How To Use The New USDA Plant Hardiness Zone Map

February 12, 2024

Has Your Garden Zone Changed?

Here is how you can use the new USDA Zone Map for your garden and how to know, if in the new version, your hardiness zone has changed. This essential tool is a guide to the coldest temperature you can reasonably expect in your garden. It will also help you decide which plants will thrive for you.

On November 15, 2023, the USDA, (Department of Agriculture) announced the first update to its USA planting zones map since 2012.

What is this going to mean to all of us gardeners?

As this map tool is used by 80 million US gardeners and commercial growers, the update does not come too soon!

Also as so many of us gardeners and growers have already felt the warm winds of change, there is a lot of interest in the output being delivered. What can the new mapping service tell us about temperature?

Working with the program has taught me that all of us can use this information in our gardens.

How To Use the New USDA Zone Map

First, I see three valuable takeaways:

Accurate-detailed- warmer

The Map is More Accurate- Has More Detail- and We are Overall Warmer!

  • The 2023 map is likely to be a lot more accurate, and this is primarily due to the considerable increase in the number of weather stations used throughout the country as well as more sophisticated mapping techniques.

-In 2012 we had data from 7973 weather stations.

-In 2023 we have data from 13625 weather stations. (Source)

  • We can all examine our specific growing zones on a much finer scale than ever before. The system is designed for internet use and when you use your zip code to find your zone, you can access a far more detailed view of how weather affects your garden. (I can see my house on a map, the roads and neighborhoods near it, and- a little surprise how my garden is a few degrees cooler than that of my neighbors!)
  • About one-half of the US is one-half zone (or 5 degrees Fahrenheit) warmer than in the last map of 2012.

Questions About The Plant Hardiness Map Data

  • First, as we know, the data is based on a yearly average and therefore does not include the absolute coldest days in the study period.
  • Next, the period over which the map data was collected was a time of warming temperature. The temperature at the end of the period is warmer than the average.
  • We do have questions about the local data. This is discussed in Florida.

We need to always keep in mind that this is based on averages.

What Do Plant Hardiness Zones Measure?

The map is called the USDA Plant Hardiness Zone Map you will also see it called PHZM.

The USDA Planting Zones Map tells us the likelihood of a specific plant thriving in our gardens. What does it measure for us? Our local planting zone measures the average annual extreme winter temperature. This is measured in 10-degree (F.) temperature zones.

*This gives us each a probability of winter extremes of coldness; it does not forecast the lowest possible temperature-it’s an average!

These maps are published showing the average coldest annual weather every 30 years. It is long data but it is not necessarily the absolute coldest it has ever been in each 30 years.

How Do The Plant Hardiness Zones Work?

There are 13,10-degree-wide temperature zones throughout the United States. They are identified by number as well as a color code.

Low number zones are the coldest and are displayed as cool colors and high number zones are the warmest and displayed as warm colors. You can start to predict the rest.

2023 USDA Plant Hardiness Zone Map
2023 National Map with Legend from USDA
  • Zone one is the coldest, and is in Alaska in chilly blue-gray. (see it above)
  • Zone thirteen is the warmest and appears in Puerto Rico and very small parts of Hawaii as hot red.
  • Each zone is divided into 5-degree wide half zones called a. and b.

-This means that zone 5 is divided into five a. the colder part and five b. the warmer. (usually but not always north and south-note warmer coastal and lower areas.

Using the prior maps our garden was in zone 10b. but the more detailed map of 2023, shows the garden just inside zone 10a. Neighboring gardens, one street, and a wide pond to the West are still in zone 10b!

UsDA Plant Hardiness Map comparison

This Version Shows The Zone Changes Between 2012 and 2023 Maps- from USDA

Check the colors to see the extent of the temperature change. White signifies no change.

How Do I Use The Planting Zone Maps-Especially New Features?

