How important is it to refer to the hardiness zones I see listed for plants I would like to try in my garden?
— Alicia Anderson, Highland Park
Choosing plants that are well-suited to the local climate is an important step to having a successful garden. Considering the zones where the plant can perform at its best is a good place to start as you’re researching plants to purchase this winter. When I evaluate new plants for my garden, I always consider their hardiness zones.
Occasionally I’ve also seen plants for sale locally that are not hardy in this region but are mixed in with plants that are. Most gardening catalogs that use hardiness zones in plant descriptions use the United States Department of Agriculture (USDA) Plant Hardiness Zone Map, which was updated in 2023; it’s a system for rating the cold hardiness of plants. The 2023 map incorporates data from 13,412 weather stations compared to the 7,983 that were used for the 2012 map, and it’s available online as an interactive GIS-based map.
This ranking system divides the United States and its territories into 13 zones that are based on the average minimum temperatures in the winter over 30 years. The map divides the country into 10-degree Fahrenheit zones and further divides into 5-degree Fahrenheit half-zones. When compared to the 2012 map, the 2023 version reveals that about half of the country shifted to the next warmer half-zone, and the other half of the country remained in the same half-zone. The annual average minimum temperature represents the coldest night of the year, which can be highly variable from year to year, depending on local weather patterns. Some changes in zonal boundaries are also the result of using more sophisticated mapping methods and the inclusion of data from more weather stations — so, they’re not necessarily reflective of global climate change. The designations do not reflect the coldest it has ever been or ever will be at a specific location but low temperature during the winter is a crucial factor in the survival of plants at specific locations.
Plants will often have a range of hardiness zones noted, such as four to six, which means it’s hardy and suitable for zones four, five and six. The hardiness zone for a particular plant is not necessarily a fail-safe single method for determining how hardy it will be in your garden, though. Variables such as sun, shade, wind, humidity, elevation and rainfall can all affect how hardy the plant is and whether it thrives. The plant also has to be able to tolerate year-round conditions such as summer heat in your area. Then there can be microclimates in a garden, like a sheltered corner near a brick wall, that will allow gardeners to push the zone hardiness a little. On the opposite end of the spectrum, significant weather extremes — like an extended period of warm temperatures in late fall with a very quick, drastic drop in temperature in early winter before the plants have hardened off — can heavily damage or kill some plants that we know to be zone hardy. Highland Park is in Zone 6 on the new map, but I would limit the number of Zone 6 plants you purchase to minimize the risk of damage from an unusually harsh cold spell.
For more plant advice, contact the Plant Information Service at the Chicago Botanic Garden at [email protected]. Tim Johnson is senior director of horticulture at the Chicago Botanic Garden.