By C. Rae Hozer
Gardeners scouting garden centers, nurseries, seed catalogs and the Internet for suitable new vegetation to add to their home landscapes and/or determining when to plant in springtime should know two things: 1) the date the last spring frost normally occurs and 2) your zone on the USDA Plant Hardiness Zone Map. Do remember, however, that these are merely guidelines derived from climate records kept over a long span of years, not the product of an exact science. Temperature data from the past cannot predict precisely future conditions.
The date your area has the Last Spring Frost helps you estimate when heat-loving annuals (whether bedding plants or tender garden vegetables) can be transplanted and when seeds for cold-sensitive plants can be sown directly in gardens with a high probability they will not be harmed by frosts and freezes
Perennial plants are those that can survive for years as long as their needs for temperature, moisture, sunlight and soil conditions are met. Determining whether a certain perennial will thrive in your landscape has traditionally been based primarily upon that plant’s tolerance to winter cold and how chilly it gets where you live. A conservative approach is to consult the United States Department of Agriculture Plant Hardiness Zone Map to determine the zone for your location then choose perennials with that zone listed mid-way in their hardiness range. For example if you live in Zone 6, select plants known to be winter hardy in zones 5 through 7. Hardiness zones on the map are intervals with 10 degrees Fahrenheit (F) of separation, each divided into two 5-degree subzones, the cooler of which is designated as the ‘a’ partition and the warmer as the ‘b’ partition. Zones defined in our state and the corresponding average low temperatures associated with each are Zone 5b (minus 15 to minus 10 degrees F), Zone 6a (minus 10 degrees to minus 5 degrees F), Zone 6b (minus 5 degrees to zero degrees F), Zone 7a (zero degrees to 5 degrees F), Zone 7b (5 degrees to 10 degrees F) and Zone 8a (10 degrees to 15 degrees F).
Be aware that within each zone there are normally microclimates. Heat islands caused by lots of blacktop and concrete occur in cities. Temperatures are cooler at higher elevations and in valleys between small hills. Experienced gardeners may even recognize some differences in growing conditions affecting plant hardiness in various parts of their yards.
The newest version of the USDA Plant Hardiness Zone Map was published in 2012. The map zones were determined based on average minimum temperatures recorded over a 30-year period (1976-2005). View the 2012 USDA Plant Hardiness Zone Map using a computer at http://planthardiness.ars.usda.gov/PHZMweb/. Select the state of Tennessee in the text box at the top right to get a closer look at hardiness zones in our state. In most but not all Tennessee counties, the plant zone rating in this latest edition of the USDA Plant Hardiness Zone Map is one subzone warmer than on the previous map.
Cumberland County plant hardiness and frost dates: Expect the last spring frost about May 10 and the first frost of autumn by Oct. 10. Cumberland County is Zone 6b from Crossville north and west toward Monterey in Putnam County. Small sections along the county line to the southwest near White County, south toward Pikeville in Bledsoe County and southeast by Rhone County are rated a warmer Zone 7a. Enter your zip code at the top left side of the USDA Plant Hardiness Zone Map website referenced above then click ‘Find’ to verify your local plant hardiness zone.
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Plateau Gardening is written by Master Gardeners for gardeners in Tennessee’s Upper Cumberland Region. Contact UT Extension Cumberland County at P.O. Box 483, Crossville, TN 38557 (931-484-6743) for answers to horticulture questions, free publications and how to become a Master Gardener. Send email comments or yard & garden inquiries to Master Gardener Rae, MGardenerRae@frontiernet.net.