Cold stratification – part 2, FAQ

The post prepared for today was too long (not again! I hear) so here’s just half of it. A FAQ and few interesting facts about snow insulation.

FAQ

First, which are the plant species that require this treatment in order to germinate well? Many of them, too many to list them here. There is data available for many of the cultivated species. In our seeds offerings these species have the germination info saying: sow in late fall/early winter or, cold-moist stratification required.

How long the cold stratification has to be?
Theoretically, the length of the required cold period can vary from a couple of weeks to a few months (see the last post). In some cases, it is impossible to tell the exact length period because it depends on the type of dormancy, the temperature applied and other factors.

How cold it has to be for the cold stratification to be effective?
 It has been proven that for many species regular fridge temperature around 0-4C is OK for the dormancy break and only some species (best known those from fam. Ranunculaceae, various Gentiana. probably other species as well) will actually require below 0C stratification.

However, not all species I’ve tried in the fridge reacted to the treatment. Not only the temperature matters, but also how it is applied. In nature, the temperatures vary during the winter months, and also from day to night. We cannot ‘reproduce’ this in our home fridge and this may be the reason why not all the species react well to stratification in the fridge.

How low is too low, temperature wise?
This question has preoccupied my for quite a while because I usually sown and keep all my pots outdoors. I still do not know the answer; probably no one has gone into this level of details. 
In any case, keeping in mind that these seeds are imbibed in water, and in part some physiological processes are already happening inside the seeds, we can assume that for example a range of: -10C, -15C would be too low.

Do we really need snow?
It depends on the stratification method and your location in regards with the winter minimum temperatures. Probably gardeners on the west coast do not need to worry about snow. The insulating benefits of snow are well-known, especially the fluffy kind of snow because the air that is trapped in between the flakes.

Doing a quick search AI kindly informs us that:
“Snow acts as a natural insulator, like a blanket, because its intricate crystals trap a large amount of air (90-95%), which slows heat transfer from the soil to the cold atmosphere, protecting plant roots and soil life from extreme temperature fluctuations and winds. The effectiveness depends on snow quality: fresh, fluffy snow is a great insulator, while hard, compacted snow is a poor one”.

There is also good info on various aspect like:
Depth thresholds: For significant insulation, a certain thickness is required. Measurable warming effects typically begin at 25–30 cm of snowpack. Research in 2026 suggests site-specific thresholds for ground stabilization can be as low as 4–7 cm in alpine regions.

Albedo effect: Snow’s high reflectivity (albedo) reflects 80–90% of sunlight back into space, keeping the surface cool during the day.

Average conditions: In moderate winter climates, 10 cm of snow can maintain ground surface temperatures up to 9°C higher than bare ground.

Extreme cold: Studies show that while air temperatures might drop drastically (e.g., to -15°C), the soil under just 10 cm of snow often stays near or slightly below freezing (0°C to -5°C), creating a difference of 10°C or more.

Apparently, 10 cm of snow has an R-value of approximately R-4 (roughly R-1 per inch; comparable to the insulating value of a layer of wood or a thin layer of fiberglass insulation).

We do not always have winters so cold like this one and we are lucky to also have copious amounts of snow, just like last year. So, even if the low was -14C in our area yesterday, with the +/-30 cm of snow the temperature at ground level should have remained decent!
Apparently, even 10 cm of snow are enough to do that.
It explains why the frozen pots I always find in very early spring, either buried in the ground or in the cold frame (while I had it), were not negatively affected germination and/or seedlings growth wise.

It also explains the buds apparition of the very early flowering species seemingly from the frozen ground in late February or early March, depending on the snow melt: the snowdrops, various Corydalis, Cyclamen coum, Eranthis, some peonies buds and even Helleborus.

In between the snowfalls, temporarily thaws are happening; we just had one last week. The snow melts in some degree and sheets of ice are forming. They are not good insulators and also the weight can be considerable (especially on top of pots above ground). But do not worry, over the years I noticed that there will be no damage.

Above ground container with Sarracenia purpurea two days ago and today. Drosera seeds were also sown in it in late fall – the easiest method of providing cold-moist stratification for seeds :) 

Next in Part 3 –  cold stratification, the practicals