Gardening Tips

Planting Zones and Seasonal Gardening Guide

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Planting Zones and Seasonal Gardening Guide

USDA Plant Hardiness Zone Map (USA)USDA Plant Hardiness Zone Map (USA)

Planting Zones

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 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

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 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.

Seasonal Gardening Guide


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