First: bring up the Plant Hardiness Zone Map; Enter your zip code in the space provided. This will bring you to a section of the map with a color-coded guide showing your plant hardiness zone. That is how it has always started. You will notice that the detail is finer this year.

Use this link to bring up the Map Homepage: It will have the zip code entry at the top, zone code on the right, and on the top left some instructions based on symbols.

Note: this map and the numbered zones, are a guide to choosing and growing perennial plants. When you open a seed packet or buy a perennial plant there will be a tag or label showing the range of hardiness zones that should be safe areas to grow your new plant.

Your State Map: you can also download your state map. You will see a ‘Map Downloads’ section at the top of the page. You can also download the old and new versions of your state map, at the same time. This is a fascinating exercise in what has happened. Is it better data and/or environmental change?

USDA Plant Hardiness Zone Map - Florida
Florida Plant Hardiness Zones 2023 Map- from USDA

What Your State Map Will Show You

If you garden in the deep south you will quickly notice that for the first time, very little of Florida is in zone 8b. You can see your own state and download any of the maps to use as you wish. I have put the copyright rules in the summary at the end.

Take a look at the changes from the last USDA Plant Hardiness Map-2012

USDA Plant Hardiness Map 2012 Editiion

An Example-Plants and Planting Zones

Our garden is in zone 10, a tropical-subtropical zone. Many plants are rated for groups of zones. For us a common grouping is zone 9-11. Having gardened in both zones 9 and 10 we reasonably expect that in zone 9 we would periodically cover plants and bring a few pots inside on rare cold winter nights. Zone 10 winters are milder, with the rare cold surprise between late December to early February, and zone 11, starts with the Florida Keys and is even warmer.

New Features

This map is designed to have some new interactive features. Look below the Zip code search. Try out these features:

  • The map has layers, showing roads, place names, and the zone itself. Use the little symbol that looks like a stack of papers. Use this to turn any of the layers on or off.
  • Next to the layers, there is a slider that lets you adjust the transparency of any layer you are working with.

How To Apply The Hardiness Map Information To Your Garden

Knowledge of your own garden is up to you, just as it has always been. Also, the zone system is based on extreme minimum temperatures and many other factors can affect the survivability of a plant. But a plant’s ability to survive winter is a reliable place to start and this tool provides the base of information we need to get planting.

In addition, the guidance we have always had from these maps is now a little more granular. This should be an accurate view of your growing environment.

So, how much do I change? The map’s advice does advise us not to pull out plants that have always performed for us, those plants are proving themselves. It does help us with future planting decisions. There are two issues to consider at this point:

  • Are you going to plant only according to the plant grower’s zoning advice or will you take some risks?
  • Are there microclimates in your garden that can advise you in plant selection?

Are You a Gambling Gardener?

Some gardeners will test the common wisdom about a plant. And there are some microclimates in which even conservative gardeners will make a contrarian choice.

Here is an example. In our zone 9 garden we loved camellias, in a cool spot with morning sun they bloomed all spring. Camillias are expected to grow best in zones 7-9 and not in coastal areas. Our garden was in the southernmost reasonable zone and in a coastal area. The plants grew successfully.

In zone 10 our local Botanical Garden is sampling a few camellias, only in pots, with partial shade. It’s an interesting little test as this is one zone too warm and also in a coastal community.

Do You Know Your Microclimates?

Sometimes a microclimate can justify (or indicate) a change in the planting zone.

Climate vs Microclimate

  • A climate is a place with a certain temperature and precipitation.
  • A microclimate is a place within that climate that differs from the norm.

Microclimates are created by features, such as topography, bodies of water, and urban areas. Sometimes very small features can create microclimates. Small sheltered areas, sheltering walls, and even sides of the house will cause changes in growing conditions.

We garden on the edge of a large pond. When strong winds cross this pond on the north side of the property plants feel the effect. We are always impressed by how dramatic the change in our placid pond can be!

A planting bed with larger shrubs and some trees protects a grouping of tropical plants behind them. So far that seems to be enough.

