In 2004, the National Arbor Day Foundation released a revision of the USDA’s Plant Hardiness Zone Map. This map, which you can find at www.arborday.org/media/zones.cfm, shows that the zones have shifted northward since 1990. Some people see this change as a clear sign of global warming; others claim that the Arbor Day Foundation’s 15 years of data collection is statistically inconclusive. Most gardening sources still use the USDA’s map.
The USDA is reportedly revising its map, though it hasn’t released a timetable for when the map’s due out. If global warming 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.
The USDA is reportedly revising its map, though it hasn’t released a timetable for when the map’s due out. If global warming 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.
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