Gardeners can utilize plant hardiness guides to create thriving gardens

By Beth Neels | The Associated Press

In 2023, the United States Department of Agriculture (USDA) updated its Plant Hardiness Zone Map (PHZM) for the first time in 10 years. The new Map draws on more than 30 years of data, utilizing better technology. But it’s the work of 8,500 volunteers who comprise the NWS Cooperative Observer Network, collecting daily weather observations across all fifty states, that ensures this is the most accurate map to date.

For almost a century, the illustrated PHZM has been a vital tool, the standard for deciding what to plant and where for gardeners, farmers, and horticulturists. The choice of a locally hardy plant is the key to its health and yield success. There are very few rules in gardening, but perhaps the most important one is to work within your zone.

While shopping at the local garden center, select any plant, and the tag will list the plant hardiness zone range. Garden center managers typically only purchase varieties known to be hardy in their local area to make shopping easier for consumers.

What is the PHZM?

The PHZM is a scale growers use to determine a plant’s ability to survive in given climates and conditions. The newest version of the hardiness map utilizes Geographic Information System-based (GIS-based) data, and the digital version’s finer scale improves accuracy.

The map indicates a given area’s lowest average winter temperature. For instance, orange trees are hardy in zones 9-11, meaning they thrive in warmer climates. Apple trees prosper in zones 3-8, as they require cold weather (below 45°F) for at least 500-1,000 hours in the winter to bloom and produce fruit.

Utilizing hardiness data lets growers skirt avoidable plant deaths due to unsuitable conditions and promotes a better understanding of one’s local environment.

How is the map created?

W.C. Woodbridge’s earliest global climate map, published in 1823, identifies global zones as torrid, hot, warm, and cold, among other descriptors.

The newest map compiles information from the National Weather Service (NWS), USDA Forest Service, and the U.S. Department of the Interior. This map depicts weather observed for the past 30 years. More than 13,000 stations contributed information to the PRISM Climate Group of OSU based on the 30-year average annual extreme minimum temperature, bolstering data accuracy.

These stations include two types of gathering equipment: “First-order stations,” which are more accurate, have more sophisticated equipment, and are located mostly at airports, and “co-op stations,” which comprise a network of daily weather observations nationwide.

Why is this important?

Like tomatoes, many vegetables grown in northern climates are hardy in zones 9 and 10. This distinction treats tomatoes as annuals in colder areas, as they cannot survive the winter. These vegetables can grow in movable pots that growers can relocate to greenhouses amid harsh winter weather.

Georgia, or the aptly named “ Peach State,” has experienced abusive heat in recent years, negatively impacting fruit survivability. Distribution of famed Georgia peaches accounts for $30-40 million in revenue annually.

Conversely, historically cold New York State areas can now produce this crop. Monitoring dramatic weather changes allows growers to determine which crops will work in a given area.

The changes to the map do not necessarily indicate global warming. The USDA states that average temperature trends recorded over several decades contribute to climate change data. The PHZM only covers 30-year extreme weather averages, so experts cannot draw scientific conclusions from the map’s data to support or oppose global warming.

The newest hardiness map notes new zone boundaries that contrast earlier releases. The new PHZM is generally about one-quarter-zone warmer than reported in the 2012 PHZM throughout much of the United States. Some of the changes in the zones are the result of additional data sources and improved interpretation methods.