Planting Zones and Seasonal Gardening Guide


Planting Zones and Seasonal Gardening Guide

The USDA Plant Hardiness Zone Map

Planting ZonesThe USDA Plant Hardiness Zone Map code

In order to help gardeners determine which varieties of plants will grow best in local climate conditions, the U.S.D.A. has devised a system of hardiness zones.

Hardiness zones, developed by the U.S.D.A., provide a means of comparing prevailing atmospheric conditions and the durability of various species of plants. These detailed computations and projections unique to a given place describe the climate by quantifying the range of temperatures during the cold months over a period of years. Horticulturists then have a gauge to figure out whether or not a certain kind of plant can survive winter conditions in such a place.

Hardiness zones, along with reliable recommendations of plant species based on the appropriate use of the maps generated by detailed climate statistics, guide residential gardeners in selecting all manner of plants that will thrive and prosper in their specific region of the country. From flowers and flowering bushes to shrubs, succulents, and trees, hardiness zones provide gardeners with a valuable tool that aids in selecting, planning, and achieving vibrant and beautiful landscape projects.

Hardiness zones, implemented by the U.S.D.A., use annual climate statistics to quantify the severity of winter in a given region for the purpose of determining which plants would best flourish there.

U.S. Plant Hardiness Zones

Getting in the Zone of the United State of America

Hardiness zones help gardeners determine which plants will thrive and prosper in a given climate. A hardiness zone is a geographically defined zone in which a specific type of plant is capable of growing, as determined by temperature hardiness, or its ability to withstand the minimum temperatures of the area. These zones were first implemented by the United States Department of Agriculture (U.S.D.A.), and have subsequently been adapted for use in other countries. The zones are differentiated from one another based on the average of the lowest temperature recorded each winter, termed the “average annual minimum temperature.”

In North America, the official hardiness zones map was revised and published by the U.S.D.A. in 1990 and serves as a guideline for selecting annual plant varieties. Each zone in the United States is labeled with a number ranging from one to 11. In 2003, a preliminary draft of a new map was produced by the American Horticultural Society (A.H.S.), using temperature data collected from July 1986 to March 2002, a period of atypically warm winters in the eastern part of the United States. The 2003 map places many areas a zone higher (warmer) than the 1990 map. The proposed map also shows a finer level of detail, for example reflecting urban heat islands by showing the downtown areas of several major cities as a full zone warmer than the outlying areas. As of the current date, the U.S.D.A. still endorses the older version of the map.

Plants vary to an extreme degree in their ability to withstand extremes of climate. The ability of a plant to withstand cold is the most basic and pervasive measure of its hardiness. The selection or breeding of varieties capable of withstanding particular climates can be challenging even to the most experienced agriculturalist or horticulturist. As living organisms, plants can adapt to some extent to changes in climate. Much of the work of nursery growers of plants consists of hardening (or hardening off) their plants to prepare them for likely conditions later in their lifespan.

For Best Results

The hardiness zones are effective in that, for many situations, extremes of winter cold are the most important factor in deciding whether a plant species can be cultivated outdoors at a particular location. However, some discrepancies exist in the system, requiring aspiring gardeners to approach plant selection cautiously. Hardiness zones generally address issues of cold and conspicuously ignore summer heat levels. Paradoxically, a site that may have the same average lows over the winter but markedly different summer highs will still be accorded the same hardiness zone. Gardeners attempting to grow plants at the extreme ends of the spectrum of hardiness zones in which that plant can survive may find that an unusually cold winter or hot summer threatens the success of new plantings.

In addition, many plants and plant species have special characteristics that may make them harder to grow in certain climates. Many plants will not live through severe winters; others will wither in the heat, and many spring-flowering bulbs and trees need a cold period to stimulate their growth cycles. Plants usually survive the cold better in a dry area, where moisture trapped underground cannot compromise root systems. Deciduous plants tolerate more exposure to sun and summer heat. Evergreens prefer a sheltered area with more humidity and cooler summers.

Zoning Out

There are also different climates and frost dates within planting zones due to topography, lakes, rivers, canyons, or mountains. These can cause altered airflows that can raise or lower the temperature, changing the zone in a localized area. Densely populated urban areas often experience warmer climates than surrounding areas due to the influence of man-made objects and architecture. The chemical balance and texture of the soil, exposure, altitude, rainfall, humidity, sunlight levels, and wind chill factors can also alter the local climate subtly, potentially skewing the predictive accuracy of plant hardiness zones.

