When the U.S. Department of Agriculture released its updated Plant Hardiness Zone Map in January, it prompted quite a bit of conversation in the horticultural world.
The new map had been long-awaited and was the first update since 1990. It indicates the average minimum winter temperatures for all areas of the United States. As you may know, one way plants are categorized is by the hardiness zones for which they are best adapted.
The new version of the U.S. map includes 13 zones, with the addition for the first time of zones 12 (50-60 degrees) and 13 (60-70 degrees). Each zone is a 10-degree temperature band, further divided into 5-degree zones – a and b.
The new map also offers a Geographic Information System (GIS)-based interactive format and is specifically designed to be Internet-friendly. The map website also incorporates a “find your zone by ZIP code” function. Static images of national, regional and state maps also have been included to ensure the map is usable by those who lack broadband Internet access. The new hardiness zone information from USDA is located at http://planthardiness.ars.usda.gov.
In developing the new map, USDA requested that horticultural and climatic experts review the zones in their geographic area, and trial versions of the new map were revised, based on their expert input.
So, what does the map show compared to the 1990 version? Hardiness zone boundaries have shifted in many areas.
The new map is generally one 5-degree Fahrenheit half-zone warmer than the previous map throughout much of the United States. This is mostly a result of using temperature data from a longer and more recent time period; the new map uses data measured at weather stations during the 30-year period 1976-2005. In contrast, the 1990 map was only based on temperature data from 1974-1986.
The previous map had Louisiana located in USDA hardiness zones 8 and 9. Average minimum temperatures are: zone 8a – 10 to 15 degrees, zone 8b – 15 to 20 degrees, zone 9a – 20 to 25 degrees and zone 9b – 25 to 30 degrees. We are still located in these hardiness zones, but with several noticeable changes.
In the prior map, all of northern Louisiana along I-20 was located in hardiness zone 8a. Now, a significant portion of northeast Louisiana is comfortably in hardiness zone 8b. Portions of Caddo and Bossier parishes are also now located in zone 8b.
Hardiness zone 9a extends more northerly up I-49 from the Lafayette area, and all of the metropolitan areas of Baton Rouge, Lafayette and Lake Charles are in hardiness zone 9a. The previous map had these areas on the border between zones 8b and 9a. The new map also shows a warmer New Orleans area, which is now in zone 9b and surrounded by zone 9a, while in the previous map it had been in zone 8b. Zone 9b is the warmest zone in the state and includes the coastal regions of St. Bernard, Plaquemines, Lafourche and Terrebonne parishes.
When the 1990 USDA hardiness zone map was released, the decade of the 1980s had been brutally cold in Louisiana, culminating in the devastating freeze of December 1989. So it was not surprising that the new 1990 hardiness zone map tended to move zones south from where they had been. In Louisiana, for instance, zone 9 dropped from just south of Alexandria to only a few locations along the coast (and an odd blob to the west of Lake Pontchartrain).
So, what does all this mean? Many weather observers have been saying that the climate is warming. On the other hand, although the new map is quite different from the 1990 map, it is strikingly similar to the USDA map used prior to 1990.
Based on this new map, we have to revise our approach for horticulture, gardening purposes.
Citrus are growing more reliably in areas of Louisiana where many home gardeners did not grow citrus before due to the possibility of cold damage. Recommendations for fruit tree variety growing regions may need to be adjusted because trees such as figs, peach, pears, plums, apples and others have chilling hour requirements. Tropical plants are also overwintering in more northern areas in the state.
The bottom line is, we need to keep in mind that the temperatures posted in the new map are average minimum temperature; however, colder weather can still occur. So we need to be sure to continue to consider growing conditions like hardiness zones when selecting our ornamentals, fruit trees, vegetables and other plants.
For more information, contact Dr. Chris Robichaux, county agent/area horticulturist, St. Martin/Iberia Parishes, at 332-2181 or 369-4440.