Choosing a perennial begins with understanding the place where it will grow. The USDA Plant Hardiness Zone Map provides a reference for winter cold, helping gardeners compare a location with a plant's hardiness information. Read the zone together with the plant description, then consider the conditions in the intended planting spot. A useful garden plan brings those pieces of information together rather than relying on a zone number alone.
A hardiness zone represents the average annual extreme minimum winter temperature at a location. For the 2023 map, that means averaging the lowest winter temperature recorded each year over a 30-year period. It describes a particular measure of cold: the average of those annual low points. It does not describe the coldest temperature ever recorded there, or the coldest temperature that could occur in a future winter.
The map contains 13 zones across the United States and its territories. Each zone covers a 10-degree Fahrenheit interval, with two half zones covering 5-degree intervals. The half zones carry the letters A and B; USDA gives zone 7, divided into 7a and 7b, as an example. Keep the letter with the number when recording your result so you retain the map's more detailed designation.
The distinction between an average and an absolute low matters when comparing plant choices. A rating can help organize a shortlist, but it cannot promise that every winter will stay within the average range. Treat the zone as a starting reference for a perennial's winter exposure and keep notes on your own garden alongside it.
USDA released the 2023 map on November 15, 2023, its first revision since 2012. The map was jointly developed by USDA's Agricultural Research Service and Oregon State University's PRISM Climate Group. It uses weather-station measurements from 1991 through 2020 and incorporates data from 13,412 stations, compared with 7,983 stations in the 2012 version.
According to USDA's explanation of the 2023 map, about half of the country shifted to the next warmer half zone, while the other half remained in the same half zone. A shift to the next warmer half zone represents warming somewhere within a range of 0-5 degrees Fahrenheit. Some locations also experienced warming within that range without crossing into a different half zone.
If you have a zone recorded from the 2012 map, look up the location again before using it for a planting decision. Keep the map version with your notes, especially when comparing an earlier garden plan with a current plant label.
USDA says the national differences in zone boundaries mostly result from incorporating temperature data from a more recent period. More sophisticated mapping methods and additional weather stations also contribute to some boundary changes. The annual extreme minimum is the coldest night of the year, and that temperature can vary considerably from year to year with local weather patterns.
The map developers therefore caution against using temperature changes between map editions as reliable indicators of global climate change. A comparison of zone labels concerns annual extreme minimum temperatures and the methods used to map them. Keep that limited purpose in mind when interpreting a changed label: use the map for the winter-hardiness question it addresses, and avoid drawing a broader conclusion from a single location's designation.
Start a plant-selection note with your zone, the proposed planting location, and the plant's stated hardiness range. Then compare its light, soil, and drainage requirements with that location. Ask practical questions: what light reaches this bed, what soil conditions are you working with, and how does water move through the planting area? These considerations belong beside the winter rating in your decision.
For an unfamiliar perennial, keep the plant label or description with your garden notes. Record where you place it and what you observe rather than assuming that a matching zone settles every growing question. The perennial selection and care guide provides a starting point for thinking through the planting site.
Use the 2023 USDA Plant Hardiness Zone Map to look up your location. Enter a ZIP code in the Quick Zip Code Search box, or click on the map to view the corresponding interactive map. Record the complete number-and-letter result before comparing it with plant information.
A simple planning routine is to look up the zone, read the plant's requirements, and write down any unanswered questions about the planting spot. Return to those notes when choosing among plants. This keeps the map's winter-temperature information useful without asking it to stand in for every observation about your garden.