Break Out of Your Hardiness Zone

The Launch of a New Garden Hardiness Zone Map

Are more gardeners developing a fixation for hardiness zones and comparing notes across the back garden fence?  The recent launch of an updated plant hardiness zone map probably is causing more people to ponder the fine print on the plant label looking for the height, spread and hardiness zone details. Why the fixation with hardiness zones? Plant hardiness zones are valuable pieces of information that can save gardeners from lots of grief.

Not infallible, hardiness zones are far from 100% accurate though. There are restrictions in the types of data that are being used to create hardiness zones and most often these are just average minimum temperature weather records. It should be noted that plant hardiness involves so many more factors. Whether a plant survives the winter may be because it is growing in a microclimate right in your garden.

Microclimates play a big role in modifying the weather patterns of a garden. Temperatures may be colder or warmer because of a proximity to hedges, buildings, water or cities. The closer a garden is to a city or large body of water, the warmer it is. Changes in elevation and the location of a garden in a valley create microclimates that produce different weather patterns. Pockets of protection allow gardeners to grow plants that should not survive in their area. This growing “out of the zoneâ€? has tempted many a gardener at the garden center who spots an exciting new plant just off the truck from a balmy southern locale. Gardeners should be encouraged to look for hardiness zone information and then find microclimates to grow plants if they want to push their zone boundaries; the challenge is addictive but beware there’s a gamble in doing so – you will know the answer after the first unseasonably cold night. Always look for the hardiness zone so you can know where best to position your new garden plants. So what is causing this heightened interest in hardiness zones?

The Newest Hardiness Map

Early in 2012 the USDA released a new plant hardiness zone map, exciting news for gardeners and growers. The plant hardiness zone map is particularly helpful in deciding which plants will survive in a particular area. The latest USDA hardiness zone map is based on the average annual minimum winter temperature using weather data collected over 30 years from 1976 through 2005. Even though thirty years of data was used to create the new map, it is important to note that the map does not use the lowest temperature for any particular location that has ever occurred. Meaning hardiness zones listed on plant tags or in books should still be used as guidelines.

About using this weather data, the USDA explains, “The longer period (30 years) of data was selected by the group of horticultural, botanical, and climatological experts who led the review of the latest revision as the best balance between smoothing out the fluctuations of year-to-year weather variation and the concept that during their lifetimes, perennial plants mostly experience what is termed ‘weather’ rather than ‘climate.’â€?

Like the older map, the newest version is divided into color-coded five degree F sections (13 zones in total further subdivided into “aâ€? (colder) and “bâ€? (warmer) regions) that identify annual minimum temperatures from –60 F to 70 F and illustrates this detail down to county-scale.

One new feature of the recently released map is an interactive option to simply type in a zip code and find the corresponding hardiness zone and temperature range. You can also click on your state (or part of your state as is the case with Texas) and a map will appear with more detail about your region. This is a nice quick reference for those that don’t want to browse the full map to find what they need. In addition, state, regional and national versions of the map can be downloaded from the internet. Check them out at

The New USDA Plant Hardiness Zone map now defines 13 zones for gardeners with average annual extreme minimum temperatures from -60 F (-51 C) to 70 F (21 C).

New zones have been added (zones 12 and 13) for warmer regions in Hawaii and Puerto Rico that have extreme minimum temperatures above 50 and 60 F. Even though zone 12 plants will thrive outdoors in only these two locations, this information is important for many gardeners in colder areas to decide when to best bring these same plants indoors from the patio or garden when temperatures start to drop in the fall.

The longer period (30 years) of weather data used in the new hardiness map has resulted in most of the United States generally changing ½ zone (5 F) warmer (although some areas did change to colder zones because of the more accurate elevation depiction). The USDA says that this shift is attributed to more sophisticated mapping methods, and greater numbers of station observations, which has greatly improved accuracy (especially in mountainous regions), than from global warming.

Does this new map mean gardeners and growers will have to rip up their garden and change their plants if their hardiness zone has changed? Of course not. Plants that grew well up to now, will probably keep on growing just fine. What should be done as a precautionary measure is to carefully consider which plants are added to the garden if they are “pushing the hardiness zone boundaryâ€? and are not hardy enough for your location. But the hardiness zone map, while it is the most detailed one yet (down to the county level) doesn’t show microclimates that might exist in your garden or property. So look for sheltered spots on the south side of a building or evergreen windbreak if you want to try a few new plants that are not rated for your hardiness zone. Other cold weather strategies include putting on extra mulch, adding cut boughs to collect the snow, planting early so plants are well established and planting deeper. There are many factors that play into the hardiness of plants including light exposure, soil moisture, duration and timing of extreme cold, and humidity or plant moisture conditions.

One big difference shown on this new USDA map is the exclusion of Canada and Mexico which, although not as accurate as the U.S. portion on the old map, were still represented on one big North American map – making it easier to see where specific zones flowed over broader areas. The justification for not reproducing the USDA map like this again, they said, was that Canada has a more detailed map produced by Natural Resources Canada – Canadian Forest Services (see below).

