What Gardening Zone Is San Antonio Texas

Global warming is hitting not just home, but garden. The color-coded map of planting zones often seen on the back of seed packets is being updated by the government, illustrating a hotter 21st century.

It's the first time since 1990 that the U.S. Department of Agriculture has revised the official guide for the nation's 80 million gardeners, and much has changed. Nearly entire states, such as Texas, Ohio and Nebraska, are in warmer zones.

The new guide, unveiled Wednesday, arrives just as many home gardeners are receiving their seed catalogs and dreaming of lush flowerbeds in the spring.

It reflects a new reality: The coldest day of the year isn't as cold as it used to be, so some plants and trees can now survive farther north.

Texas has eight hardiness zones, which are warmer classifications than they had been under the old map, officials said.

As a result, more warm-weather plants were described as suitable for growing farther north in the state.

Bexar County is split between two zone categories, 8B for the northern half of the county and the slightly warmer 9A in the southern half. Previously, all of Bexar was in 8B.

Michael Brown, owner of the Grass is Greener Landscape Inc. in San Antonio, said the city is situated in a transitional area between the Gulf Coast and the Hill Country that makes it a tricky place for some plants. Extreme freezes like the one that killed palm trees in San Antonio last year might not be part of the calculations that produced the new zones, he said.

The new plant guide uses better weather data and offers more interactive technology. For example, gardeners using the online version can enter their ZIP code and get the exact average coldest temperature.

Also, for the first time, calculations include more detailed factors such as prevailing winds, the presence of nearby bodies of water, the slope of the land, and the way cities are hotter than suburbs and rural areas.

The map carves the U.S. into 26 zones based on five-degree temperature increments. It includes two new warmer zones: 12, for climates with average lows of 50 to 60 degrees Fahrenheit, and 13, for 60 to 70 degrees.

Those differences matter in deciding what to plant, but just up to a point.

"The map is not a good instrument for determining climate change," said Kim Kaplan, a spokeswoman for the USDA's Agricultural Research Service. "In some cases where areas changed zones there was less than a one-degree change in temperature."

David W. Wolfe, a professor of plant and soil ecology at Cornell University, said the USDA is being too cautious and that the map plainly reflects warming.

The revised map "gives us a clear picture of the 'new normal' and will be an essential tool for gardeners, farmers and natural resource managers as they begin to cope with rapid climate change," Wolfe said in an email.

Plant experts also cautioned that additional information about soil makeup and growing conditions should be obtained to feel confident that different plants will survive different regions, officials said.

"It's merely a guideline that says the typical winter temperatures might be," said Michael Arnold, a professor of landscape horticulture at Texas A&M University. It's "not an absolute."

Express-News Business Writer William Pack, San Francisco Chronicle Staff Writer Stacy Finz and the Associated Press contributed to this report.