Seed stratifying in the garden are visible again as the snow melts. They are covered with a double window frame which I lift off when it rains or snows…
Stratification is also known as cold treatment. Most perennials are adapted to germinating irregularly, sometimes over several years, to maximise the chances of favourable spring weather conditions. The seed coat is broken down as a result of the winter weather, freezing and thawing. I germinate my perennials by putting them out in the garden to “stratify” naturally. Some people bring the seed inside (a greenhouse) to germinate in early spring. I wait for them to germinate naturally outside as I don’t have much space inside in spring and I’m in no hurry as these are perennials that will be around for many years and saving a month is of little importance :)
See more here  https://en.wikipedia.org/wiki/Stratification_%28botany%29