Using records of the lowest winter temperatures across the country, the United States Department of Agriculture divides the Country into 11 hardiness zones. A plant's known cold hardiness is then used to assign it to one of these climate zones, to guide the purchases of plants that will survive winter temperatures in your area. This zone represents the lowest temperatures that the plant can endure and be expected to emerge from dormancy in the spring.
When you design a garden, it is helpful to know which USDA zone you live in and then using only plants rated for your zone (or, in most cases, for those zones with higher winter temperatures than yours) to insure that they will overwinter in your garden. Among horticultural professionals there are many differences of opinion about the reliability and practicality of the USDA Zones but for most gardeners in North America it’s the best guide for acquiring non-native plants for the home landscape that will survive the winter temperatures.
Plants native to certain climatic ranges often thrive only within those ranges. While factors such as moisture, humidity, sunlight, heat and wind are important too, these can be moderated by the placement of the plants in the landscape, soil amendments and irrigation. Cold presents a bigger challenge. Vascular tissue in woody plants can freeze and be destroyed in trees and shrubs unaccustomed to winter temperatures beyond those of their natural range.
The nursery industry realizes that its products migrate far beyond the climes in which they were raised and they try to prevent consumers from buying specimens that may not survive in their home gardens. Most reliable growers make sure that their plant tags contain growing information for their plants, including the plant’s known USDA zone of hardiness. Check the USDA map for your location, note the color and look up the color key to find what Zone your garden is in. There are many interactive Zone look-up websites.
If you live in the Western United States, you can also use the Sunset Climate Zones, which are smaller and include more than just average historical minimum winter temperatures to in their determination.
A perennial question is whether a plant can thrive in zones higher than the one that it is rated for, e.g. can a plant rated for Zone 6 flourish in Zone 9? This very much depends on the characteristics of the zone in question, and the key cultural requirements of the plant you are considering adding to your garden. Plants that need winter chill, for example, won't do well (and may not flower or set fruit) in a warmer zone without the chill requirement. Plants that come from mountainous areas that have cool nights, even in summer, often languish or die in warmer zones, such as in the Southeast, where the nights are barely cooler than the days. If you have questions about whether a certain conifer that is native to a zone cooler than yours will grow successfully in your garden, join the ACS and connect with conifer lovers in your zone!
From decades of records of the average coldest temperatures across the country the United States Department of Agriculture created gradients in a map that reflect zones which correlate with plant species’ ability to overwinter. Called hardiness zones, they represent a geographical area of known minimal winter temperatures usually calibrated with a range of 10 degrees F. for each Zone. Gardeners use this information as a guide – but not a guarantee – when selecting a particular plant for a specific geographic location.
For example, a New York City terrace gardener wants to put out dwarf conifers in containers on her balcony overlooking the Hudson. New York is comfortably in Zone 7 where the lowest winter temperature outside can get down to 5 degrees F. She purchases plants rated at Zone 7. Fine. But then she gets a job in Zone 5 Syracuse, 250 miles inland where it can reach down to -15 F. in the dead of winter. Take the job but leave the plants for the next tenant because they are unlikely to survive their first winter outside on a patio in upstate New York.
Although there were attempts by various organizations to chart plant hardiness as far back at the 1920s, it wasn’t until 1960 that the USDA’s Agricultural Research Service published it first Plant Hardiness Zone Map, initially as a resource for agronomists but eventually botanists, natural resource managers, farmers and horticulturists realized its value.
As more data was compiled the maps have been refined and have changed in subsequent editions. The original map divided North America into ten zones but in 1990 they were effectively doubled by splitting each Zone in two. (Zone 5a was five degrees colder than 5b) and, even later, Zone 11 was added. The most recent update in 2012 acknowledged global warming when it moved many gradient lines to reflect climate change.
