loaves) or in a store-bought "greenhouse" flat. Depends on how much room you have in your fridge. I have done this as well, for instance, with mandrake seeds before I started using cold water soaking. Top

Planting in jiffy pellets and then putting them in the fridge also works very well for seeds that need just a little boost of cold instead of a whole "winter." In that case, you only keep them in there for a few weeks. For instance, with poppy seeds that require cool temperatures to grow, planting them in jiffy pellets and putting them in the fridge for a week or so will cause them to germinate with great gusto once you take them out and put them in indirect light. I suspect refrigerating for just a few weeks would greatly help the germination of some annual seeds that usually are simply planted outside in the spring. Top

Fall Planting

 

Fall planting has also been a traditional method both for most Europeans presently and for old-timers here in the US. Seeds are planted either directly in the ground in a cold frame or in pots left in a protected area. If you have a cold frame, you probably already know plenty about cold stratification. You can jerry-rig a cold frame with old windows or old storm windows leaned against the side of the house (I did this one winter in northern Minnesota and was able to grow snow peas against the east side of the house much of the time). To cover the sides, tape up folded-up cardboard boxes to the windows or use straw bales for more serious protection. You can put uncovered pots with seeds in them into a cold frame, but check on your coldframe regularly, because mice like to be warm. One thing about a coldframe--it will not allow any moisture in, so seeds that need snowmelt to germinate won't like it. Coldframes are more useful for season extenders than seed germinators. Top

With fall planting, seeds are planted outside, sometimes directly in the ground - for instance, you can just broadcast cover crops and allow frost heave to bury them. More often, however, fall planting is done with pots before temps fall below 8C/45F. The pot is covered with either a piece of glass (if you have very rainy or dry winters) or a piece of screen held on with string or a rubber band (if you have snowy winters). The screening allows snow to come in close contact with the soil in the pot, which insulates the soil from extremes of cold, and snowmelt is able to trickle down through the holes in the screen. Lots of seeds are triggered to germinate by snowmelt. Place the pots up against a building. The east side of a house is good, or if seeds are of a plant that is borderline in your area, choose the south side. Then make sure that if the pot is covered with glass you raise it up a bit when the sun is bright and temperatures begin to warm, or it will get too hot in there. Someone emailed questioning the use of terra cotta pots outside in the winter. You can use them, as long as you keep them close to the wall of the house and they do not fill up with water and then freeze (keep away from the roof's dripline). I have used them outside here in upstate NY and in northern Minnesota without a problem, but you have to make sure that you get good ones. The terra cotta pots that are slipware are too thin. You need the heavy old-fashioned kind. Last year I had one pot crack outside; it was made of that paper stuff. Also, handle plastic carefully when it is bitter cold; it cracks more easily then. Top

Winter/Solstice Sowing

 

A modified version of fall planting is to plant later in the year, from Winter Solstice (December 21) all the way up through February, depending on the severity of your winters. The harder your winter, the later you can plant. This takes advantage not only of the natural swings between cold and warm that occur in late winter but of the growth energy of the Earth that starts once the days begin to lengthen at Solstice. So this method combines temperature fluctuation with snowmelt and increasing daylength to help seeds grow. Europeans have been winter sowing for decades--I've seen a mosaic from Roman times depicting Frenchemn winter sowing beans--so it was not invented by an American. Americans simply tend to use throw-away containers instead of terra cotta or plastic pots, and to use the plastic tops instead of screens or glass. By the way, don't waste your time planting annuals in the winter. It will just stress them, and a lot of them will die or be knocked out by late frosts. Annuals will germinate just fine planted in the spring.Top

You can incorporate planting into your Yule celebration or just do it as a private affirmation that even on the darkest day of the year, even in the very deepest of sleep, the Earth is getting ready to awaken. If you are in the Southern Hemisphere, this same method will work on Summer Solstice. Top

Any covered container works - including terra cotta pots, as described in fall planting section. The kind of foil and plastic-topped baking pans you can find in the supermarket in the baking aisle are convenient (although at a dollar a pop, not cheap).  Regular little 25-slot planting trays that come with a plastic cover work well and are not expensive if you get them on sale in the late summer at garden centers. You can use any sort of plastic carry-out food containers with a plastic lid as long as the bottom part is deep enough--at least an inch. Top

For drainage, make some slits in the bottom of the pan or tray with a knife or poke holes with the tip of a Philips screwdriver. Fill the pan with seed starting soil (or use jiffy pellets) and tamp down gently. Don't use dirt from the yard, as it is usually compacted, which makes it hard for some seeds to break through. Water well before sowing--until it is mudpie consistency (this is the way you want to start out, not the way you want it to stay permanently). If your planting mix has got any peat moss in it, it will get wet more easily with warm water--just make sure if you use hot to let it cool off before you put the seeds in. Top

When the soil has drained, take the seeds you've had soaking overnight and toss out any floaters. Pour the rest into your hand, letting the water drain through your fingers, and sprinkle them *thinly* on top. If the seeds are very small, like wormwood seeds, you don't need to soak them overnight. If you plant the seeds too thickly, you will end up with spindly seedlings that will tend to die from damping off. It's a good idea to sow at most only half of a packet at a time. Try other methods of sowing with the rest of the seeds so you can figure out which method works best for you, for your seeds, and for your climate. I have noticed that everyone has their own best way to plant. Top

Gently pat the seeds down with your hand. You want a good contact between soil and seed. Then sprinkle a thin layer of soil (some use sand or grit) over the seeds, barely covering them, and gently pat that down as well. You don't need that covering soil to be wet--it sprinkles more easily if it is dry, and it will wet quickly from the soil beneath it. If the seeds are very tiny, like for wormwood or other dust-like seeds, don't cover them with any extra soil, just very gently pat them in (trying not to let them get stuck to your hand!). Top