The First Hardiness Maps

The first cold hardiness map was published in 1927 by Alfred Rehder in his classic Manual of Cultivated Trees and Shrubs. He used eight zones which were calculated based on the lowest mean temperature of the coldest month. The zones were marked in increments of 5 degrees F. Even with this information the boundary lines between zones were quite arbitrary. Donald Wyman in 1938 created a more accurate hardiness map. He used 40 years of data and based his map on the average annual minimum temperature. Surprisingly, his zones were created 5, 10 or 15 degrees F. apart. This lack of uniformity caused still another hardiness map to be developed twenty years later. In 1960, the United States Department of Agriculture (USDA) created its first map. The USDA map has since become the standard for the horticultural industry in North America. Most nurseries, reference books and catalogues use this map.

The Sunset Hardiness Map

Western gardeners came to feel that the USDA map was too restrictive with its cold temperature focus and didn’t reflect the true nature of the plant growing environments on the coast. So Sunset magazine created a climate zone map that took into account many more factors such as the length of the growing season, rainfall amounts (and timing), summer high temperatures (and winter lows), wind and humidity. Sunset’s map divides the country into 45 climate zones – most of which are located on the West coast. Going east and north, the zones become much broader. As an example, with the Sunset map, Florida is divided into just three zones; on the new USDA map this state shows seven zones.

The sunset map can be found at

A First Time Look at Heat Hardiness

An interesting alternative to the cold hardiness maps that have been developed for over 60 years, and of interest to southern gardeners, is the AHS Heat Zone Map. This map, first published in 1997, was created to indicate the frequency of high temperatures. The map is divided into 12 zones which have been determined from weather records. Researchers used the average number of days per year that the temperature is above 86 F to create their map. The map shows locations that have less than 1 day above this temperature to those locations that have over 210 days. Data for the map came from the daily maximum temperature records of 7,831 weather stations from the years 1974 through 1995.

Unfortunately the heat zone map is just for the US (including Alaska and Hawaii).  It was created because cold temperatures are not the only weather factor that affects the survival of plants. In general the nursery industry has placed a greater focus on cold tolerance and breeding cold hardy plants than on heat tolerance. There are many plants that will not tolerate being grown where they do not get adequate cold temperatures in the winter. Eastern Hemlock will not do well south of zone 7, and white spruce will grow poorly if planted in zones higher than 6. It has been found that high temperatures kill plants slower than cold and are more likely to be misdiagnosed. The AHS Heat Zone Map can provide valuable information to prevent this.

Take the plant zone information presented on the maps, printed on nursery labels or published in books as a guideline only. It is important to realize that hardiness is based on more than weather records of average minimum or maximum temperatures. Soil types, exposure, rainfall, humidity, proximity to windbreaks, soil moisture, snow cover, winter sunshine, plant types and the age of plants all contribute to survival ability. Also add one more criteria to your choice of hardiness map, which map is most accurate for your location. There are some good choices for gardeners, check them all out. For cold hardiness: USDA, Sunset, or Natural Resources Canada or for heat tolerance, the AHS heat zone ratings.

Canada’s Hardiness Map — Combining Climate Data with Indicator Plants

Canada has a hardiness zone map that was created by Agriculture Canada in 1967.  It is reputed to be the most detailed for Canadian latitudes.  Some people debate this statement and contend that the USDA map is better. The map was revised by Natural Resources Canada in 2000 using weather data from almost 30 years (1961 to 1990).

The map uses 9 zones to mark the hardiness of the most populated areas of Canada with 0 being the coldest and 8 the mildest (warmest). Each zone is divided into “aâ€? (colder) and “bâ€? (warmer) sections. The Canadian zones are not the same as the USDA zones. Canada plant hardiness zone 6 ? USDA zone 6. The Natural Resources Canada map has the added benefit of having indicator plants (trees and shrubs) that are representative of each zone – a helpful feature for gardeners.

Zone 1 Saskatoon Serviceberry (Amelanchier alnifolia)

Zone 2 Siberian Peashrub (Caragana arborescens)

Zone 3 Staghorn Sumac (Rhus typhina)

Zone 4 Ginkgo (Ginkgo biloba)

Zone 5 Early Forsythia (Forsythia ovata)

Zone 6 Japanese Maple (Acer palmatum)

Zone 7 Sweetgum (Liquidambar styraciflua)

Zone 8 Japanese aucuba (Aucuba japonica)

The map authors used a formula that takes into consideration several meteorological factors affecting the cold hardiness of a plant.  The most important being the minimum temperature during the winter.  They also considered the frost-free period, summer rainfall, maximum temperatures, snow cover and winds that would affect a plant’s growth.

Natural Resources Canada has taken on an additional project “Going Beyond the Zonesâ€? to expand the indicator plants beyond just trees and shrubs to include perennials as well. They want to develop a climatic profile for each plant using continent-wide mapping.  They have asked for help from gardeners across Canada and the United States to identify plants from a comprehensive list that will survive at their location. Once enough data is entered online, a range map will be generated (and updated when additional reports are submitted).


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