In 1960, the United States Department of Agriculture (USDA) devised a map that divided North America into 11 separate hardiness zones based on average minimum winter temperatures. Each zone is a guide for the types of plants that can grow comfortably in that particular zone.

A Note for Canadians

If you’re in Canada near the border, you can go by the US state zones nearest you. See also the section below under ‘How Many Hardiness Zones Are There?’ for information on how you might also subtract 5-10°F (or more if you’re further north) from the US zone nearest you.

Also, here’s a link to Canada’s Plant Hardiness Zones resource.

A Quick Note on the Hardiness Meaning

I grew up in Hawaii, and I never heard of hardiness zones there. After transplanting to the mainland US, I used to think that a “hardy plant” is one that grows well in all conditions.

But it turns out that the definition of hardy that applies to people, (capable of enduring fatigue, hardship, exposure, etc.; sturdy; strong); isn’t the same meaning for plants.

The definition of hardy when speaking of plants means:
Able to withstand the cold of winter in the open air.

So now that we’re clear on the meaning of hardy in the various contexts, what is a hardiness zone?

What is a Hardiness Zone?

A hardiness zone, in the context of horticulture and gardening, is a geographically defined area identifying the specific set of climatic conditions relevant to the survival and growth of various types of plants and trees in that area.

Plant hardiness zones are primarily defined by the average extreme minimum temperature that occurs within the region.

The concept of a hardiness zone was introduced to provide gardeners and landscapers with a useful framework to understand the potential success of planting certain species in their specific region. So knowing your hardiness zone means that you can better choose plants that are likely to thrive and survive the coldest winter temperatures in your garden.

The USDA Hardiness Zones is based on plant cold hardiness.
Plant heat tolerance is classified by the American Horticultural Society’s (AHS) Heat Zone Map.

What is a Hardiness Zone Used for?

In essence, the hardiness zone is a guide to the lowest temperatures a plant can withstand in a specific location, which significantly aids in the selection of plants that are best suited to a given climate.

Other Factors to Consider When Choosing Plants

While knowing your hardiness zones can help your garden grow, of course it’s not the only factor. You’ll naturally need to take into account other crucial local factors affecting plant growth such as soil conditions, wind exposure, rainfall, humidity, light exposure, and specific weather events.

Updates and 2012 Revision

The hardiness zone map has undergone several revisions since its creation in 1960. The most recent update as of this writing (in 2023), was in 2012.

The 2012 revision used advanced weather data collection and analysis methods towards greater refinement of area climates. The update included temperature data from more weather stations and considered factors like elevation changes, proximity to large bodies of water, and terrain position.

How Many Hardiness Zones Are There?

With these additional factors for improved delineation of the accuracy of plant hardiness zone boundaries, the USDA changed it from 11 zones to 13 different zones. Each zone represents a 10-degree Fahrenheit difference in the average annual minimum winter temperature. But there’s more.

Plant Hardiness A and B Zones

Then the zones were further divided into “A” and “B” to provide even more specificity, with each subsection representing a 5-degree Fahrenheit difference.

This system of hardiness zones has been widely adopted in many regions of the world, though adaptations and modifications have been made to account for local climate variations. These adaptations allow for even more accurate guidance when selecting plants for gardening and landscaping.

Many plants have a wide hardiness zone. Where the finer details come into play are for the more temperate and tropical plants as well as for garden vegetables and fruits.

For instance, when planting tomatoes, an ideal soil temperature is around 60°F. We start ours between 50-60°F, but use the red plastic mulch, which helps to warm up the soil temperature enough to get away with that.

You can find more details and free tools for measuring soil temperature here.

Each USDA plant hardiness zone represents a 10°F jump in the average annual low temperature, and each “A” or “B” zone represents an average 5°F differences.

A Note About Soil Temperatures

Many other vegetables have minimum soil temps to heed before planting as well, so knowing the hardiness zone as well as your first and last frost dates is a key ingredient to gardening success.

So we were using a laser thermometer for testing soil and ground level temperatures. However, we’ve come to learn that the most accurate soil temps, it’s best to use an in-ground soil thermometer.

Alternately, we found a great resource for you to be able to determine soil temperatures in real time online.

Hardiness Zone is a Helpful Guide, Not a Definitive Predictor

Despite updates, the map is a guide, not a definitive predictor. It indicates where a plant could possibly survive based on temperatures alone.