International planting zones extend the work of the U.S.D.A. in creating hardiness zones to help gardeners all over the world find the right plants for their own use. Useful for cultivating exotic plants and planning novel species, these systems can help you get the best garden ever.

International Planting Zones

The best-laid plans


International planting zone maps and charts help gardens stay in peak condition all year. With all the considerations that influence a plant’s success (water needs, light requirements, and soil conditions, along with a host of other important issues) it can be difficult to know which plants will grow well in any given area. A good place to start is in knowing a plant’s winter hardiness. Thankfully, this information has been compiled in easy-to-use maps.

Planting zones, also known as hardiness zones, were first developed by the United States Department of Agriculture in the 1960s. Since the 1960s, the U.S.D.A. has been compiling data and creating maps to aid large-scale agricultural producers and hobbyist gardeners alike. These maps separate the country into many smaller micro-zones based on minimum annual low temperatures. Hardiness maps and their corresponding zones give gardeners information about which plant species are most likely to survive and thrive in their respective areas. Each zone in the United States is labeled with a number ranging from one to 11. Each of these numbers corresponds to a mean annual low temperature. For example, areas in zone 1 have average low temperatures of about -50 degrees Fahrenheit, while those in zone eleven average lows of 10 degrees.

Taking the Show on the Road

Since the system used in the United States is founded on such simple principles, other nations around the globe have made similar maps based on their winter low temperatures. Many of these international hardiness zone maps are accessible online and in print through specialty garden retailers and through international garden societies like the United Kingdom’s Commercial Horticultural Association. A word of caution: since maps created in different international locations may not adhere to exactly the same system developed by the U.S.D.A., not all spots in any given zone will have exactly the same mean winter low temperatures.

If a gardener has concerns about the reliability of a zone map’s information or the compatibility of different international hardiness zones, there is a simple way to calculate the correct zone. To approximate a hardiness zone, find out what the average low temperatures are for the area you want to label. Information about average annual lows can be gathered at national weather services, as well as agricultural extension offices. This information translates to a corresponding hardiness zone label of 1 through 10.

Final Calculations

Part of the beauty of hardiness zones is in their simplicity, but that lack of complexity does make for some limitations. In the gardener’s vernacular the term hardiness takes into account many things. In addition to the cold, hardiness factors include heat, drought, and wind tolerance. While knowing one’s zone illustrates which plants are more likely to survive the punishment of winter, the zones only address the coldest climatic conditions. This means that while a plant may survive a garden’s winter temperature, that same plant may die in that climate’s summer heat. In short, these zones take no account of soil conditions, rainfall or other factors essential to the success of garden plants. That said, hardiness zones are useful when the information they contain is utilized properly.

Power Tips

The best way to use planting zones is as a tool to determine a plant’s likelihood of surviving winter low temperatures. This can be especially constructive when dealing with exotic, unfamiliar, or non-native plant materials. For example, a gardener living in Dublin, Ireland (in zone 8) could expect a non-native plant from other zones also labeled 8 to over-winter in his or her climate. This information is increasingly useful as international plant distributors move species around the globe at a record pace and exotic plants find their way into gardens all over the world.

Know your garden zone

The U. S. Department of Agriculture created a map of North America in the mid-1900s that separated the continent into eleven growing zones based on the lowest annual minimum temperature. This was done to aid farmers and gardeners in selecting plants that would do well in their region. This map is especially helpful when choosing trees, shrubs, and perennial plants, but is also useful as a guideline when planting seeds for crops and gardens. Since growing seasons vary by region, it is beneficial to know what seeds require longer periods to reach maturity and produce grain, fruits, and vegetables for harvesting.

The map has been modified several times since its inception, due to changes in climate and to give it more granularities in certain areas. The AHS (American Horticulture Society) created a plant heat zone map in 1997 to be used along with the USDA zone map, to help aid in selecting plants according to their ability to sustain high temperatures. Recently there was funding granted to AHS to update the USDA map to accommodate global climate changes. The updated map is expected to be released this year.

According to a Minnesota Pollution Control Agency report, “parts of northern Minnesota have warmed five degrees F or more in winter” in the last 100 years; and “precipitation has already increased 20 percent in the southern half on the state since 1900.” It is generally thought that for every 1.8 degrees F that the climate heats up, the vegetation zone shifts north approximately 60 miles.

Scientists predict that Minnesota weather will become more humid and wetter, and may potentially be more like the climates of Nebraska, or Missouri. For Minnesota gardeners, this could mean that plants and shrubs that were previously off-limits in your backyard, would now flourish in the warmer climate. While this might seem like a wonderful thing, it generates concerns for the effect that it will have on not only plants, but insects, birds, and the rest of the food chain as it depends on vegetation for nourishment.

