Wednesday, January 25, 2012

Movin' On Up -- New USDA Plant Hardiness Zone Map

The USDA recently released a new Plant Hardiness Zone Map (

2012 USDA Plant Hardiness Zone Map.
What type of data is this mapped based upon, and how is it different from previous years? Here's what the USDA website states:

"Zones in this edition of the USDA Plant Hardiness Zone Map (PHZM) are based on 1976–2005 weather data. A trial check did not find that the addition of more recent years of data made a significant difference in the definition of these zones. Each zone represents the mean extreme minimum temperature for an area, calculated from the lowest daily minimum temperature recorded for each of the years 1976–2005. This does not represent the coldest it has ever been or ever will be in an area, but it simply is the average of lowest winter temperatures for a given location for this time period.

The previous edition of the USDA PHZM, revised and published in 1990, was drawn from weather data for 1974–1986. The longer period (30 years) of data was selected by the group of horticultural, botanical, and climatological experts who led the review of the latest revision as the best balance between smoothing out the fluctuations of year-to-year weather variation and the concept that during their lifetimes, perennial plants mostly experience what is termed "weather" rather than "climate."

Because this map was created digitally with GIS technology, it has a higher level of resolution and can show smaller areas of zone delineations than ever before. For example, cities tend to hold more heat because they have large amounts of concrete and blacktop, so a city or town may be assigned to a zone warmer than the surrounding countryside. Higher elevations tend to be colder than surrounding lower areas, so the top of a mountain may be an area of cooler zones. A location near a large body of unfrozen water may provide milder winter weather and be in a warmer zone."

So what does this mean for gardeners in Virginia, particularly the Tidewater area? Minor zone changes for some gardeners, probably not too much change for most other gardeners. Here is the detailed map of Virginia reflecting the above changes:
2012 USDA Plant Hardiness Zone Map for Virginia.
Many of the inland areas and some outlying coastal areas retained their previous zone designations. But it is interesting to note that GIS technology has allowed the USDA to define smaller zone delineations than in past versions of the map. 
.09 Acres is located in Newport News, Virginia, two blocks from the James River and near the end of the Virginia Peninsula. The southern exposure of my neighborhood on the north bank of the James River and warmth-retaining properties of the river and Chesapeake Bay are factors that influence my hardiness zone. In previous years, I was in Zone 7b. The new map has moved up my location a notch to 8a, reflecting these characteristics and suggesting we have slightly warmer winter lows than indicated on past USDA hardiness maps. In reality, my fenced/protected backyard is likely another half-zone or zone higher, reflecting an even more defined micro-climate.
It is important to note that there are always variations within these types of designations, but the USDA Plant Hardiness Zone Map can be a helpful general reference for gardeners. 
Did your hardiness zone remain the same or change from the past USDA map? Do you provide micro-climates for your plants to stretch the boundaries of your hardiness zone? Or do you say, "Phooey to all this this scientific mumbo jumbo! Mother Nature does what she wants and I have no control over her whims. This arbitrary designation doesn't mean a thing to me or my plants."
I'm looking forward to hearing your thoughts.


Ed said...

Very interesting, my zone hasn't changed, it's still 6b. It's good to see these maps getting more precise. My former garden less than 20 miles away is now a zone higher.

Thad said...

It appears that we may have moved up a zone since we are now right on the 7a/b line. In the previous map, we were always 7a. Don't think that it will really change any of my plans for the new garden.

Sybil Mays said...

It's like our conversation here at the farm about microclimate - the dramatic changes in "zones" we all have, depending on wind protection, sun orientation, shade and all of that. Here at the farm in south Virginia Beach, but well inland and in open fields, I'm an easy 8b to cool 9 - allll most 10 on the south side of the house, out of the wind, next to the brick and an easy 7b on the cold NE side out in the open field.

PS Your darling backyard garden looks great in that photo!


Laura said...

I've also moved from 7b to 8a. Makes me wonder what else I might try to grow that I was afraid to before. I still can't seem to get artichokes to overwinter, though. Of course, it would help if I got them to really grow better in the first place.

I'm still hoping that someday USDA will come out with heat and humidity zones.

Sybil Mays said...

Laura, the USDA hasn't but the American Horticultural Society has! Check out their heat zone map here:

Sybil @

Laura said...

Thanks Sybil, I forgot about that one. I guess I'm jealous of the Sunset zone mapping that was developed for the Western U.S. The AHS map puts tidewater VA in Zone 7, but so is lots of Nevada.

.09 Acres said...

Thad, I agree about not necessarily changing my grow plans. I likely wouldn't unless there were significant consistent changes in the mildness of winter around here. Speaking's been very mild this year.

Sybil, thanks for chiming in with your wisdom and info. How's the "sap-suckered" tree? Hopefully it's on the mend.

Laura, I agree that heat and humidity play a significant role in how well or poorly things grow in a certain climate. I'll have to look at the AHS map Sybil referenced.