Seasonal Changes In A Zone That Is Otherwise Perfect

If you are using a plant at the end of its planting zone will seasonal changes put it at risk? It is never necessary to always accept what Mother Nature brings! This is where ‘the right plant, in the right place’ comes into play.

imaggs of environmental issues

Light Conditions: your plant prefers partial sun, can that become too much or little sun for part of the year? Is it a plant that should be container-grown for movability? Our zone 10 environment is too hot for several favorite herbs. They will last longer in summer if we move them to partial shade.

Moisture: if your plant performs best in moist soil, will seasonal winds dry the soil too much? You may succeed with a seasonal change in the watering schedule. Our summers are wet and humid, but also stormy. Our container plants need more water than you would think.

Plant Varieties with different requirements: every plant genus has its favorite temperature and conditions, but within species, and even varieties the range may be smaller.

Cold Exposure: many plants accept a few days of cold during every climate’s winter. Sometimes an extended cold snap is too much and plants that did not need protection overnight need it if the cold is extended in one winter.

Changes in humidity: humidity can limit cold damage by preventing moisture loss from leaves and branches. If the humidity drops in your area, plants can be damaged. You might need to water when a freeze is expected and push potted plants together, or cover them for cold protection.

Useful Facts To Know About The Newest Map

What Makes The Map More Helpful

The new map used a sophisticated algorithm to improve how the data between the weather stations was used. This helped to account for factors such as changes in elevation or closeness to water that can affect the zones. The team also used a board of gardeners and growers to fine-tune the results.

Note that in the new map, our garden and those nearest to it now appears in a slightly cooler half zone. You might find some similar changes in your garden.

New Zones: If you are expecting 11 plant hardiness zones, note that there are now 13. These are warm zones with average cold temperatures above 50 and 60 degrees F. This will be good information in view of the growing popularity of tropical plants both in our gardens and as house plants.

Higher Resolution: As the last two maps, (2012 and 2023) were made using GIS technology they can show zones in much higher detail. A city covered by hardscape will be a little warmer than the nearby countryside and can be given a warmer number. A high elevation can be given a cooer number. We could not do that before.

The Time Period for the development of the averages has lengthened. For example; the 1990 map averaged data from 1974-1986, 12 years. The newer editions used 30 years of data. This smooths out the annual weather fluctuations.

Canada and Mexico have been studied only since 1990.

Are We Looking At Climate Change?

We are warned against making this assumption, here are the reasons I found.

  • Climate trends are developed using averaged data of over 50-100 years We are using 30-year data and the zone changes are not considered reliable evidence.
  • The new map is overall about one-quarter of a zone warmer. We are told that this is due to factors like different averaging time periods, more new data sources, and new ways to interpolate the data points.

What The Plant Hardiness Zone Map Does Not Show You

The coldest temperature your garden can reasonably expect to experience is essential to plant selection. We need this fact to succeed. But it is not all we need. These maps, supported by an extensive network of weather stations provide a tremendous amount of data but they can’t cover every eventuality. The USDA plant hardiness zone map won’t tell us these significant facts:

  • How long the cold will last? One cold night will do cellular damage to our plants, but if the temperature warms, the plants will begin to repair themselves. But if the cold snap lasts for days more plants will die.
  • How humid the garden can be! Some plants thrive in humidity, orchids for example, but many others do not.
  • How cold it might get; we are dealing with averages, the extremes of cold are not included. Also, our data is from the last 30 years. For example, the extreme cold periods we experienced in the 1980s were included in the 2012 map but not the newest map of 2023.
  • How suddenly the cold appeared. A cooling fall acclimates our plants to the coming winter, a warm fall followed by a sudden influx of cold will not. Plants will die from the suddenness, not the extremes of cold.
  • Winter rain. Many plants can be healthy in extremes of cold but fail in rainy winters that are much milder.
  • Soil preference of the plants.
  • Nutritional issues. Adding potassium in the fall can extend the plants’ hardiness, but adding nitrogen in the fall can increase growth, making the plants more susceptible to cold damage.
  • If late pruning encourages growth the plant will be more susceptible to cold.
  • Any microclimates you have or create in your garden.
  • How hot it can get! ‘Some Like It Hot: Measure Heat In Your Garden’

Zones are good information, but there can be variations. Here is a good example. We garden in zone 10 in South Florida. It is the same zone as gardeners experience in San Diego. I recently noticed that Vancouver Canada and Tampa Florida, both coastal climates have been in the same zone! Those are very different garden places.

