What the USDA Zone Map Can (and Can’t) Tell You
In an effort to help farmers and gardeners determine which plants can survive in their climate, the USDA created a nationwide hardiness map based purely on the harshness of a region's winter.
The map, which was updated in 2023, ascribes each region to a "Zone" which is divided into 10-degree bands based on the average annual minimum temperature. The colder the winter, the lower the zone number. Each zone is further divided into 5-degree increments labeled "a" and "b," with "a" being the colder of the two. For perspective, Zone 1 (with a low of -60°F…brrr) is only found in a section of remote Alaska and Zone 13 (60°F… balmy!) exists in just a teeny tiny portion of Hawaii.
Most of Greater Philadelphia is classified as Zone 7a, with an average yearly minimum temperature of 0°F.
These plant hardiness zones provide insight, then, into just one of many important climactic factors for gardening: the harshness of the winter. Let's take the climate and flower-growing potential in Bend, Oregon, as an example. While Bend is considered the same hardiness zone as Greater Philadelphia because they have similar winter lows, their climate is very different. Like much of the Pacific Northwest, Bend residents enjoy a lengthy cool spring season season, as opposed to Philly’s "blink and you miss it" transition from a freezing winter to a sweltering summer! Their summer temperatures are also considerably cooler than ours. With temperatures averaging between 55° - 75° for more than four months, Bend residents are far more likely to easily succeed growing cool-weather loving cut flowers - including popular varieties like ranunculus, sweet peas, snapdragons and anemone. Bend (and the rest of the Pacific Northwest, for that matter) is a far more favorable location to grow these coveted blooms. While these varieties can typically be grown in our region with some success, it often involves more strategic planning and, ideally, a hoophouse or minimally heated greenhouse - or just very good luck!
Hardiness zones only become a relevant metric in the world of cut flower growing when it comes to planting perennials, overwintering certain varieties, or when to plant cool-weather flowers. Ranunculus lovers, for example, will know to plant their corms in the fall if they’re located in Zones 8-11, and in the late winter or early spring if they’re in Zones 3-7; similarly, gardeners in the south are lucky enough to cultivate certain tropical perennials and vines that those of us in the north can’t successfully grow.
Cool-weather ranunculus and other spring flowers, including tulips and clematis ~ Photography by Ashley Meier
