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    Plant Hardiness Zones

    Authored by Leah Chester-Davis

    Knowing your Plant Hardiness Zone is helpful information when selecting perennials and shrubs. It is the standard by which gardeners and growers often determine which plants are likely to thrive in various regions of the country. 

    Often, gardening articles, nursery catalogs, and plant tags will include Plant Hardiness Zone information. The term refers to the various growing zones that are part of the United States Department of Agriculture (USDA) Plant Hardiness Zone Map. The map is divided into 13 different zones across the country and each zone has either an “a” or “b” designation, which takes into consideration slight differences in temperatures within each zone. The “b” designation means the area is typically 5 degrees warmer that the area designated “a.”

    USDA created and updates the map based on average annual minimum winter temperatures over a period of several years. Each zone changes every 10 degrees (or every 5 degrees if you take into consideration the “a” and “b” designations of each zone. Knowing your Plant Hardiness Zone can be a good starting point when selecting plants though it has limitations. It is based on cold weather analysis and not heat and humidity.

    Find your Plant Hardiness Zone

    Plant Hardiness Zones range from 1a, which has the coldest temperatures, to 13b, which has the warmest winter weather. Parts of Alaska are in zone 1 which dips to -60 degrees F, while hot climates such as Puerto Rico are zone 13, which is 60 to 70 degrees.

    In the 48 contiguous states, zones range from 2 to 11 with northern Minnesota, northern North Dakota, and northern Montana being in zone 2, which see an average winter temperature as low as -40 degrees F. Southern Florida, southern California, and southern Arizona are in zone 11 where temperatures usually don’t dip below 40 degrees F. Other regions fall somewhere in between. 

    For example, the midpoint of the contiguous United States is just northwest of Lebanon, Kansas. It’s in zone 5b. Generally, in much of the midpoint of the country, running from east to west except for the coastal areas, the zones are 6a to 7b. The higher your zone, the longer your growing season.

    The best way to find your zone is to visit https://planthardiness.ars.usda.gov and type in your address or zip code. The color-coded map corresponds with a legend that also provides zone and average coldest winter temperature.

    How to read hardiness zone information

    When you determine your zone, use that information when you look at plant tags at the local nursery or order plants online. Most plant tags will include the zone information and frequently it will include a range. If you are in zone 4, you may want to bypass a plant that is labeled for zones 6 to 9, for example.

    Here is a look at an actual plant tag from Proven Winners for Oso Easy Italian Ice, a landscape rose. The tag indicates that it grows in USDA 4-9 zones, which means it can withstand temperatures as low as -30 degrees F. While it may withstand colder temperatures, the plant hardiness zones are simply a starting point for plant selection. It may be possible that the plant will thrive in zone 3 if in a protected area, for example. Protection from fluctuating temperatures can be just as important as protecting a plant from cold temperatures. The zones are simply a guide to help determine what plants may grow where.

    Considerations beyond Plant Hardiness Zones

    Even if you select plants based on the Plant Hardiness Zone, there are other considerations that affect a plant’s performance. Among them are summer temperatures, precipitation, and length of growing season. Plants need the required cultural growing conditions to thrive. Cultural conditions include starting out with good quality soil that has organic amendments, suppressing weeds so that your plants don’t have to compete for nutrients, siting your plant so that it receives the required light conditions such as full sun or shade, and water as needed particularly during long, dry periods. 

    Even within the various zones, microclimates can make a difference in temperatures and other conditions that affect a plant’s growth.  A protected area on the South side of your house, for example, may allow you to grow a plant that may require a temperature that is warmer than the rest of your landscape. Sometimes higher elevations may stay a little warmer than low points or valleys. On the flip side, an area with harsh winter winds can severely stress some plants, even if the plant is recommended for your growing zone.

    Changes in climate

    Plant Hardiness Zone information is simply a useful starting point guideline. As you garden, take note of the microclimates in your landscape and how your plants might best benefit from these areas. Employ good cultural practices such as building good soil by adding compost every year and siting your plants, so they receive their preferred light requirements. 

    While you may have plants perfectly suited for your zone, extreme weather events can be detrimental. If you are in a region that is experiencing extreme cold temperature fluctuations, consider protecting your plants with some type of winter protection such as a burlap or frost wrap that is anchored to the ground. If you are in a region with extreme rainfall levels, consider mulching around plants with straw or other organic material to help protect roots and reduce soil compaction.