Your Hardiness Zone Is Not a Planting Date
A USDA hardiness zone is a winter-minimum statistic: the average annual lowest temperature. It answers what survives your winter, and it answers that well. It was never a growing-season statistic, and measured across this site's 7,125 NOAA stations it does not behave like one.
Two ZIP codes, one zone, 140 days apart
Both of these are USDA zone 6a. Both figures are the growing season NOAA publishes for the station each ZIP code actually cites.
- ZIP 81620
- 77 days Vail, CO (USC00058575) , 13.1 km away · 2,531 m
- ZIP 44139
- 217 days Cleveland Burke AP, OH (USW00004853) , 24.9 km away · 178 m
Elevation is the mechanism: 2,353 metres separate them, and the higher of the two (ZIP 81620) is the one with the shorter season. A hardiness zone is set by how cold the coldest night gets, so a mountain valley can share that figure with a lakeside city while having 140 fewer days to grow in.
These two are the ends of the range, picked by the data rather than by us — they are the shortest and longest seasons among the 4,573 zone 6a ZIP codes whose station publishes an ordinary season. The rest of this page is about the middle.
It is not two outliers — it is every zone
Across the 17 zones with at least 25 stations behind them, the median zone spans 54 days between its 10th and 90th percentile station, and annual growing degree days vary by a median factor of 2.02× inside a single zone.
Each band is the 10th to 90th percentile of published growing seasons across the NOAA stations that zone's ZIP codes actually cite — not its two extreme stations, which would overstate the case. Zones served by fewer than 25 stations are left out entirely — 9 of them — because a handful of stations is not evidence.
The bands overlap because the zone is not measuring this. Zone 9b alone spans 95 days between its 10th and 90th percentile stations.
The same figures as text
| Zone | Season, 10th–90th percentile | Spread | Stations |
|---|---|---|---|
| 3b | 98–140 days | 42 days | 92 |
| 4a | 112–148 days | 36 days | 298 |
| 4b | 103–157 days | 54 days | 477 |
| 5a | 110–165 days | 55 days | 718 |
| 5b | 117–177 days | 60 days | 752 |
| 6a | 122–187 days | 65 days | 1,004 |
| 6b | 132–198 days | 66 days | 991 |
| 7a | 150–208 days | 58 days | 941 |
| 7b | 175–222 days | 47 days | 679 |
| 8a | 195–239 days | 44 days | 689 |
| 8b | 195–261 days | 66 days | 705 |
| 9a | 220–302 days | 82 days | 472 |
| 9b | 255–350 days | 95 days | 382 |
| 10a | 311–365 days | 54 days | 190 |
| 10b | 365 days everywhere | 0 days | 79 |
| 11a | 365 days everywhere | 0 days | 34 |
| 12b | 365 days everywhere | 0 days | 47 |
⚠️ That median is conservative, and it is worth knowing why: 3 of the 17 zones are frost-free everywhere, so their spread is 0 by nature and they pull the middle down. The zones where the question means anything are wider — 9b spans 95 days and 9a spans 82.
And the zones overlap
If a warmer zone reliably meant a longer season, each zone's stations would freeze later than the one below it. They do not. Comparing first-fall-freeze dates:
- 26% of zone 5b freezes before the median zone 5a station, which freezes on October 2
- 29% of zone 6a freezes before the median zone 5b station, which freezes on October 8
- 30% of zone 6b freezes before the median zone 6a station, which freezes on October 14
- 33% of zone 7a freezes before the median zone 6b station, which freezes on October 20
So roughly 33% of zone 7a — a warmer zone — has its season ended earlier than half of zone 6b. Taken as one group, zones 5 through 8 record a first fall freeze anywhere from August 28 to December 17.
Inside one state it is worse
A state is a smaller place than a zone and the spread survives it. Across the 49 states with more than one station:
- Washington spans 182 days, from 94 to 276, across 154 stations
- New York spans 147 days, from 97 to 244, across 176 stations
- the median state spans 93 days
- even the narrowest — Delaware — spans 31 days, from 200 to 231
There is no state in the union where one planting date is right everywhere in it.
What this site does instead
Every date on USGrowersGuide is computed from the NOAA station your ZIP code actually cites, and every station is named with its identifier and its distance so you can check it. Where a place is served by stations that disagree, the page gives the range and says how many stations it is over — it does not average them into a single date that is true nowhere.
- What can I still plant? — a crop-by-crop verdict against your own station's first fall freeze
- When to start seeds — a dated spring calendar anchored on your own last spring freeze
- What survives my winter? — the question a hardiness zone is the right statistic for
Every figure on this page is recomputed from the published dataset each time the site is built, so it cannot drift from the data it describes. Freeze dates and growing seasons are NOAA 1991–2020 Climate Normals; hardiness zones are USDA PHZM 2023. Where the numbers come from.