Practical considerations for sowing in late fall and winter
For those who don’t know it yet…the devil is in the details, sometimes.
Sowing in late fall – early winter: what is different than fall sowing?
Most important is to consider that seeds must enter the cold cycle well imbibed with water. When sowing in late fall when temperatures are still well above freezing, this is not a problem. By the time the cold arrives (meaning temperatures around 0C and below), the seeds have already soaked a lot of water and will enter the cold stratification well prepared.
But when sowing in late fall (November) and advancing into the winter, even if the pots are well watered, the seeds may not be able to absorb the water fast enough when they are placed right away outdoors.
For this reason it is best to keep the new pots with sowings indoors for about 2-3 (4)weeks, and move them outdoors in the cold after that. It can also happen that some seeds will germinate, even when known to be cold germinators!
The same concept applies if you want to start the cold/moist stratification in the fridge.
Place the seeds in the moist medium of your choice (vermiculite, sand or wet towels) and keep them at room temperature first for a few days, before placing them in the fridge. You will be surprised by the change most seeds will undergo while absorbing water in only a few days.
I can exemplify with Fritillaria and Tulipa seeds that I plan to grow under lights indoors this year (2020). In this purpose I kept them at room temp. for about a week before placing them in the fridge; in some cases the embryos are already visible (hover the mouse for names).
Talking about stratifying seeds in the fridge, please consider first what species you want to germinate. Most of them germinate best when sowed and placed outdoors for the winter.
In the case of species known to require low temperatures for dormancy breaking (like various Gentiana, Saxifraga, most Ranunculaceae), stratification in the fridge at +/-4C is not a good option (unless you have a fridge dedicated to seeds and can vary the temp.).
True that in case of a mild winter seeds may not germinate in the first spring, even if sowed outdoors! Breaking the dormancy requires temperatures below zero for various periods of time, and if specific cold period requirements are not satisfied in the first winter, the seeds will need an additional cold cycle to germinate. In this latter case, keep your pots and take care of them until the second spring.
Talking moist packed seeds: a reminder that these seeds are in full development by late fall (their warm cycle completed and the cold one started), and in some cases there may be signs that the radicles are just about to emerge (classic examples: Asarum canadense, Trillium grandiflorum…).
The moist packed seeds require a cold cycle for breaking the shoots dormancy at this point, so they should be sown asap and placed outdoors. The best option is to use a cold frame, a cold garage, or in absence of these options bundle the pots together and fingers crossed for snow (alternatively, cover the pots with a thermal fleece, tarps, in case it gets very cold with no snow cover).
