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growing zones north america
The 2012 USDA Plant Hardiness Zone Map is the standard by which gardeners and growers can determine which plants are most likely to thrive at a location. The map is based on the average annual minimum winter temperature, divided into 10-degree F zones.
For the first time, the map is available as an interactive GIS-based map (above), for which a broadband Internet connection is recommended, and as static images for those with slower Internet access. Users may also simply type in a ZIP Code and find the hardiness zone for that area.
No posters of the USDA Plant Hardiness Zone Map have been printed. But state, regional, and national images of the map can be downloaded and printed in a variety of sizes and resolutions.
What Do Hardiness Zones Mean?
The 2012 USDA Hardiness Zone Map
The latest version of the USDA Zone Map was jointly developed by USDA’s Agriculture Research Service (ARS) and Oregon State University’s (OSU) PRISM Climate Group, and released in January of 2012. To help develop the new map, USDA and OSU requested that horticultural and climatic experts review the zones in their geographic area, and trial versions of the new map were revised based on their expert input.
Compared to the 1990 version, zone boundaries in the 2012 edition of the map have shifted in many areas. The new map is generally one 5°F half-zone warmer than the previous map throughout much of the United States. This is mostly a result of using temperature data from a longer and more recent time period; the new map uses data measured at weather stations during the 30-year period 1976-2005. In contrast, the 1990 map was based on temperature data from only a 13-year period of 1974-1986.
However, some of the changes in the zones are a result of new, more sophisticated methods for mapping zones between weather stations. These include algorithms that considered for the first time such factors as changes in elevation, nearness to large bodies of water, and position on the terrain, such as valley bottoms and ridge tops. Also, the new map used temperature data from many more stations than did the 1990 map. These advances greatly improved the accuracy and detail of the map, especially in mountainous regions of the western United States. In some cases, they resulted in changes to cooler, rather than warmer, zones.
The 1990 USDA Hardiness Zone Map
This webpage was originally based on the 1990 USDA Hardiness Zone Map, which is one of several maps developed to provide this critical climate information. The USDA map is the one most gardeners in the eastern United States rely on, and the one that most national garden magazines, catalogs, books, websites and nurseries currently use. This map divides North America into 11 separate zones. Each zone is 10°F warmer (or colder) in an average winter than the adjacent zone.
Eastern Zones vs Western Zones
Eastern Zones – The USDA map is great at its job of outlining the different gardening climates of the eastern half of North America. This is a comparatively flat area, which means it can be mapped out by drawing lines approximately parallel to the Gulf Coast around every 120 miles as you progress north. The lines will start to tilt northeast as the Eastern Seaboard approaches. The USDA map also accounts for the special climates caused by the Appalachian mountains and the Great Lakes.
Western Zones – A range of factors including winter lows, elevation and precipitation determine the growing climates in western North America. The weather in the west floats in from the Pacific Ocean, becoming less humid as it moves around the mountain ranges to the west. The growing climates in the west can vary quite a bit compared to the east, where cities in similar zones can grow the same plants in the same climates. The weather – and the plants that can be grown in it – in coastal Seattle are much different than what you can
expect in the higher and more inland Tucson, Arizona. This is in spite of both cities being a part of zone 8 of the USDA map.
How to Use Your Planting Zone?
Planting zones are most useful to gardeners growing perennial plants, since perennials are meant to live beyond just one growing season. Perennials need to be able to survive winter in your area, so it’s important to know how cold it typically gets in your area and whether a particular plant is hardy enough to survive those temperatures.
Perennial flowers, shrubs, and trees grow best when planted in the appropriate zone. You’ll find that winter damage occurs most often when plants are out of their range or “comfort zone.” When you choose plants for a garden or landscape, avoid selecting plants that are only marginally hardy for your region; that’s when you’ll see winter damage, poor growth, and a reduction in flowering.
Planting native species is a surefire way to achieve a stable garden. Native plants are which occur naturally where you live! So, naturally, they will thrive in their habitat. See our article on natural landscaping.
For annual plants, like most vegetables and some flowers, it’s far more important to pay attention to things like the length of your growing season and the typical dates of your first and last frosts. (See local frost dates here.) Because annuals are only meant to last the length of one growing season,
planting zones don’t necessarily factor into the equation.
What is Plant Hardiness?
Plant hardiness is the ability of a plant to survive adverse growing climates such as drought, flooding, heat and cold. Plant genetics determine the ability of a plant to withstand cold temperatures without damage. Each cultivar of a plant may have different hardiness levels based on their adaptations and genetics. Even different parts of a plant may be hardy in different ways. For example, the cold may cause your beloved perennial to die; however, the roots might be hardy enough to bring new growth in the spring.
Many plants require very specific growing environments to thrive. You can adjust many things, such as soil type, moisture levels and amount of sunlight in your garden, but temperature can be hard to control. Choosing plants with hardiness levels appropriate to specific planting zones gives you the best chance of gardening and landscaping success.
Breakdown of Plant Hardiness Zones
In the simplest terms, USDA hardiness zones keep a grower in Alaska from making the mistake of planting peach trees. Understanding the different plant hardiness zones gives you the ability to narrow down your gardening choices. Keep in mind, when a plant is hardy to a specific zone, it will usually perform well at any zone number that’s higher as well. For example, most plants hardy to Zone 3 will perform well in Zone 7, as long as other plant requirements are met. Use the information about individual growing zones as a starting point for planning what to plant in your garden.
How are hardiness zones determined?
When in doubt, opt for native plants?
Plants that are indigenous to an area are always good choices. These native plants are well suited to the average
minimum winter temps and the typical amount of rain, type of soil, and average summer heat in the region.
In addition to being good options for thriving in the area,
native plants also play a vital role in xeriscaping, the practice of growing plants that do not require a lot of extra
water, fertilizer, or pesticides,
because they’re well adapted to the region. In this way, using native plants will help conserve water and cut
down on the number of chemicals introduced in the environment.