This is the usual way of sowing seeds of perennials, trees, and shrubs, in Europe and elsewhere, and simply involves sowing seeds outside in the autumn, usually in pots, to allow them to experience the ups and downs of winter temperatures to encourage them to germinate in the spring.
It might seem obvious, but only sowing seeds in the autumn or winter can be called 'winter sowing', and you sow seeds at this time for particular reasons. When I see references to 'winter sowing' in spring or summer on US Forums, I conclude that this new discovery by American gardeners is just the rediscovery that seeds don't need artificial light or heat to germinate.
Whilst sowing outside in natural conditions is the method most usually practised throughout the world, sowing seeds outside in the autumn or winter is not the best way to treat all seeds. A few seeds of some species might germinate if sown at this time, but that might be despite, not because of, this treatment. Surely the whole point of sowing seeds yourself is that you can choose the best way for the particular seeds you want to germinate?
You should always consider the type of seeds that you want to sow, to make sure there is a reason for sowing them over the winter. There is no point in sowing seeds of tender or tropical plants outside in the colder months. They do not need a period of cold to induce germination, and they will not germinate until the warmer weather arrives in spring. In the meantime, they may be killed by the cold, or be eaten by birds or animals, or they may rot. If you are sowing seeds of tender or tropical plants outside in areas where you have frosts, you would do better to wait and sow them when the weather warms up.
There are two quite different reasons for sowing seeds in the autumn or winter, and before sowing seeds this way, you should understand why you are doing it, so that you can choose the right sort of seeds and know what sort of results to expect.
Sowing seed of Hardy Annuals for early flowering the following year:
Seeds of hardy annuals, or perennials that flower in their first year, can be sown in the autumn and left out over the winter, instead of sowing them in the spring, to give them a head start over plants sown in spring, so that they will flower earlier. If you also sow another batch in the spring, you can have a longer period of flowering than if you sowed only one batch.
These seeds are generally easy to germinate, and it's quite possible that they will germinate within a few days of sowing, and continue to grow (slowly) throughout the winter, so that they are ready to start flower production as soon as the weather warms up in the spring.
These winter-sown annuals can be sown in situ (in the place they are to flower), or they can be sown in cells, pots or trays to be put into their flowering positions in the spring.
Because they are hardy plants, and will not ever have been kept in warmer conditions, they will not need 'hardening off'.
Some flowers that can be treated in this way are:
- Agrostemma (Corncockle)
- Alcea (Hollyhock)
- Calendula (Pot Marigold)
- Clarkia
- Consolida (Larkspur)
- Eschscholzia (California Poppy)
- Iberis (Candytuft)
- Lathyrus (Sweet Pea)
- Matthiola (Stock)
- Nigella (Love-in-a-Mist)
- Papaver somniferum (Opium Poppy)
- Viola x wittrockiana (Pansy)
Remember, if you sow plants that are not hardy, they may be killed by the cold weather over the winter. Virtually any plant that is hardy in the worst weather in your area can be sown in the autumn, but anything that would be killed as a plant in your conditions will be killed as a baby seedling.
On the other hand, because these seeds are designed to germinate and grow quickly so that they complete their life-cycle in one year, they do not need very cold temperatures during the winter. It is the extra time you are giving them that matters.
Sowing seed of Hardy Perennials that require a period of cold to break dormancy:Seeds of most alpines, many perennials, most shrubs and trees from temperate or cold regions, and some desert plants, need a period of cold to break dormancy and encourage them to germinate.
This period of cold is called 'stratification', because the seed was traditionally sown in layers (strata) of sand and kept outside over the winter.
It is important to note that this period of stratification must be carried out when the seeds are in a moist medium, such as peat, soil, sand, perlite, or some other type of compost. Storing seeds in cold conditions is not stratification. The medium in which they are kept must be moist, not wet, nor must it be freezing. Freezing wet seeds for any length of time will kill them.
Seeds sown outside for stratification are usually sown in pots. This makes it easier to remove those that have germinated or store those that have not germinated. Sometimes 'difficult' seeds may need several years of alternating seasonal temperatures before they germinate, so it is best to keep pots for at least two or three years, and seeds have been known to take five years or more to begin to show signs of life!
Alternatively, seed may be mixed with moist compost and kept in a small plastic bag until there are signs of germination. It is also possible to keep seeds, either in pots or plastic bags, in a fridge instead of outside.
Seeds of some species may need scarification (nicking or abrading the seed coat) before sowing, to allow moisture to penetrate. This is difficult with small seeds, and it is possible that the expansion and shrinking of seedcoats and/or moisture in the sowing medium during freezing and thawing may have the same effect as rubbing the seedcoat on sandpaper, puncturing the seedcoat. If this is the case, it may be important that the seed is actually sown in a layer of sand rather than compost.
Seeds requiring stratification are those which will not germinate readily if sown by other methods.
Seeds which seem to require stratification and take many weeks or months to germinate include:
- Anemone
- Clematis
- Corydalis
- Dictamnus
- Eremurus
- Eryngium
- Hebe
- Helleborus
- Iris
- Muscari
- Nomocharis
- Saxifraga
Seeds which are kept outside for stratification do not need protection from frost or snow. They may need protection from very heavy rain or birds/animals, but they should not be kept warm. The purpose of stratification is to expose the seeds to alternating freezing and thawing temperatures, so they should be left outside for several weeks at a time when this fluctuation in temperature is likely to occur, to allow this to happen.