However, as indicated earlier, other factors like soil type, wind, precipitation, humidity, and light exposure also affect plant growth. All of these factors are important to consider for the optimal plant life.

Growing Zones Can be Tricky

Knowing your USDA hardiness zones can make a huge difference in your gardening success, so that you’re able to provide the best growing environments for your plants.

What makes it a bit tricky is that often, people search for information by state and yet there are states with more than one growing zone. Further, you can have neighboring states (and beyond) with the same USDA hardiness zone, while other parts of your state may be a different one!

To add further complexity, there can be microclimates within an existing zone that changes things to make it warmer or colder. Similarly, there are generally even microzones within the same yard!

But then that’s also good news, because it may mean that you can grow something in your zone or area of your yard that may not thrive in another area!

USDA Hardiness Zones are guidelines, not absolutes.

First and Last Frost Dates

Another important factor to consider and to know for successful gardening is your first and last frost dates.

While it’s possible to give general estimates of frost dates based on USDA hardiness zones, these dates can vary significantly within each zone due to factors like altitude, proximity to bodies of water, and local weather patterns.

While it’s more accurate to look up frost dates based on location or ZIP code, here’s a rough estimate of first and last frost dates by hardiness zone. Remember, these are only estimates and actual frost dates can vary widely within each zone, and even by each zip code!

USDA Hardiness Zone	Estimated First Fall Frost Date	Estimated Last Spring Frost Date
Zone 1	Early August	Late June
Zone 2	Late August	Early June
Zone 3	Early September	Late May
Zone 4	Mid September	Mid May
Zone 5	Late September	Early May
Zone 6	Early October	Late April
Zone 7	Mid October	Mid April
Zone 8	Late October	Early April
Zone 9	Early November	Late March
Zone 10	Mid November	Mid March
Zone 11	No frost	No frost
Zone 12	No frost	No frost
Zone 13	No frost	No frost

Climate Regional Names of Descriptions

The USDA hardiness zones primarily use numbers (e.g., Zone 1, Zone 2) to categorize regions based on their average minimum winter temperatures. These numbers are often what gardeners and horticulturists refer to when discussing plant hardiness and planting recommendations.

However, while the USDA itself doesn’t assign specific names to each zone, general descriptions based on climate and geography sometimes accompany these zones in various publications or references.

We’ve included these because we enjoy knowing the regional terms for the various states and zones, but the specific zones are the most reliable for your planting purposes.

Plant Hardiness Zones Regional Names or Descriptions

• 1-3 Zones: Often referred to as “Subarctic” or “High Northern Regions“, this includes places like:
  • Northern Alaska
  • Parts of Canada.
• 4 Zone: Sometimes described as “Cold Northern Regions”, this would be areas like:
  • Southern parts of Alaska
  • Northern parts of states like:
    • Minnesota
    • North Dakota
    • Montana.
• 5 Zone: Could be termed as “Upper Midwest” or “Cold Temperate Regions” representing states like:
  • Iowa
  • Illinois
  • Nebraska
  • parts of New York
• 6 Zone: Often referred to as “Lower Midwest” or “Moderate Temperate Regions”, this covers parts of states like:
  • Missouri
  • Kansas
  • Pennsylvania
  • Ohio
• 7 Zone: Could be described as “Upper South” or “Warm Temperate Regions” and includes states like:
  • Oklahoma
  • Arkansas
  • Tennessee
  • North Carolina
• 8 Zone: Often termed as “Deep South” or “Subtropical Regions”. This would be states like:
  • Texas
  • Louisiana
  • Mississippi
  • Alabama
  • Georgia
• 9-13 Zones: Frequently referred to as “Tropical” or “Warm Coastal Regions” and includes parts of:
  • Florida
  • Southern Texas
  • Coastal California
  • Hawaii

Please note that these descriptions are quite generalized and are not universally standardized. The primary purpose of the USDA zones is to provide guidance on plant hardiness based on temperature, and the zones themselves are a more reliable metric than any accompanying descriptive terms.

Frost Dates by Zip Code

Farmer’s Almanac website has a very handy tool where you can look up frost dates by zip code.

Also, you can find more on plant heat tolerance here along with a heat zone map.

Whatever hardiness zone and wherever in the country you may be, wishing you the very best gardening and growing success!