How Do I Use Hardiness Zones For Container Plants?

Plants in containers have less protection from cold than those in the ground and their roots will be damaged sooner. Most experienced container planters expect to add two zones cooler to protect container plants. Also, when covering plants during cold weather wrap the planter itself to protect the roots.

Summary,

The USDA Plant Hardiness Zone Maps have always been the source of cold hardiness information for us gardeners and this last iteration is the most informative yet. I hope this proves as useful for your garden as it has been for us.

History Of Zone Maps In The US

The first geographical zone map was designed to aid gardeners by researchers at the Arnold Arboretum in Boston. Published in 1927 it was updated until 1971. Following that the USDA system began at the National Arboretum in Washington DC and was first published in 1960. Its key improvement was the use of consistent 10-degree zones. Five-degree half zones were introduced in 1990.

Can I Download The Maps?

The maps are made to be downloaded and you can access your state map in various resolutions and you can print them. Go to the first page USDA Plant Hardiness Zone Maps and you will see Maps Download at the top left.

What Happens Elsewhere In The World?

The USDA pioneered the concept of constant ten-degree plant hardiness zones. The idea has been adapted to the differing needs of places in different climates. Here is a pictorial example of how things look elsewhere.

World Hardiness Zones
World Hardiness Zones Tara Krause, Public domain, via Wikimedia Commons

The following is the information on the copyright status of the website. I have also added how to ask questions of the USDAF

Copyright

“Map graphics. As a U.S. Government publication, the USDA Plant Hardiness Zone Map itself (as a graphic) is not copyrighted and is in the public domain. Map graphics may be freely reproduced and redistributed. The USDA-ARS and OSU logos must be prominently displayed in any reproduced or redistributed map graphics. Zone boundaries and designations and other features may not be altered without an explicit and prominently displayed disclaimer that the map is not the official USDA Plant Hardiness Zone Map and USDA-ARS and OSU logos must not be displayed in the modified version.

Files in varying resolutions and formats may be downloaded and/or printed from this website.”

Other Ways To Understand Your Weather

First And Last Frost Dates

When spring arrives the cold season’s last frost date in my plant hardiness zone tells me it is safe to plant for the new growing season. Conversely, the first frost date in my zone is a signal that it is time to harvest my crops or take cuttings or bring my delicate plants indoors for the winter.

First and last frost dates are average like zone temperatures and need to be used as a guide. Weather data comes from NOAA and the frost dates are not related to your plant hardiness zone. Use the sources below to find you first and last frost dates.

First and last frost dates with freeze temperatures for gardens

Resources You Can Use

Use this address to ask about the zone mapping phzminfo@usda.gov

Ask Usda-You can use this site to ask direct questions

Here is an interesting discussion from Rutgers University.

Sources of First and Last Frost Dates:

National Weather Service: This is the source of our weather data. You can customize your view by putting in your city or zip code. (My city covers two hardiness zones so I use my zip code to get data from the closest weather station.

Many other websites offer good helpful information: Here are some to use.

The Old Farmers Almanac: this link will take you to 2023, the latest information. This will give you the nearest weather station location, the altitude the first and last frost date, and the number of days in your growing season.

“How To Find Your USDA Plant Hardiness Zones For Florida.”

‘Some Like It Hot: Measure Heat In The Garden’

‘How To Acclimate Plants Indoors For The Winter

‘The Tools You Need If You Want To Love Gardening.’