As the days get longer and the weather gets warmer most of us gardeners are itching to get outside and get our hands dirty. As spring approaches, our mailboxes are flooded with catalogs tempting us with beautiful pictures of every plant under the sun.
Before we start giving into our cravings to order exotic flowers, its important to ask ourselves, “What planting zone am I in?”
Choosing a plant that is labeled for your zone, helps to ensure that your plant will survive the winter. Here in northeast Ohio its common to find a tropical variety hibiscus for sale at the big box stores, garden centers, and even outside grocery stores. Many people buy them to take home and plant them in their front yard. They look beautiful all spring and summer, but never survive the winter. Its very profitable to sell the same $20 tree to the same people year after year.
We can bring the tropical hibiscus inside over the winter…if we have room for a small tree in our house. A better option would be buying a hardy variety, like Lord Baltimore Hibiscus, that will survive the winter here in Ohio. This is why its important to know what planting zone you are in.
In 1960 the USDA came out with the first hardiness map that broke the United States into different growing zones based on 10 degree (F) differences in average low temperatures. Over the years the map has evolved to take into consideration more precise measurements, but the map still only reflects one thing: the average low temperature.
This zoning does not take into account soil condition, humidity, heat, wind or snowfall. Here is what the USDA cautions:
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 greatly 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 longer 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.
Nevertheless, plant hardiness zone maps have become a useful guideline in determining what grows where. Most plants have their zones listed on tag.
The newest version of the map was released in 2012. It was compiled using weather readings from the past 30 years. The newer map has more specific results than the previous versions because of better mapping technology and many more weather observation stations.
The new map takes better notice of micro-climates inside larger zones. This helps set guidelines for larger cities that (because of all the heat stored in concrete) stay warmer than the surrounding countryside or cooler temperatures resulting from higher elevations.
The newer version of the map added zones 11 and 12 to represent 50 and 60 degree lows found in Hawaii and Puerto Rico. There is also an option to look up your area by zip code for more specific results.
Click here to visit the USDA website.
Click here to see an interactive plant hardiness map for Canada.
Here are a few other useful tidbits that will help you decide which plants to grow.
Date of Last Expected Frost:
Zone 3 Mid-June
Zone 4 Early June
Zone 5 Late May
Zone 6 Mid-May
Zone 7 Late April
Zone 8 Early April
Zone 9 Early March
Zone 10 February
Common Effects of Freezing Temperatures on Non-Dormant Plants:
Light freeze: 29-32 degrees (F) kills tender plants
Moderate freeze: 25-28 degrees (F) destroys most vegetation
Heavy freeze: 24 degrees (F) and lower causes heavy damage to most plants.
Full Sun: 6+ hours of direct sunlight
Partial Sun: 4 to 6 hours of direct sunlight
Partial Shade: 2-4 hours of direct sunlight, or filtered sunlight
Full Shade: Less than 2 hours of direct sunlight
(The terms “partial sun” and “partial shade” are sometimes used interchangeably. Potato, po-tat-o. )