Frost date probability, honestly.

The average isn't the answer.

A USDA hardiness zone tells you the average annual minimum winter temperature. That's useful for deciding whether a perennial plant survives — and useless for deciding when to set out a tomato transplant. The decision space every spring planter is navigating is the gap between the earliest possible last-frost date and the latest possible last-frost date. That gap can be 4–6 weeks.

Each county page on this site shows you that gap explicitly. Five risk tiers — very conservative, conservative, median, aggressive, very aggressive — at the killing-freeze (28°F), light-freeze (32°F), and hard-freeze (24°F) thresholds. Pick the row that matches your appetite for risk and the value of the crop you're protecting.

• Very conservative
  Frost has occurred this late in only 1 of 10 historical years. The safe-side bound for high-value transplants with no protection.
• Median
  50/50 — half the years were past their last frost by this date, half weren't. Useful for direct-seed crops that tolerate light frost.
• Very aggressive
  Only the luckiest 1 in 10 years was frost-free this early. Useful only with row cover, low tunnels, or other season-extension tools.

Where this data comes from.

• NOAA U.S. Climate Normals 1991–2020, Annual Frost/Freeze Probability product — the PRBLST and PRBFST variables computed by NOAA from ~9,200 long-record COOP and ASOS stations.
• US Census 2024 Gazetteer — county and ZCTA centroids for matching stations to geography.
• Nearest-station mapping — each county is paired with its closest station meeting the 30-year completeness threshold. Confidence is graded by station distance: high (≤25 mi), moderate (≤50 mi), low (>50 mi).

Microclimate variation within a county can be 2–4 weeks. Read the station data as a regional anchor, then adjust for what you know about your own ground — elevation, cold-air drainage, water proximity, and urban heat.