US Hardiness Zones
What are the US Hardiness Zones?
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| USDA's 2012 Hardiness Zone Map |
The current version of the USDA Hardiness Zone Map was jointly developed by USDA's Agricultural Research Service and Oregon State University's PRISM Climate Group. It was released in January 2012. The new map is generally one 5°F half-zone warmer than the 1990 plan throughout most of the United States. This change resulted from several factors including advances in technology, the inclusion of data from many more weather stations, and the use of temperature data from a longer and more recent time period. New algorithms that include factors such as changes in elevation, nearness to large bodies of water, and position on the terrain, such as valley bottoms and ridge tops. The new map uses data measured at weather stations during the 30-year period 1976 - 2005. The 1990 map was based on temperature data from 1974 - 1986, a 13-year period.
Identifying Your Plant Hardiness Zone
The first step in understanding what plants will thrive in your garden is to know what plant hardiness zone you live in. If you're unsure what this is, find your location on the map to get your hardiness zone level. What you will see is a scale 1 - 13 that categorizes every location in the United States based on its average annual extreme minimum temperature during a 30-year period in the past, not the lowest temperature that has ever occurred in the past or might happen in the future. The coldest regions further north and higher in altitude are 1 on the scale, while the hottest regions are 13. This scale is further subdivided with 1a being colder than 1b, and 2a being colder than 2b, etc. These smaller zones represent 5°F differences. So this is the type of number that you will receive when you check what hardiness zone you live in. The majority of the northeast tends to fall somewhere between 3 and 6.
How Your Plant Hardiness Zone Helps You Choose Plants for Your Garden
Every plant has been identified for what zones it will survive in. All you need to do is know what zone your garden is in and then select plants that are labeled to be in that range. Knowing your zone is critical before selecting plants because it allows you to purchase the ones that have adapted to thrive in your specific location. Choosing the right plants for the right spot gives you a fuller, brighter, healthier garden that typically requires less work.
Identifying Microclimates in Your Plant Hardiness Zone
Now that you've narrowed down your options into a plant palette that survives in your hardiness zone, we can zoom in even farther to microclimates - an area of your yard that differs significantly from the environmental conditions of the remaining landscape. Differences could be warmer or cooler; wetter or drier; and windier or calmer. Courtyards are obvious examples of microclimates. They offer protection from wind and the warming effects of nearby structures. Microclimates are often due to their proximity to structures such as fences, houses, garages, patios, sidewalks and driveways. Structures warm faster and retain heat longer than soil. Nearby structures help to warm the surrounding soil, hence daffodils and other plants develop and emerge earlier in the spring.
Urban areas create a widespread microclimate. Cities with their heat collecting rooftops, parking lots and pavements tend to have less extreme low temperatures in winter and warmer soils than the surrounding rural area where I live. Buildings and paved surfaces absorb heat during the day and then radiate back into the air at night which can reduce the chances of frost and moderate low temperatures during winter.
In the summer the other side of this warming effect becomes apparent as cities become heat islands. Urban microclimates can trap heat which can create a scorching environment for us and our plants. Buildings may also offer protection from wind but can also create wind tunnels.
We can use microclimates to grow plants that may be marginally winter hardy here. Gardeners are known for their zonal envy as we gravitate to plants rated to a warmer zone.
We also create microclimates when we mulch with shredded leaves or bark mulch. Mulch moderates soil temperatures, keeping them warmer in winter and cooler in summer. Raised beds also create microclimates as the soil warms and dries faster in the spring making for quicker planting in the spring.
Look around your landscape now. Which plants are progressing faster than their neighbors? You may be seeing the effects of a microclimate.
Your property is most-likely not a "one-size fits all" in terms of planting spots. Take a moment not to visualize all of the different microclimates you have. Some examples include:
- North Side of Home/ Under a tree (Full Shade)
- Open Field/South Side of Home (Full Sun)
- East Side of House (Part Sun/Morning Sun)
- Rocky Hill (Quick Drainage and Sun)
- Stream Bed (Clay and Wet)
- Forest Floor (Dense Shade and Dry)
To do this, first recognize where you want to plant. Then analyze how much sunlight that location gets in a day, scaling from dense shade to full sun. Next, determine how dry or wet the location is; is it in a lowland spot that fills with water after a rain, or a dry hilltop? From there, you can narrow down the plant palette you will work from. Make sure to look at the information provided for each plant online or ask a professional at the nursery if you're unsure.
- Even though this edition of the USDA Hardiness Zone Map is drawn in the most detailed scale to date, there might still be microclimates that are too small to show up on the map. Microclimates, which are fine-scale climate variations, can be small heat islands- such as those caused by blacktop and concrete or cool spots caused by small hills and valleys. Individual gardens also may have very localized microclimates. Your entire yard could be somewhat warmer or cooler than the general zone for your area or for the rest of your yard, such as a sheltered area in front of a south-facing wall or a low spot where cold air pools first. No hardiness zone map can take the place of the detailed knowledge that gardeners pick up about their own gardens through hands-on experience.
Many species of plants gradually acquire cold hardiness in the fall when they experience shorter days and cooler temperatures. This hardiness is typically progressively lost in late winter as temperatures warm and days become longer. A period of frigid weather early in the fall months injure plants even though the temperatures may not reach the average lowest temperature for your zone. Similarly, exceptionally warm weather in midwinter followed by a sharp change to seasonably cold weather may cause injury to plants as well. The map does not take these factors into account.
Growing plants at the extreme of the coldest zone where they have adapted means that they could experience a year with a rare, extreme cold snap that lasts just a day or two, and plants that have thrived happily for several years could be lost. You need to keep that in mind and understand that past weather records cannot be a guarantee for future variation in weather.
Always Remember: "Right Plant, Right Place!"
Remember, plants have adapted to grow and survive over millions of years. If you select the right plant for the right place it will thrive with very little input from you. It will give you more growth, more blooms, and less work in your garden. Alternatively, if you select a plant that is out of your hardiness zone level, or is planted in the shade when it wants sun, it will suffer, underperform and eventually die.
Understanding these limitations may seem overwhelming when selecting plants for your garden, but don't worry! Simply start with knowing your hardiness zone and shopping for plants that fall within that range. From there, the label and planting information will tell you everything you need to know about if that plant will thrive in your specific location. In no time at all you will become familiar with the plant palette adaped for your garden and, after planting it all out, you will be ready to sit back and watch everything grow!
Other Factors
Many other environmental factors, in addition to hardiness zones, contribute to the success or failure of plants. Wind, soil type, soil moisture, humidity, pollution, snow, and winter sunshine can significantly affect the survival of plants. The way plants are placed in the landscape, how they are planted, and their size and health might also influence their survival.Light
To thrive, plants need to be planted where they will receive the proper amount of light. For example, plants that require partial shade that are at the limits of hardiness in your area might be injured by too much sun during the winter because it might cause rapid changes in the plant's temperature.
Soil Moisture
Plants have different requirements for soil moisture, and this might vary seasonally. Plants that might otherwise be hardy in your zone might be injured if soil moisture is too low in late autumn and they enter dormancy while suffering moisture stress.
Temperature
Plants grow best within a range of optimum temperatures, both cold and hot. That range may be wide for some varieties and species but narrow for others.
Duration of Exposure to Cold
Many plants that can survive a short period of exposure to cold may not tolerate long periods of cold weather.
Humidity
High relative humidity limits cold damage by reducing moisture loss from leaves, branches, and buds. Cold injury can be more severe if the humidity is low, especially for evergreens.
Click here to view the USDA Agricultural Research Service's interactive map to find your Hardiness Zone.
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