Zone 4A Planting Calendar — May 14 to September 27

NOAA provides station-level Climate Normals. NOAA's National Centers for Environmental Information (NCEI) records daily minimum temperatures at thousands of U.S. weather stations and publishes, for each station, the probability that the last spring frost has occurred by each calendar date — based on 30 years of observations (1991–2020). These per-station probability percentiles are the primary data.

SoilStack aggregates those station-level normals into a per-USDA-zone composite. For each ZIP code in Zone 4A, we identify nearby NOAA stations, weight each station's contribution by inverse-distance squared to the ZIP centroid, build a per-ZIP composite from those weighted station percentiles, and then aggregate every per-ZIP composite in the zone into a single zone-level composite using a weighted circular mean for dates and a weighted arithmetic mean for numeric values like growing-season length.

For Zone 4A specifically, this composite was aggregated across 742 contributing ZIP codes (of 783 total ZIP codes assigned to this zone in the USDA 2023 Plant Hardiness Zone Map).

Read the full methodology →

NOAA NCEI 1991–2020 U.S. Annual/Seasonal Climate Normals — the authoritative U.S. climatological dataset, released by NOAA in 2021 and covering frost-date probabilities at six temperature thresholds and nine probability percentiles for thousands of stations nationwide. Dataset identifier: gov.noaa.ncdc:C01619.

Methodology paper: Palecki, M., Durre, I., Applequist, S., Arguez, A., Lawrimore, J. (2021). U.S. Climate Normals 2020: U.S. Annual/Seasonal Climate Normals (1991–2020). NOAA National Centers for Environmental Information.

Station ↔ ZIP crosswalk: built from NOAA’s GHCN station inventory and the U.S. Census Bureau 2024 ZCTA Gazetteer (public domain) using great-circle distance with inverse-distance-squared weighting.

Live freeze outlook: National Weather Service (NWS) public forecast API, for the location of a ZIP you enter. Used only within about 7 days of a forecast freeze, the range an NWS point forecast reaches; without a ZIP, or outside that window, the chart shows the NOAA climate normal alone.

View the NOAA NCEI dataset →

The curve shows the cumulative probability that the last spring frost has occurred by each date. It rises from 0% (no chance frost is done that early) to 100% (essentially certain frost has occurred by that late date).

Three reference points are highlighted:

• P10 (May 1) — the earliest date by which there’s a 10% chance the last frost is already done. Gardeners willing to take risk on early planting anchor decisions here.
• P50 (May 14) — the median. Half of historical years had their last frost on or before this date. This is the “Typical” reference.
• P90 (May 28) — the date by which there’s a 90% chance frost is done. Risk-averse gardeners planting frost-sensitive crops use this date as their go-ahead signal.

For most warm-season crops (tomatoes, peppers, basil, beans), waiting until the P50 to P90 window is the conservative call. Hardier crops can go in around P10.

The typical last-frost line. The P50 marker is the climate-normal center — the median last-frost date from 30 years of NOAA data. If you enter your ZIP and a freeze is within the 7-day National Weather Service window, a separate marker shows that forecast freeze for your location. A forecast freeze is not necessarily the season’s last: later freezes remain possible.

Climate normals plus a short-range forecast blend. The probability curve reflects the climatological baseline from 1991–2020: what’s likely in a typical year. The optional freeze outlook adds the live NWS forecast for your ZIP when a freeze is within about 7 days, the range where a genuine point forecast exists. Without a ZIP, or beyond that horizon, the chart stays on the climate normal, because no forecast can reliably see further out. An early warm spell does not move the estimate earlier on its own, since last frost dates do not reliably track spring warmth.

Microclimate variation. A zone composite represents the weighted average across hundreds of stations. Your specific ZIP may run several days warmer or cooler than the zone average due to elevation, water proximity, urban heat island, or local topography.

Station density. Zone 4A has dense NOAA station coverage. Some zones with sparser ZIP-level coverage (Alaska, parts of the desert Southwest) rely on fewer mapped ZIPs and should be interpreted with slightly more caution.

Data refresh cycle. NOAA releases new Climate Normals on a 10-year cycle. The current 1991–2020 dataset will be superseded by the 2001–2030 release expected in 2031.