How we work out frost dates, zones and planting times
Everything on hortyard starts from two public data sets and one rule: use the weather station nearest the reader, and say which one it is. This page explains each step and where the numbers can mislead.
1. Frost dates
NOAA's National Centers for Environmental Information publish the U.S. Climate Normals for 1991 to 2020. For thousands of weather stations they include freeze probabilities: for each threshold (32 °F, 28 °F and others), the calendar dates by which the last spring freeze has happened in 10%, 50% and 90% of years, and the same for the first fall freeze. We use the 32 °F values.
- Average last frost: the 50% date. In Columbus, OH it is Apr 18: in half of the 30 springs, the last 32 °F night came later.
- Late year: the date the last frost had passed in 90% of years (May 5 in Columbus, OH). Only one spring in ten had a frost after it.
- Early year: the date by which the last frost had passed in only 10% of years (Apr 4).
- Frost-free season: NOAA's count of days between the average last and first frost (191 days in Columbus, OH).
Some stations in the warmest areas see a 32 °F night in only a few years. When a frost happens in fewer than half of years, an average date means little, so we show how often frost happens there instead of a date.
2. Matching your town to a station
Town locations come from the U.S. Census Bureau's 2024 Gazetteer: one point for each incorporated place and census-designated place, and one for each postal code area (ZCTA). For each point we take the nearest station with freeze data, up to 40 km (about 25 miles) away, with a small preference for airport stations, which have the longest and most complete records. The station's name and distance are shown with every result.
If two or more stations within that distance sit more than 300 meters (about 1,000 feet) apart in height and their average last frost differs by more than two weeks, we add a warning. Cold air drains into valleys, and a hillside garden can have a very different spring from the airport down the road.
3. Growing zones
The familiar zone map divides the country by the average of each winter's lowest temperature over 30 years: 10 °F per zone, 5 °F per half zone (a is the colder half). The official USDA map for 1991 to 2020 is drawn from a gridded climate model whose files carry their own terms of use, so we do not use it. Instead we apply the same definition to NOAA's Global Summary of the Year, which records each station's lowest temperature of every year. We average the 1991 to 2020 values at the nearest station with at least 20 of those 30 years, up to 25 miles away. In tests on big cities the result matched the official zone or was half a zone away. It is our own calculation, and every zone on the site says so.
4. Planting, harvest and pruning times
University extension services publish planting guides that time each crop in weeks before or after the average last frost, or before the first fall frost. We built one table of about 70 crops from those guides. Every figure (weeks, depth, spacing, days to harvest, soil temperature) is checked against at least two guides, and where they disagree we show the range that covers them. Each crop page lists the guides it used.
The dates you see are those weeks counted from your station's frost dates. For example, a crop started indoors 6 to 8 weeks before a last frost of Apr 18 is started between 6 and 8 weeks earlier. Harvest dates add the days to harvest to the planting date and stop at the first fall frost for crops that frost kills.
Limits
- Frost dates are probabilities from past records, not a forecast. Always watch the forecast in the weeks around your dates.
- Normals describe 1991 to 2020. Recent springs in many places have run a little earlier.
- A station can sit in a different setting from your garden: a city center, an airport, a hilltop.
- Extension guides are written for a state or region; your county extension office knows local conditions best.
If a date looks wrong for your area, email [email protected] with your town and what you see locally.