Winter sowing is a method of putting seeds out into the garden well ahead of spring to sprout the seeds of plants that require cold-wet stratification in order to germinate.
What is cold-wet stratification?
Many plants need to feel the sting of winter and the kiss of spring warmth in several rounds in order to be convinced that false spring is well and done and it is safe to emerge into another season of bounty.
Have you noticed how terracotta pots will spall and crack or even burst when left outside in the winter? This is because of the expansion due to freezing of the water that has been absorbed into the porous material. This can also be seen on bricks and is a primary process of how boulders become sand, too.
And this spalling to due freezing water also happens with seeds - when they lay in a not freezing and wet environment they absorb moisture (the "wet" in wet-cold stratification) and when the temperatures drop below freezing this water freezes (the "cold" in wet-cold stratification) and eventually helps the hard coating of the seed to open.
Not all seeds need freezing conditions, and there are also many chemical processes that happen - read more about the fascinating processes of seed germination in this Nature article.
Indeed, some plants need a complex process of cold as well as warm periods in order to germinate, and the information on this page focuses on cold-wet stratification for germinating seeds of plants native to the north-west of North America, and specifically the Carolinian forests and wetlands of Essex County, Ontario, Canada.
Benefits of winter sowing in containers vs in the ground
Winter sowing aims to mimic the conditions that seeds would experience in the wild, and in these conditions they thrive. The seeds can indeed be winter sown directly in the ground and for large seeds like nuts and fruit stone for trees and shrubs this may be the ideal way so long as the spot is protected from squirrels with some caging.
Winter sowing perennial seeds aims to imitate nature as well as giving it a little help. Sowing in covered containers has the added benefits of protecting the seeds from being eaten or moved by animals, disturbed by water or wind, and also to maintain adequate moisture.
Larger upcycled food containers are best for winter sowing
The best containers to use for winter sowing are large plastic jugs from milk, juice, ice tea, and the like. Having been used for food, they are food safe. If you don't purchase these items, they can easily be found in the bins of your neighbours on recycling day - or ask friends and coworkers to save their containers for you.
These jugs that used to hold liquid are great because they already have the perfect sized opening at the top for air circulation while protecting from wind.
The large size of jugs that used to hold milk, juice, or iced tea makes it easier to both maintain the moisture in the soil and at the same time have adequate air circulation in the air space above, as well as room for the seedlings to grow after sprouting. It is not required to have the jugs be air tight and lids are not needed.
If you are using other containers, just make sure they are not toxic, food safe, and that the lid has a 1 to 2 inch (2 to 5 cm) hole in the top.
How to prepare containers like juice or milk jugs for winter sowing
To prepare the containers, first add drainage holes! This is most easily done BEFORE the container is cut in half, so do it first.
Using an awl, a kitchen knife, utility knife, a nail and hammer, or even a drill, poke many holes in the bottom of the container. Make sure the holes are at least 1/8 inch (about 4mm) so that they cannot easily get plugged up. If you are using a knife, use a screwdriver or other tool to pry the hole to stay open, do not rely on it being just a cut slit in the plastic as an opening. It is very important that excess water has a chance to drain away so that rotting doesn't happen when the temperatures turns warmer.
To make a bottom half and the lid, cut the jugs in half horizontally just below the angle of the jug handle, usually about 3 or 4 inches from the bottom (see diagram).
To most easily and safely cut the jugs, use both a knife AND scissors. You can use a utility blade or a kitchen knife to "stab" the plastic and start the cut, and then poke in scissors and continue the cut.
Cut ALMOST all the way around, leaving about an inch attached to form a hinge.
How to fill the containers with soil and water
Any light, well draining soil can be used. If you have light sandy soil or make your own soil mix, this is wonderful. Avoid soils heavy in clay as these may settle out in the containers with the clay forming a plug at the bottom.
If using a commercial soil mix, these can sometimes be very dry and hard to rehydrate - it helps to mix the soil and water in a larger container before adding to the winter sowing jugs. Add the soil mix and plenty of water and either let it sit for a few hours or helpt the rehydration process by mashing the water into the dry soil.
Fill the containers with 3 to 4 inches of light, well draining soil and make sure the soil is quite wet and also that it is draining well. Pat the soil down gently without compressing and smooth the surface lightly.
That's it! You are now ready to sow a large variety of seeds that need cold-wet stratification to germinate.
Sow the seeds
In the wild, most seeds would simply fall on the surface of the ground. Indeed, many seeds require to be right on the surface or only very lightly covered in order to know how many daylight hours are happening!
So to sow the seeds, simply scatter loosely on the surface of the surface of the soil, cover only with a light sprinkling of soil if at all, and pat down gently so that they make good contact with the wet soil.
It is easiest to sow into quite wet soil and not water after at all, as this will disturb the seeds the least. It is also ok to water very gently and lightly, such as with a spray bottle, as this will also help press the seeds into the soil.
Close the containers
To close the containers, just hinge the top over and put a single piece of tape to secure.
You can also use a twist tie threaded through holes poked into the container bottom and lid.
It is not necessary to tape all the way around, not at all. Also leave off the screw cap. The jug pouring opening and the crack in the sides makes for just about a perfect amount of air circulation and rain and snow precipitation falling in.
Label the containers
Possibly the easiest way to write the name of the plants on the plastic is with a grease pencil. These are available in most hardware stores as well as garden and art supply stores. The writing goes on easily, and remains permanent until washed off.
Regular pencil will not fade in the sun. Regular hardness of HB is fine and a softer artist pencil such as 4B will make a thicker, darker line.
Tags that are put just inside are fine as well since the information is needed in the spring after things sprout and it isn't relevant what's inside for the months of winter and early spring care of the containers.
Tags made from yoghurt containers
An easy source of material for garden labels is the plastic of food containers. Just wash and cut into strips.
I find grease pencil or a soft 4B lead pencil write well on the white plastic from yoghurt and soft cheese containers.
Set up and forget the containers
Set up the containers where they will be out of the way of human, animal, and machine activity for the winter and early spring months, and also where the containers will be exposed to both light and precipitation in the outdoors. The seeds need to feel cold and even frozen, some are sensitive to the changing length of the daylight hours, and in general they do not need any special care at all. Once they are set up in January, you can forget about them till the spring.
What about winter care and snowdrifts and howling wind?
It can feel odd to watch our precious seedy expectations lying naked under a blanket of snow, out in the cold dark of winter's hardship. It is important to remember that these are our own projections and for the seeds this is their natural, most comfortable situation, and they are fine!
Perhaps the only thing to look out for is drought - if your containers are standing in a full sun condition and got well heated up and possibly dried out, they may need extra water. Open the container and water very gently, or water from the bottom by standing the jub in a pan of water and letting it absorb up into the soil.
Germination and sprouting
These two photos were taken on March 21 and March 31. In the first photo it is clear how the seeds are germinating on the surface of the soil, and few have sprouted. Ten days later, they have mostly all sprouted and are starting a healthy growth.