What zone 4a means
In zone 4a, the average of the coldest night of each winter (1991 to 2020) is -30 to -25°F (-34 to -32°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 4a, the plant should survive a typical winter here. If you are in the "a" half, you are in the colder part of the zone.
Frost dates in zone 4a
Median 32°F freeze dates across 792 ZIP codes with a NOAA weather station within 60 km, from the 1991 to 2020 climate normals. Frost dates vary a lot inside a zone, because zones measure winter cold, not season length, so look up your ZIP code for your own dates.
Vegetable planting calendar for zone 4a
Purdue Extension groups vegetables by how much frost they can take and times planting from the average last spring frost. Applied to this zone's typical last frost of May 12:
| Group | Plant outdoors | Crops |
|---|---|---|
| Hardy crops (tolerate a hard frost) 4 to 6 weeks before the average last spring frost | March 31 to April 14 | Asparagus, broccoli, Brussels sprouts, cabbage, collards, horseradish, kale, kohlrabi, leek, mustard, onion, peas, potato, radish, rhubarb, spinach, turnip |
| Semi-hardy crops (tolerate a light frost) 2 to 4 weeks before the average last spring frost | April 14 to April 28 | Beet, carrot, cauliflower, celery, chard, Chinese cabbage, lettuce, mustard, parsnip |
| Tender crops (damaged by frost) after the average last spring frost, once nights stay above 50°F | From May 12 | Bean, sweet corn, tomato |
| Very tender crops (warm season) at least 2 weeks after the average last spring frost, once nights stay above 60 to 65°F | From May 26 | Cucumber, eggplant, melons, okra, pepper, pumpkin, squash, sweet potato |
Spring planting for direct seeding or transplants. A late cold snap can still happen, so watch the forecast and protect tender plants. Want today's answer for your ZIP code? Use what to plant now.
Sources: NOAA NCEI U.S. Climate Normals 1991 to 2020; Purdue Extension, Indiana Vegetable Planting Calendar (HO-186-W).
Zone 4a next to its neighbors
Zone 3b, one half zone colder, averages -35 to -30°F (-37 to -34°C). Zone 4b, one half zone warmer, averages -25 to -20°F (-32 to -29°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 3b are now in 4a.
Using zone 4a when you shop
Look for the hardiness range on plant tags and in catalogs. A plant listed for "zones 2 to 6" includes zone 4a. 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 4a ZIP codes
Cities in zone 4a
Frequently asked questions
What does zone 4a mean?
Zone 4a covers places where the average coldest winter night is -30 to -25°F (-34 to -32°C). On the 2023 USDA map, 814 ZIP codes are in zone 4a.
What is the difference between zone 4a and 4b?
Each 10°F zone is split into two 5°F half zones. "a" is the colder half and "b" the warmer half. Zone 4a averages -30 to -25°F (-34 to -32°C); zone 4b averages -25 to -20°F (-32 to -29°C).
Source: 2023 USDA Plant Hardiness Zone Map ZIP code listings, PRISM Climate Group, Oregon State University; USDA ARS. Not the official USDA map.