USDA zone 6

What zone 6 means

USDA's Plant Hardiness Zone Map is based on the average annual extreme minimum winter temperature, in 10°F zones and 5°F half zones (USDA ARS). USDA's ZIP code search labels the two halves of zone 6 like this:

Some places, as extension services describe them: Ohio State says Ohio has four zones on the 2023 map, 5b, 6a, 6b and 7a. UW–Madison's Laura Jull writes that the 2023 map added zone 6a to Wisconsin along the western shore of Lake Michigan, from the Chicago area and Kenosha County into southeastern Sheboygan County. UConn's Dawn Pettinelli notes that Storrs, Connecticut, was 5b on the 1990 map, 6a in 2012 and is now 6b. For your own address, use the ZIP search on USDA's map; every zone below was checked that way in October 2026.

Zone 6 does not set your frost dates

A zone is a winter-cold number for choosing perennials. University of Idaho Extension: "Knowing your hardiness zone will not tell you how many frost-free days you will have." Frost dates and season length come from NOAA's Climate Normals, which give the last spring and first fall 32°F freeze at stated probabilities. The hardiness zones guide explains the difference.

Six zone 6 cities, side by side

Zone: USDA ARS ZIP search for the city ZIP shown (2023 map, 2012 zone in brackets); the airport ZIP of each station has the same 2023 half zone. Frost data: NOAA NCEI 1991–2020 normals, 32°F threshold. The 50% date is the median; a 10% spring date means a later freeze about 1 year in 10, and a 10% fall date a freeze that early about 1 year in 10 (NC State's guide to these tables).

Stations: Kansas City Intl AP, Milwaukee Mitchell AP, Port Columbus Intl AP, Hartford Bradley Intl AP, Albany AP, Denver Intl AP. NC State calls the 90% season the conservative one: at least that long in about 9 years out of 10.

What the table shows:

• The median last spring freeze runs from April 13 (Kansas City) to May 5 (Denver), and the median season from 193 to 156 days, a 37-day gap inside one zone.
• Milwaukee, in the colder half zone 6a, has a 192-day median season and the latest median first fall freeze of the six (October 29), longer than Hartford's 177 days in 6b.
• Four of the six moved up a half zone between the 2012 and 2023 maps; Albany and Hartford did not. Their frost dates come from NOAA's normals either way.

Crop timing examples

Planting guides count from your own last frost date. UNH Extension's fact sheet sets lettuce transplants out 3–4 weeks before last frost, kale 4 weeks before, tomatoes and cucumbers 1–2 weeks after, peppers, basil and squash 2 weeks after, with cucumber and squash seed sown "when soil warms to 70°F." Applied to the NOAA 50% dates of the earliest and latest cities above (our arithmetic):

Same zone, three weeks apart. A cautious grower in Denver who uses the 10% date (May 19) instead would push every row back two more weeks. Enter your own dates in the planting calendar for 25 crops.

A zone 6 planting table, and why it needs a date check

Virginia Cooperative Extension's home garden planting guide (Jadrnicek and Hessler, 2025) is one of the few that keys planting dates to half zones. Selected rows from its zone 6a and 6b table:

The table rests on frost ranges in the same guide: an average (50%) last spring frost of May 5–15 for 6a and April 25–May 5 for 6b, and a first fall frost of October 5–15 for both. Ohio State's Ohio Gardening Calendar prints the same ranges and bases its dates on zone 6a, moving spring dates one week earlier for 6b. Compared with NOAA's medians above, those ranges run late: Milwaukee (April 19) and Albany (April 27) sit well before the 6a range, and Kansas City, Columbus and Hartford (April 13–21) all fall before the 6b range. Only Denver's May 5 lands inside its range. Virginia's guide itself notes that Roanoke County spans four hardiness zones with a 30-day planting difference, and that actual frost dates vary with local conditions.

Where the sources disagree

Using this page in a zone 6 garden

1. Confirm your half zone with USDA's ZIP search; four of the six cities above changed between the 2012 and 2023 maps.
2. Use the zone for perennials, the plants that stay in the ground over winter; that is what USDA designed the map for.
3. Get frost dates for the station nearest you in distance and elevation; see how to find your frost dates.
4. Pick a probability: NC State calls the 10% spring date the low-risk choice, which suits frost-tender transplants.
5. Check a zone-keyed chart's frost dates against your own; for the cities above, the chart ranges began up to 16 days after NOAA's median (Milwaukee).
6. Let soil temperature and the forecast override any calendar date. The zone 5 page compares six cities one zone colder, the zone 7 page six cities one zone warmer.