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Plant hardiness maps increase state's average low temperature

STARKVILLE — New plant hardiness maps released by the U.S. Department of Agriculture increased Mississippi’s average low temperatures by 5 degrees.

Eric Stafne, Mississippi State University assistant Extension professor for fruit crops, said the maps provide good average information, but individuals still must make careful planting decisions.

“With fruit crops, I am more concerned with individual weather events than averages,” Stafne said. “It takes only one or two individual extreme cold events to put someone out of business, especially if they chose the wrong plant variety.”

Commercial producers and recreational gardeners use the USDA plant hardiness maps to determine the best plants to grow in different regions. Mississippi has four USDA plant hardiness zones, from zone 9a along the extreme Gulf Coast, 8b and 8a through central Mississippi and zone 7b in the northeastern third of the state.

USDA last updated the map in 1990, drawing from data collected from 1974 to 1986. The 2012 map reflects weather station data collected from 1976 to 2005. The average low temperature for that period was warmer than before.

“The ramifications of this can be mild or profound, depending on where you live,” Stafne said. “It could mean fewer chilling hours, causing erratic and early budbreak and bloom times. It could lead to more frost injury in northern areas, and it could lead to poor fruit quality on plants growing in now hotter areas.”

Stafne said for the plant hardiness map is a useful tool for home gardeners when they decide what plants to purchase and use. And although the map has changed, the temperature change has been gradual.

“I don’t believe the plant material that can be grown will change too much, so gardeners have no need to buy all new plant books or replace their landscape,” Stafne said.

Gary Bachman, Extension horticulturist at the Coastal Research and Extension Center in Biloxi, said savvy gardeners should check to see if their area has been reclassified.

“I don’t think we will see much difference in the ornamental landscape plants we grow during the summer months,” Bachman said. “Most of the flowering plants in the landscape are annuals, and there is little expectation beyond summer beauty.”

The biggest change, he said, will be that some plants previously classified as borderline for a hardiness zone can now overwinter just fine.

“This has occasionally occurred in most gardeners’ experiences, and I’m sure it will be a more frequent occurrence with the shift in hardiness zones,” Bachman said.

As the state’s zones have become slightly warmer, gardeners are more likely to try plants they previously thought were too sensitive to cold temperatures. While the maps indicate the lowest average temperatures, cold damage will still occur when temperatures drop to the low 20s along the coast and the teens and lower in North Mississippi.

“Gardeners should look at this as a situation where the glass is half full,” Bachman said. “It gives us the opportunity to plant something besides what we’ve normally planted.”

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