Why a big chill is cold comfort for dormant seeds
There are a number of misconceptions about dormancy, which is why it helps to understand the science behind it
Look online for advice on germinating seeds that need a period of winter chilling to break dormancy, and it won’t be long before you come across the following, or something like it: “Sow your seeds in a pot and leave outside to let frost break down the seed coat.” This is reasonable enough advice, and quite likely to work, but I’m a firm believer that you’re a better gardener if you know not just what you’re doing, but why you’re doing it, and that isn’t quite right.
If a seed needs cold, it’s generally to do one of two things, and sometimes both. Some seeds have an immature embryo that will only grow to full size at low temperatures.
Other seeds (the majority) simply have a chemical switch that is turned from “off ” to “on” by a cold period. In neither case does this have much to do with the seed coat.
Nor does it have anything at all to do with frost. If a seed needs cold to break dormancy, the exact temperature required varies from species to species, but anything below 10C is usually OK. Mentioning frost could lead you to believe your seeds need to be frozen, which they don’t; frozen seeds will just stay dormant. Something else that I’ve been guilty of assuming is obvious, but perhaps isn’t, is that for the cold to work, seeds need to be wet, or “imbibed”, technically speaking. Low temperatures will not break the dormancy of dry seeds.
GREEN SHOOTSThe sequence of germination, main; alpines, below, have few species with any seed dormancy. Bottom: chilling seeds, known as stratification