In reality, how cold it gets in the winter has almost nothing to do with when you can actually plant in the spring. When you can plant is more related to when the frost has left your soil, hard freezes have stopped, and when the ground drains and dries out quickly and is workable. Some areas of zone 4, which gets really cold in the winter, actually thaw out almost 2 months before other areas that are zone 5 or 6. And then you have the snow belts where the snow takes forever to melt, so the soil stays frozen a lot longer. Basically areas with much longer winters may not be all that cold (zone 6) but take longer to thaw and dry out so planting can be a month or more later than some in zone 4.
So what do I do in these cases? When I get one of these requests I look up the average last frost dates and average temperatures for that exact location and schedule the dormant plant shipments when the average low temp is just at or slightly above freezing and the average daytime temps are close to 60 degrees. Based on that, some people say that is too early, others say that is too late, and the rest say that is perfect, but we get it all worked out
I guess that looking up all of this data over the years has simply made me realize that there is no such thing as a proper planting time for any one zone. Yet it seems a lot of mail order companies try to lump everyone into one set shipping time for each zone. The best for me is when people just pick a date, or a date range. I can schedule those a lot easier