Make slits in the plastic cover or plastic wrap, or just use a piece of screen so heat and air can get in and out, or seeds will cook in the sun even in winter. Put the lid on and fold the foil pan edges around it so it is secure and can't be blown off or removed by hungry critters. Don't forget to label the pan with the seed name(s)--I can't tell you how many times I have forgotten to label plantings and had to wait until they got big before I could figure them out! You can write with a grease pencil (Sharpies fade in the sun) on the pan bottom or even the plastic cover, or use regular address labels on the side. Cover the labels with tape to help keep them dry. I usually print out my labels on my laser printer, which uses waterproof ink, and they stay pretty readable, considering. Top

If it is very cold out, below 30F/0C, condition your seeds for a week or two in your fridge before putting them outside. This will decrease the shock of going from room temperature to below freezing. Top

Put your containers outside in a safe place. On the ground next to the north or east side of the house is best (next to a building should give you at least another zone's worth of protection). It is good if snow can get on the pans and melt, because the snowmelt can help trigger germination. They should not be in standing water or right in the drip line under the eaves of the house. Too much water, especially in winter, will kill the seeds. If it snows and does not cover them, pile snow over their tops or move them to a sheltered place where snow will cover them--snow's an important part of this process. Top

Alternately, you can use regular pots and just put them into a plastic baggie, which Deno described in 1993. Then put the whole shebang against the north side of the house. Open the baggie some when the seeds start to germinate, and gradually allow them more light and air. Be careful not to cook them. This method does not make use of snowmelt. It is especially aimed at seeds that like to get a little colder, like aconites. Top

Seeds should germinate in late winter. They won't come up all at the same time, like the seeds of most cultivated plants are bred to do. Seeds that use cold to germinate are closer to the wild and so have a good reason to stagger their germination - more are likely to survive that way, and you will get more genetic variation. That means there will be a greater likelihood of getting plants that will survive and prosper in your conditions. Top

Once they come up, check them regularly to see if they have enough water. Make sure to put the lids back on securely, because even warm wind can quickly kill seedlings. Each week make the slits in the lid larger, gradually "hardening off" the seedlings so they will be ready to pot up or put them in the ground after they have their first set of true leaves (this is the second pair of leaves they will get). Transplant before they hit the lid or just remove it, if it's warm enough. Transplanting is best done on a cloudy day so that the sun doesn't bake the seedlings. Seedlings are just like children and should be treated gently but firmly. Top

"Outdoor Treatment"

Norm Deno's "Seed Germination Theory and Practice" (1993) is well known for its description of using gibberellic acid to jump-start germination and for advocating the use of paper towels and baggies for starting seeds. One type of cold moist stratification that he calls simply "Outdoor Treatment" makes use of temperature fluctuations in winter for especially recalcitrant seeds. You "plant" as usual with the paper towel method, but instead of putting the baggies in the fridge, you put them outside in a shed during winter. This gives you fluctuation in temps instead of steady temps. You only need to keep them away from direct light (and hungry mice!). If you want to cold stratify a bunch of different seeds, this method sounds really helpful, as it saves enormously on work, soil, space, and containers, and you end up planting only the seeds that actually germinate, which is the advantage of the paper towel method in the first place. I tried this with over 150 species during the winter of 2006, 2013-2007, 2013, and it was incredibly successful. Ninety-five percent of the different varieties had germination, as opposed to using the refrigerator, where I often get the situation where I can tell the seed is alive (it doesn't rot), but it won't germinate. I will be using Outdoor Treatment to cold stratify from now on. Top

Snow Planting

This isn't strictly cold stratification but it's a wonderfully mysterious method and should delight children. It works for hardy annuals, biennials, and short-lived perennials--seeds that enjoy some cold to germinate and grow but don't truly require cold stratification - for instance, members of poppy family, heartsease, and  sweet alyssum. During the winter after a heavy snow, go outside with your seeds and just broadcast them out on the snow. Believe it or not, they will come up in the spring. I tried this with California poppy, and it works wonderfully well. Just make sure you are broadcasting them in an area where they will be able to grow. This is a great way to beat the dulls of winter. Top

 

Whichever method you try, remember that seed germination of perennials is an art, not a science. Patience is definitely required, and even the most experienced growers fail sometimes. That's why you always keep some seeds back to try again.:) Top

2004, 2013, 2009, 2013 Alchemy Works; No reproduction without permission

Seeds That Need a Winter

Anise Hyssop

Belladonna

Betony

Black Cornflower

Black Henbane

Black Nightshade

Calamus Root

Climbing Nightshade

Columbine, Wild

Cowslip

English Bluebell

Green Wizard Coneflower

Grey Sage

Harebell

Heather

Hellebores

Hops

Italian Cypress

Jack in the Pulpit

Old English Lavender

Lily of the Valley

Monkshood

Mandrake (Black, White)

Masterwort

Moonwort

Motherwort

Mugwort

Myrtle

Penny black nemophila

Poison Hemlock

Pokeweed

Red Pasque Flower

Rose Milkweed

Rowan

Russian belladonna

Sage

Tansy

Valerian

Vervain

Wild Rose

Wolfsbane

 

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Seeds of annual plants usually don't need a winter to germinate. For them, it's more a question of whether they survive the cold to germinate the following spring instead of needing the cold the germinate the following spring. So don't waste your time and seeds using these methods on annuals. Instead, plant annual seeds in the spring. Top