Hardiness Zones

As a gardener or a plant enthusiast, you have probably noted the ‘hardiness zone’ on plant tags, seed packets, or garden catalogs. It’s an essential factor to determine whether a plant can survive in your specific region or not. In North America, a hardiness zone map delineates various zones based on the average minimum temperature of an area. The United States Department of Agriculture (USDA) released the first version of this map back in 1960, and it has been updated several times since then.

The hardiness zone map is based on the concept of climatic zones. Every zone is defined by a 10°F temperature interval that categorizes areas using different plant hardiness ranges. These ranges, in turn, help gardeners and landscapers understand which plants can survive in specific temperate ranges across the United States and Canada.

Understanding the Map

The hardiness zones range from 1a to 13b, with zone 1 as the coldest and zone 13 as the warmest region. Zone 1a is the coldest zone, with an average winter temperature that falls below -50°F, and it stretches northwest from Alaska to the upper Midwest. Conversely, zone 13b with a range of 65°F to 70°F includes the southern parts of Florida, Hawaii, and Puerto Rico. Notably, Hawaii is the only U.S. state located in zone 13.

The hardiness zone map has several practical applications. For instance, if you’re a gardener in the United States, you can use this map to understand the best time to start planting your crops or the most suitable type of vegetables, flowers, or trees to grow based on your region’s hardiness zone. Likewise, the zone map is crucial to commercial nurseries and gardening centers, as it enables them to stock plants suitable for each region.

How is the Hardiness Zone Map Determined?

To create the hardiness zone map, USDA scientists analyzed temperature data gathered from 1976 to 2005. The map is derived from the average annual extreme minimum temperature that is recorded over a 30-year period. The data collects several factors such as historical weather data, the elevation of the region, proximity to large bodies of water or other typical temperature-altering geographic features, and other microclimatic variations within a region.

To determine a region’s hardiness zone, USDA scientists collect this data and then classify it into ranges of 10°F differences. They then identify the upper and lower limits of each range and develop borders around each range to create the distinct zone map.

Impact of Climate Change on Hardiness Zones

It’s notable to mention that the hardiness zones aren’t static entities. As global temperatures increase, the zones’ boundaries shift to reflect the average rising temperatures. This shift means that some of the traditionally colder hardiness zones could become warmer in the future, allowing gardeners and landscapers to plant more varied and interesting plants. Additionally, zones normally reserved for tropical plants could move northwards, making formerly delicate species more prevalent in new regions. Therefore, it is essential to monitor the changes in these zones to ensure that gardeners and researchers are updated on how they should respond to these global temperature increases.

So, what does the hardiness zone map entail?

Using the hardiness zone map, gardeners and garden retailers can select plants, shrubs, and trees that are best suited to the local climate. This means that the plants can survive frost, snow, and heatwaves of the given region, thereby significantly improving these plants’ long-term health and growth. The map brings a lot of value to the gardening industry, allowing novice gardeners and seasoned professionals alike to select the right plant for the right environment.


In conclusion, the hardiness zone map is a critical tool for gardeners and landscapers throughout North America. Understanding the climate and ideal planting conditions in different regions of the continent is essential for cultivating healthy plants and maintaining proper garden care. Therefore, if you are a gardener, it is important to check your hardiness zone and choose the cultivars that are best suited to your area to ensure success and the most diverse plant life possible.

About Author