As mentioned above, the USDA Zone map is only a guide because it can’t take into consideration micro climates that may exist on your property due to the hours of exposure to direct sun, moisture, drainage, prevailing winds, proximity to rocks or structures that reflect heat, etc. Most knowledgeable horticulturalists – and that includes veteran conifer collectors in the ACS – place greater emphasis in knowing where the plant was grown and what success others have had growing a particular cultivar in their area.
Unless you have reliable local information on a particular plant’s winter hardiness it’s best to err on the side of caution and get a plant that will survive in the next less temperate (colder, i.e., lower number) USDA Zone. If, for example, you live in Zone 4, don’t tempt fate buying a new plant tagged Zone 4 but instead buy yourself some insurance by getting another known to survive in Zone 3. If you decide to buy a Zone 4 plant consider it an experiment where failure is always a possibility.
The USDA map is less reliable in the western states where the Pacific currents and the Rocky Mountains combine to produce temperature, rainfall, soils and altitude levels markedly different from the rest of the country. Recognizing this, Sunset, a regional lifestyle magazine published in Menlo Park, CA, created a climate zone map with 24 different zones that account less for extreme cold and more for elevation, ocean influence and local geography, among other factors. The Sunset maps are tailored to western regions but also include areas outside that.
In an attempt to provide more accurate plant hardiness information, particularly to gardeners in the warmer parts of the US, in the late 1990s the American Horticultural Society created a Heat Zone Map based on the number of heat days (defined when the daily temperature reached 86 degrees or above.) Like the USDA zones for cold hardiness, AHS’ heat data was gathered from thousands of recording points to create a gradient map of heat zones 11 (indicating the most heat days) through 1 (the fewest).
Although the program is still being developed, the intention is to chart the mortality of all garden plants for their viability to sustained heat as measured in heat days a particular zone of the country experiences annually. It is hoped that someday this data will be incorporated by the nursery industry such that plant tags will include two pairs of numbers to help consumers determine a particular plant’s viability in their gardens, such as 4-10, 11-3. The first set of numbers on a plant tag would be the USDA recommended range for cold hardiness with Zone 4 being the coldest in this example. The second set would be the AHS heat zone where this same plant can tolerate the heat of Zone 11. However, much testing still needs to be done on this system and its implementation.
Conifers and Zone Maps
While the individual plant records in the Conifer Database contain a map that represents that conifer’s minimum range of hardiness, it does not include the range that this plant will survive in more temperate zones as there is, of yet, no reliable data on how heat tolerant all the conifers in the Database are. Generally speaking, conifers can survive four or five zones warmer than its minimum listed USDA Zone but, relative to the lower 48 states, the range of conifers becomes more limited in the lower latitudes where the heat and humidity of the southeast and the extreme heat of the desert takes its toll on many conifer species and cultivars. There is some promising work being done grafting heat sensitive conifers to heat tolerant rootstock such that, over time, the nursery industry may produce more cultivars that survive in these higher heat zones but the supply of these trees is limited.
The ACS recommends that if your garden is in the southeast or southwest and you are considering adding a conifer to your collection that is rare or unusual to your area that you check with a knowledgeable local nurseryman, arborist, forester, etc., for advice about the cultivar’s local viability and if there is any evidence of it surviving in conditions that mirror your garden. More information on this topic is becoming available in, among other places, ACS’s Conifer Quarterly. The recently published Landscaping with Conifers and Ginko for the Southeast was written by two ACS members.
[Ed. Note: In keeping with the ACS’ educational mission, the Conifer Database contains many records of conifers native to the tropics and the southern hemisphere which are not common to domestic horticulture and are unlikely to be found in North America outside of a botanical garden or conservatory. They are included here to give the conifer enthusiast a broader understanding of gymnosperms world wide. Nonetheless, with their integration into the database, their hardiness is recorded and displayed in a USDA Zone map with their record. However, the considerable differences in climate, moisture, soils in our region, etc., make it unlikely that many of them would prosper here – nor would they be allowed to be imported for fear of their environmental impact on domestic plant life.]