New Plant Hardiness Map For Rose Growers

This guest article by Mary Treacy discusses the impact of the new hardiness zone map for rose gardeners in the Upper Midwest.
As the days lengthen and the sun shares just a bit of warmth Minnesota rose growers are busily adjusting to the recent announcement from the U.S. Department of Agriculture (USDA) and the Arnold Arboretum at Harvard University. The colorful and interactive map of Plant Hardiness Zones inspires unbridled hope in the heart of intrepid North Country gardeners. As every gardener, farmer, horticulturalists and meteorologist knows, the long-awaited map depicts and affirms that it really is getting warmer.
Admittedly, it will affect roses and rose growers more than it will mean to the man or woman on the street. The hardiness zones reflected in the USDA map are based on a scale of 13 hardiness zones, two of which have been added to reflect increased temps in Zone 11. Zone 1 is really frigid (temps as low as -50 degrees) with the Zone numbers rising with the temps to 13B (no lower than 40+ degrees).
The new map, based on the publication of the nation’s new climate normal, means those rose lovers in Southern Minnesota will till the soil in Spring 2012 in Zone 4B, a warmer geography. They will share Zone 4B with lowans who have long boasted of their rose-growing prowess while Minnesotans have struggled to coddle their winter-threatened plants.
Though some “urban heat islands” in the metro area skyrocketed to the 5A Zone local horticulturalists are skeptical about that data. On the other hand, the 2B Zone, where Northern Minnesota was earlier classified, has thawed to 3B status where much of the state is now rated.

In spite of the warming trends reflected in the USDA maps, some optimistic rose growers were disappointed. Because it was a 22 year wait for the revised hardiness zone map, and probably because all the talk about global warming, some horticulture groups had already created unofficial maps that put Southern Minnesota into Zone 5, a tropical zone by horticultural standards. It is important to note that meteorologists and climatologists are disinclined to factor global warning into the USDA calculations.
The validity of the new map rests on the USDA affirmation that the data are based on 39 rather than 13 years of data collection. Catherine Woleki, undersecretary for research education and economics at USDA, is widely quoted as assuring the public that the new map depicts readings from 8000 widely distributed temperature-observing stations takes into consideration factors such as slope, terrain and proximity to large bodies of water. Fortunately for growers, statistical information was matched with what USDA calls “ground truth”, the observations of experts who are actually on the grand, in the field or in the garden. Statistics may lie, but horticulturalists do not.
Gardeners are advised to remember that this is still Minnesota and that -25 is still a bit nippy. Area experts in both weather and gardening express some skepticism that reassignment to a warmer hardiness zone is any guarantee. State director of the Master Gardener program through U of M Extension, Julie Weisenhorn, says that “people are welcome to try stuff, but they have to be prepared that it might not make it if it’s from another zone.” Meteorologist Mark Seeley acknowledges that the shifts in temperatures are measured in whole degrees, even several degrees, but reminds enthusiasts “to go back to February 1, 1996, when we were minus 33 degrees in the Twin Cities and they were minus 60 degrees up in Tower.” “That’s a little disconcerting in light of the new plant hardiness zones,” he observes.
So what is rose grower to do at the nursery? Master Gardener Weienhorn calms their angst by assuring St Paul Pioneer Press reporter Tom Webb that “when a consumer goes and purchases a plant at the garden center, it’s not going to say Zone 4A, it’s going to say Zone 4-7… So to the consumers, it not going to matter that much.”
For rose lovers, spring is already in the air as catalogs gather, garden centers beckon and thumbs take on a greenish hue. One way to bide the time is for the eager gardener to probe the intricacies of the Plant Hardiness Zone Maps it’s far more interesting than its name suggests.
Mary Treacy is an avid blogger about North East Minneapolis and environs
events and resources.
Note: See the Climate Zones and Rose Gardening article for information about the old USDA Plant Hardiness Zone Map.

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