A question for the biologists here (hobby and otherwise). What does it mean to cold condition my seeds? My fridge is about 8C or so. But permanently. If I sow seeds here in autumn I will have fluctuating temperatures if I'm lucky maybe up to minus 5 (which is about 20F). Would this be enough for plants of Noth America? Are there seeds which must be cold conditioned at lower temperatures than others?
Depends on the plant. Some need more involved stratification than others. For simple cold stratification, soak the seeds in water for 12 hours and then remove and store at just above freezing for the recommended period (one month is a good guess if you don't know). 8C is probably not cold enough for many plants that require stratification.
The only thing I would add to Bill's excellent answer is that you imitate nature. If you got some Siberian Larch seeds, you had better put them in the back of the freezer and forget about them for 3 or 4 months before you attempt to germinate them. If the plant in question is native to your area, I recommend storing the seeds outside so that they will get the full experience of winter, the one they are used to.
If you are into Zen, you can put yourself into the mind of the seed and ponder on what the conditions should be to make you awaken. The seeds of the tambalacoque tree, having adapted to passing through the gut of the dodo bird, had no one to process their seeds when the dodo went extinct. Until this was figured out, and attention was given to starting some seeds, the tambalacoque tree came very close to extinction.
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