Let me explain. To assist our country’s 80 million gardeners, the United States Department of Agriculture produces the Plant Hardiness Zone Map. The map was updated in 2012 for the first time in twenty two years to reflect temperature data, and we’ve been shifted a half zone warmer.
The map contains thirteen North American zones with Zone 1 being the coldest and Zone 13 the warmest.
Each zone is further subdivided into A and B. For most of my horticultural life North Dakota and the corresponding region of Minnesota were considered Zone 3, but we’ve warmed.
Much of North Dakota and Minnesota are now in Zone 4A with average winter minimums of -25 to -30 degrees Fahrenheit. The northern several tiers of counties are in Zone 3B.
But there’s a problem. Many labels and descriptions simply list Zone 4 without dividing into A or B. Our region is Zone 4A. Zone 4B is farther south. Zone 4 is a large geographic area. What performs well in Minneapolis might not fare well in Fargo, although both are in Zone 4. (Fargo 4A, Minneapolis 4B, but information often doesn’t differentiate)
Zone 4 material should be approached with caution, since plants labeled Zone 4 may be borderline in hardiness for some of us. Investigation is often needed to see if a tree, shrub or perennial listed as Zone 4 has a history of regional success.
This emphasizes the importance of microclimate. A microclimate is a small portion of the larger climate, such as found in a home yard that is modified favorably by sunlight or shade, moisture, wind protection, and proximity to structures and established plantings.
A plant that may perform well in the microclimate of a sheltered, established yard in Fargo or Bismarck might not survive on an exposed, windswept hillside surrounding my native Lisbon. Even within the same city, borderline plants are more likely to “over- winter” in older, more sheltered neighborhoods than in new housing developments having few trees and little protection to buffer wind and elements.
Homeowners in new developments should consider choosing trees, shrubs, and perennials hardy in Zones 3,2, and 1. Stick with these for starters. Get them well-established and you will begin creating your own microclimate that will eventually allow you to diversify into Zone 4 plants.
Well, we still aren’t in the peach belt, but there’s no where else I’d rather be. As always, “Happy gardening!” And enjoy our new-found Hardiness Zone.