For those gardeners that like to experiment with different annuals, perennials, and bulbs, the climate changes provide a continuous laboratory in the backyard garden. Vegetation, much like humans, is very adaptable, and if the changes are gradual, most will adjust and be resilient enough to not only survive but possibly even flourish. However, it is safe to say that some plants (much like insects, birds, and humans) will move on to climates that are more conducive to their survival or just die out completely.

To be sure, capturing evidence of climate change patterns and modifying plans for successful growing seasons is an ongoing challenge. We can be confident that farmers and gardeners will continue to do what they’ve done for centuries. That is, through trial and error, determine what grows best in their fields and gardens, and build on their experience, working to refine and adjust it to accommodate the weather changes year after year. And continue to hope that the rest of the food chain can succeed in doing the same.

Seasonal Gardening Guide

A Monthly Game plan for Busy GardenersPlanting Zones and Seasonal Gardening Guide

From landscape planting to ordering and sowing seeds, to the delights of spring’s first tender plants, there’s fun to be had in the garden in every season of the year. Keeping note of the best times to do different tasks in the yard also makes for larger yields in both the flowerbed and vegetable patch, and keeps a gardener’s spirit growing year-round.


Because temperatures can be the lowest in January, now is the time to ensure that tender plants that have been mulched heavily for winter are still healthy. With Christmas just past, compost the tree and other natural trimmings, such as wreaths and centerpieces. Greenery can be cut up and used as insulation on annual beds or be chipped and added to the compost pile.


February is ideal for pruning fruit trees. On a temperate day, remove any dead wood and crossing branches. Concentrate especially on young growth and shoots running vertically to old wood.

This is also a good time to prepare materials for plants sown indoors. Sanitize all pots, tools, and racks. Buy necessary materials: soils, new light bulbs for growing fixtures, and seeds. Test older seeds for potency.


As soon as the soil is workable, start preparing beds for planting. Apply time-release fertilizers (where necessary) and several inches of good quality compost. Remove early weed sprouts and tidy up raised beds. Since much of the country tends toward soggy weather in March, this is also the time to be vigilant about slug and snail eradication.


April is a great month to remove winter mulches and make way for new plants. Discard mulching materials to the compost heap. Check on any perennials divided in the fall. Make sure that their roots have remained well covered and discard materials that did not survive last fall’s transplant. This is also the month to divide perennials. Take pie-shaped wedges from established plants and reuse them in other areas of the garden.


It’s time to reintroduce plants to the outdoors that have been wintered inside. Take tropical plants, citrus, and other frost-tender species and leave them outside during warm afternoons. In many climates, this is also the month to start planting the vegetable garden in earnest. May is also the season to prune back early flowering trees and shrubs.


Tackle lawn care in June. Remove any clumps of crabgrass from the lawn and apply a nitrogen-rich fertilizer and weed killer. Also, take out the mower, clean the undercarriage, and sharpen the blades. Keeping the lawn regularly cut, removing no more than 1/3 of its total height, will keep it in top shape all summer. Plant colorful annuals for late summer displays.


July is the high season for annual weeds, so it’s more important than ever to stay on top of them. Use manual methods first and chemical methods only when absolutely necessary. Discard any seed-bearing weeds in the refuse or in an autumn burn pile.

Since this is the season of greatest glory in the garden, why not take some time to enjoy all the hard work? Plan a garden party or just enjoy a cool iced tea in your favorite spot in the garden.


Concentrate on deadheading during August. Removing faded blooms will elongate the glory of annuals and keep the garden looking tidy through the first frost. Watering intelligently is also important in late summer. In the morning or early evening, give lawns and beds deep waterings. This will encourage good root development.


Transplanting is most successful in the fall. Prepare beds for new divisions by weeding, removing faded annuals, and fertilizing. Dig up perennials for a division and separate clumps with a pair of garden forks. Replant both sections immediately and water well. September is also a good time to clear out the vegetable garden and to get it ready for a winter rest.


If it hasn’t been done already, October is the month to protect frost-tender perennials and to trim back spent foliage. Spring bulbs should also be put in the ground and fertilized. Planting crocus and daffodils in drifts will make for brilliant spring displays.


Take a well-deserved break.


It’s time to start plotting all over again. Pour through seed catalogs, using small sticky notes to mark potentials for next year’s garden. Make a drawing of your yard and consider last year’s successes and learning experiences. Make appropriate changes to the plan and order plants and seeds for the spring.

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