Zone "in," gardener. A really important batch of data have just been released by the U.S. Department of Agriculture and the U.S. National Arboretum in Washington, D.C., and it could really help you be more successful gardening here in North Texas -- or anywhere else in America.
For the first time in 21 years, the USDA Plant Hardiness Zone map has been reconfigured, and things have shifted around just a bit. This is the map to which you'll see references in almost every nursery catalog or website, and on almost every plant label. "Zone 7a." "Zone 8b." "Zone 9a." What's this all about, anyway?
Research scientists are collecting data on temperatures, county-by-county, day-by-day, forever into the future. Those temperatures are recorded and plotted, and a "connect-the-dots" type of map is drawn that aligns all areas with similar temperatures.
The map speaks specifically to the average lowest temperature reached annually in a given place. In layman's terms, "How cold does it get here in winter?"
This is the map that will show you what to expect, city-by-city, across America. It's fascinating to see how it all works. The url for the website is planthardiness.ars.usda.gov/PHZMWeb/#. If you lay the paper down and haven't committed all that to memory, just search online for "new USDA Plant Hardiness Zone Map."
But, we still haven't addressed how this is going to be of concern to a gardener. The missing fact is that every plant species has a minimum winter temperature below which it cannot survive. Some plants (tomatoes and tropical hibiscus, as examples) freeze at 32 degrees. Others (birch and blue spruce) tolerate temperatures 60 or 70 degrees colder. Over the past 100 years, nursery owners and other plant people have collected and grouped all the data, so we now know how much cold a given plant species will tolerate. It's all broken down into 5-degree increments.
Sure, length of the cold exposure and prior conditioning to progressively colder weather ("hardening") also figure in, but a plant's "winter hardiness" to a specific minimum temperature is fairly predictable -- enough so that it's a really useful piece of landscaping information.
How it affects DFW
Here are the Hardiness Zones that appear in the main circulation pattern of this newspaper, along with the minimum temperatures we're likely to see in each zone each winter.
-5 - 0
0 - 5
5 - 10
10 - 15
15 - 20
20 - 25
If you've been fairly conversant regarding the old map from 1991, one of the first things you're going to notice about the new map is that we're shown to be a little bit warmer in winter.
On the old map, Tarrant County showed to be Hardiness Zone 8a, while Dallas County was shown to be Zone 7b. In the new map of 2012, the DFW Metroplex shows to be squarely in the middle of Hardiness Zone 8a, which means that we can expect temperatures here to plunge to 10 to 15 degrees at least once each winter.
When you start buying landscape plants this spring, that means you'll want to look for those listed as being "winter-hardy to Zone 8a and northward."
For example, you can almost always succeed with a shrub listed as Zone 7 if you're gardening in a Zone 8 area. By comparison, if you try a Zone 8 plant in a Zone 7 area, you're very likely to lose it the very first winter. Buy plants listed as being suited to your Hardiness Zone or to the zone immediately to your north (slightly colder).
All of this heroic work employs Weather Service data from the past 120 years. However, we of the Half-Full Society will warn you that, just as temperatures have warmed the past few winters, so they could also fall in upcoming years. We plunged down around zero the night of Dec. 23, 1989. Regardless of the new zone map and all that research, we're capable of revisiting those temperatures in the next few years.
The takeaway, then, is that we are now considered to be Hardiness Zone 8a, but that we'd still be well advised to stick mostly with plants listed as being for Zones 7 and 6. While Zone 8 plants do appear to be more suited now than we thought they were 21 years ago, we probably shouldn't commit big parts of our landscapes to them.
Neil Sperry publishes Gardens magazine and hosts Texas Gardening noon-1 p.m. Saturdays and 9 a.m.-noon Sundays on WBAP AM/FM. Reach him during those hours at 800-288-9227 or 214-787-1820.