When you start talking gardening here in North Texas, you really need to zoom in the view. There are a lot of us here, and we occupy a big spread of real estate. However you slice it, it's a two-hour drive to cross from one side to the other. And, during that drive, you're going to pass through parts of three U.S.D.A. Plant Hardiness Zones. You'll encounter a variety of soils, and the plants that are native may vary from one mile to the next. This place is a lot less homogeneous than many might think.
About those zones
The federal government conducts ongoing surveys of every county in America to determine the lowest temperature that might be expected each winter. Based on that historic data, they plot the results onto a detailed map that charts the expected minimum temperatures in 10-degree increments. Zone 7, for example, can expect winter temperatures to drop between 0 and 10F. Zone 8 sees average lows between 10F and 20F. Zone 9's lowest temperatures fall between 20F and 30F. As data accumulates, boundaries do shift.
Trees, shrubs, vines, ground covers and perennials are grouped according to the lowest temperatures they can endure. What makes choosing "winter-hardy" plants more challenging is the fact that any given landscape has little "micro-pockets" of warmer, more protected air. If you have a sheltered atrium or an enclosed patio courtyard, it may be 5 degrees warmer there than it is out in the open in another part of your yard. That can make a big difference in the survival of borderline plants.
There also is a phenomenon weathercasters refer to as the "urban heat influence." Concrete absorbs heat during the sunny days, then releases it back at night. Cumulatively, over a large downtown area like Fort Worth or Dallas, that can mean winter lows that are consistently 6 to 10 degrees warmer than the suburbs. That can represent an entire Hardiness Zone in your plant opportunities. If you're near a downtown area, or if there are large expanses of concrete near you (industrial parks, shopping centers, etc.), don't be surprised if it's warmer in your garden at all seasons of the year.
So, going back to our Metroplex model, this urban heat impact might generally suggest that our downtown areas could almost verge on being Zone 9, particularly in protected locations. Of course, one bad cold spell could wipe you out if you had chosen too many marginally hardy plant species.
Moving outside the downtown by five or 10 miles, our close-in suburbs could easily be figured to be Zone 8, while northern, and more rural North Central Texas cities are safer being listed as Zone 7. The list of plants suited to those more rural settings will be very different from what people might use in the shadows of downtown office buildings.
For much of Dallas-Fort Worth, the last killing freeze will be somewhere in the March 15-20 period, while the first killing freeze of the fall will average Nov. 15-20. The normal rule of thumb is that you gain or lose one week at each end of the growing season for each hundred miles you travel north and south. But, that's a rural average over Texas' farmlands, pastures and prairies. When you stir that urban heat impact into the mix, you might gain as much as a couple of weeks in the spring and another couple come fall. That means that you might get one bonus month to garden when you do so near the city.
Talkin' about dirt
Soils vary amazingly here. Most of our area was originally a tall grassland prairie, and most of that was in the alkaline, blackland clay gumbo soils. However, if you're driving east or west in the Metroplex, you'll cross through bands of brown or red, sandy loam soils. Those soils are slightly acidic, so you'll be able to grow plants that would be much more difficult in the alkaline clays.
These ribbons of sandy loam soil come down from the Red River through eastern Denton County, into parts of Lewisville and Grapevine, through the Mid-Cities, Arlington and east Fort Worth, down through Mansfield and toward Cleburne. Similar bands are in Wise and Parker counties, also in southeast Dallas counties, and in almost all cases, native post oaks (Quercus stellata) are the markers you can use to know where the sandy loams are.
Soil depths also will vary. Most of our area is covered by the black clay gumbos, but the depth of that soil, before you hit white caliche bedrock, will vary from a few inches to many feet. Since you need several feet of topsoil to sustain large shrubs and shade trees, really shallow soils can present unusual problems. Let your local nurseryman, who is familiar with the area, guide you into good decisions.
All of which is to say, if you're going to be successful at a game, you really need to know the field on which you're playing. 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.