Plant Hardiness Zones On The Move

The Plant Hardiness Zone Map is used by gardeners to determine which plants are best to grow in a particular area.  The map is based on the average annual minimum winter temperature, divided into 10-degree F zones.  I mentioned in a recent post that this year might be a good time to think about starting a backyard garden and this information would be helpful.  The warming climate across the U.S. is causing the range of plants to move poleward.

Average low temperatures for the U.S. (1895-2010).  Image Credit: Climate Central.

Note that the long-term trend has been moving up, however the 25-year moving average has shown mild periods and cooler periods.  However, there has been a significant increase in the moving average since the late 1970s.  As a result the U.S. Department of Agriculture (USDA) had to update its zone map due to the warming of recent decades.

Changes in the Plant Hardiness Zone Map from 1990 to 2006.  Image Credit: Arbor Day Foundation.

South Carolina as a whole has bucked the national trend.  The long-term trend has been for slight cooler average low temperatures in the coldest month.  However, the 25-year moving average has seen similar ups and downs like the national trend, just not as much.

In the graph below, you can see how South Carolina's cooling trend emerges from the backdrop of year-to-year changes, which are even more variable than decadal swings (no surprise to meteorologists). Click here for an interactive version where you can show or hide the linear trend, exponentially weighted moving average, and annual variations by selecting on the appropriate item in the legend.

Note the variability in the average annual low temperature for South Carolina as a whole.  Image Credit: Climate Central.

The recent warming has been enough to change the Plant Hardiness Zone Map for South Carolina.  Columbia was at the north end of zone 8 in the 1990 version.  The new release has zone 8 extending as far north as Greenville and Charlotte.

Image Credit: USDA.

Notice that there is still a small area of zone 7 just north of Columbia in northwestern Richland county and southwestern Fairfield county.  This is the area around Cedar Creek which is a known cold spot.  Low temperatures are frequently 5-10 degrees colder than Columbia.  The reason is related to the topography.  Cold air drains to the lowest spot and the terrain is higher around the reporting station with the cold air draining to it on clear, calm nights.

The 1990 version of the zone map lasted for a little over 20 years.  It is likely to be updated again in 15 to 20 years due to the changing climate.