What zone 1a means
In zone 1a, the average of the coldest night of each winter (1991 to 2020) is -60 to -55°F (-51 to -48°C). Some winters will be colder than that and some milder: the zone is an average, not the coldest temperature ever recorded. When a plant tag lists a zone range that includes 1a, the plant should survive a typical winter here. If you are in the "a" half, you are in the colder part of the zone.
Zone 1a next to its neighbors
Zone 1b, one half zone warmer, averages -55 to -50°F (-48 to -46°C). On the 2023 map, about half the country moved to the next warmer half zone compared with 2012, so some gardens that were in 1a are now in 1a.
Using zone 1a when you shop
Look for the hardiness range on plant tags and in catalogs. A plant listed for "zones 1 to 3" includes zone 1a. If the range stops just above you, choose a sheltered spot, mulch roots before winter or grow it in a pot. Remember that zones describe winter cold only; heat, rainfall and soil decide the rest. How to read a plant tag.
States with the most zone 1a ZIP codes
Frequently asked questions
What does zone 1a mean?
Zone 1a covers places where the average coldest winter night is -60 to -55°F (-51 to -48°C). On the 2023 USDA map, 5 ZIP codes are in zone 1a.
What is the difference between zone 1a and 1b?
Each 10°F zone is split into two 5°F half zones. "a" is the colder half and "b" the warmer half. Zone 1a averages -60 to -55°F (-51 to -48°C); zone 1b averages -55 to -50°F (-48 to -46°C).
Source: 2023 USDA Plant Hardiness Zone Map ZIP code listings, PRISM Climate Group, Oregon State University; USDA ARS. Not the official USDA map.