What Flower Zone Is Knoxville, Tennessee?

Zones

• Although no official “flower zones” exist, two measures -- the U.S. Department of Agriculture (USDA) Plant Hardiness Zone Map and the American Horticultural Society Plant Heat Zone Map -- help gardeners decide what extremes plants, including flowering plants, must tolerate to live in their gardens. The USDA map outlines areas that share the lowest annual average winter temperature in contours of 10-degree Fahrenheit gradients, with subcategories for 5-degree Fahrenheit gradients. The map was first published in 1990 and is still in circulation, but a new map, published in 2012, covers a 30-year period and contains more details and updated zones. The AHS map charts contours using the average number of days per year that the temperature rises above 86 degrees Fahrenheit.

Knoxville Numbers

• Nestled in a valley on a northwest edge of the Appalachian Mountains, Knoxville summers and winters are warmer than those of many surrounding areas. Due in part to its latitude, location, elevation and city heat island effects, Knoxville sits in USDA plant hardiness zone 7b and AHS heat zone 7. This means that the average lowest winter temperature is 5 to 10 degrees Fahrenheit and that the area sees 61 to 90 summer days where the temperature rises above 86 degrees Fahrenheit.

Applications

• Annual flowering plants, such as zinnias (Zinnia elegans), that live and die within one growing season enjoy a long life in USDA zone 7b, where average low temperatures rise above freezing in April and don’t fall below freezing again until mid-October. Perennial plants, which return year after year, are more discriminating. Some, such as gladioli (Gladiolus spp.), hardy in USDA zones 6 through 10, cannot survive where winters are longer and colder. Others, such as peonies (Paeonia spp.), and tulips (Tulipa spp.), both hardy in USDA zones 3 through 8, require a certain number of days of cold weather in winter to set flowers the following spring.

Considerations

• Exceptional weather can catch up the most careful gardener. The lowest temperature recorded in Knoxville, minus 24 degrees Fahrenheit in 1985, is among the average coldest temperature range in USDA zone 4b, where gardeners must treat gladioli as annuals, digging up their corms and storing them indoors during winter. The temperature in Knoxville has risen as high as 105 degrees Fahrenheit, which occurred in 2012. Plants that require snow cover during winter were safe in the winter of 1959 through 1960, when 56.7 inches of snow fell in the city, but tender plants require mulching most years. Because Knoxville sits in a pocket-shaped corner of USDA zone 7b, many of its gardeners may be wise to chose their most expensive perennials from those that are also hardy in the surrounding USDA zone 6.