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The USDA revised its map in 2012. If climate change continues to accelerate as many people fear and as more and more scientists believe, the averages will rise. But for now, expect the numbers of the zones to remain the same, even if the details of each zone changes.

The most important revisions made to the existing zone maps so far have been to list finer details. For example, in the USDA map, the more central zones have been split into halves, so you can also see Zone 5a and Zone 5b and so on, with the a being somewhat colder and the b being somewhat warmer. Canada’s map includes even more split zones.

Sometimes gardeners blame heat or cold for the death of their plants, but other reasons may be the causes, such as following:

 Poor drainage: Poor soil drainage is big killer of plants especially during a wet winter.

 Dry conditions: A very dry winter can also cause a lot of damage and even plant death for evergreen plants that continue to use and lose water during the winter. These plants should be watered when the ground isn’t frozen, if needed. Sometimes it’s winter drought, rather than winter cold, that kills a garden plant.

 Winds: Drying winds can be very damaging. Some gardeners provide windbreaks to relieve this problem.

 Varmints: Don’t forget varmints, especially voles and mice that can do invisible underground root, stem, and bulb damage that occurs most in the winter when less of their natural food is around.

Climatological data comes from the horticulture industry, university researchers, botanic gardens, and so forth. Sometimes when a new plant enters commerce, the zone rating is conservative and after a while — with more people growing the plant in different areas and with more research — they find the plant to be more or less cold-tolerant than they originally thought.