It’s not often that we get to talk about breaking news in the gardening world, but this week the USDA updated its plant hardiness zone map and it has been a hot topic! In this article, we’ll break down these changes and what they mean for your gardening ventures.

The USDA Hardiness Zone Map is primarily based on the average annual minimum winter temperature in different regions of the United States. It uses temperature data to categorize various geographic areas into distinct hardiness zones, which represent the range of temperatures that plants can be expected to withstand during the winter months.

The specific criteria used for defining these zones are as follows:

• The USDA map uses the lowest average annual minimum winter temperature as its primary criterion. It looks at the coldest temperatures typically experienced in a region over a 30-year period. The map uses temperature data from weather stations across the country to calculate these averages.
• The zones are defined in 10-degree Fahrenheit (approximately 5.6-degree Celsius) increments, meaning each zone represents a 10°F difference in the average annual minimum winter temperature.
• The USDA then assigns a numerical zone designation to each region based on the temperature data. For example, Zone 1 represents areas where the lowest average annual minimum temperature can fall below -50°F (-45.6°C), while Zone 13 represents areas where the lowest temperature typically stays above 60°F (15.6°C).

It’s important to note that the USDA Hardiness Zone Map provides valuable information for gardeners, farmers, and horticulturists to select plants and crops that are well-suited to the local climate conditions, particularly in terms of cold hardiness. However, it does not take into account other climate factors such as humidity, rainfall, or summer temperatures, which can also impact a plant’s overall suitability for a particular region. As such, while the USDA Hardiness Zone Map is a useful tool, it’s just one of many factors to consider when planning and designing gardens or agricultural endeavors.

When comparing the updated 2023 map with a previous map from 2015, it is immediately noticeable that a significant part of the country has shifted up a zone – meaning less hardy plants can survive in regions they previously could not. 

Some regions of the country remained the same, primarily those regions that are impacted by mountain or lake effect weather patterns. For example, we are located in Northwest Michigan and did not experience a zone change. This is most likely because of the weather protection we receive from Lake Michigan.

Change is always a double edged sword. While this shift in growing zones will benefit some growers, certain types of crops will likely suffer.

For many gardeners who live in regions that have shifted to a warmer growing zone, they will get to enjoy the perks of growing new plants that previously were unlikely to survive their coldest winter temperatures. In some circumstances, this might mean being able to grow a plant as a perennial that was previously only able to be grown as an annual.

Shifting up a zone is not entirely positive for all growers. For example, we live in a region where many farmers grow orchards full of apples, plums, and cherries. These types of crops require a specific number of ‘chill hours’ in order to successfully produce fruit. Crops of this type will suffer with warmer, mild winters. 

Invasive species, such as the lantern fly, will also flourish with warmer temperatures. Harsh and cold winters are often the only effective tool against invasive insect species, which means a warm up will likely enable them to hibernate and survive the winter months.

The USDA hardiness zone map is an excellent tool when it comes to determining which plants to grow based on average minimum temperatures. However, the map does not account for precipitation which can greatly affect a plant’s ability to survive cold temps. For example, there are several types of succulent plants that can survive the harsh, cold temperatures of the dry Colorado mountains, but would never survive a wet and chilly Michigan winter. Soil moisture can have a big impact on how low of a temperature a plant can survive.

Additionally, the USDA hardiness zone map is based on an average of minimum temperatures for a given region. This means there is always the potential for an extreme low temperature or a sudden cold snap that might kill a plant, even if it’s rated to thrive in your growing zone.

For the average gardener, these updates to the USDA plant hardiness map will not change your overall gardening process too much. For farmers of crops that require specific chill hours, these changes will likely be significant. Adaptation will be needed on all fronts, with some being negative and some being positive. 

Above all, I think these changes point to likely more changes in the future. Meaning as gardeners, we should plan on our gardening plants being flexible and easily adaptable. With this mindset, we can continue to grow fresh food and plant gardens that keep us happily engaged with our planet.