Question 1

How is this different from the average last frost date?

Accepted Answer

The 'average last frost date' is just the 50th percentile: it means half the years will freeze later. This tool shows the full distribution. For tender crops, you really want to plant on the 10% date (90% of the time it won't freeze after that), not the average. Garden guides that only publish the average are technically correct but practically misleading.

Question 2

Where does the 30 years of data come from?

Accepted Answer

NOAA's Regional Climate Centers, through their ACIS data service. We read the PRISM daily temperature grid, which covers the lower 48 states at about 4 km resolution from 1981 onward and is built from thousands of weather stations. The 32°F threshold is the standard light-freeze definition used by NOAA NCEI and cooperative extension services.

Question 3

Does it cover the first fall frost?

Accepted Answer

Yes. Switch to Fall and the tool finds, for each of the last 30 years, the first night at or below 32°F from July 1 onward, then gives the chance that the first frost has come by the date you pick. Years with no fall frost at all count as no frost, so in warm ZIPs the chance stays low. The 10% date is the early end: from then on, cover tender crops on cold nights.

Question 4

Does this account for microclimate?

Accepted Answer

Only indirectly. PRISM is a 4 km gridded product, so it captures regional climate but not your specific south-facing brick wall or frost pocket. A south-facing slope with full sun and good drainage typically beats the ZIP average by 1-2 weeks on the last frost side. Urban heat islands can add 1-3 weeks of extension. Use the tool's number as a floor, then adjust for your site conditions.

Question 5

My zone didn't change after 2023. Is the historical data still relevant?

Accepted Answer

The 2023 USDA zone map update pushed about half the country a half-zone warmer because the climate normals shifted. This tool always uses the most recent 30 years, so the data automatically reflects the warming trend without you having to adjust. That's actually an advantage over published NCEI 1991-2020 tables, which are already three years stale.

Question 6

The tool said 90% safe is May 15, but I've seen frost on May 20. Is it wrong?

Accepted Answer

Not wrong, just stating probability. 90% safe means roughly 3 out of every 30 years WILL frost after that date. If you were gardening during one of those three years, you saw the tail event. That's why tender crops go in by the 90% safe date (the 10% row of the spring table), not the median, and why frost cloth is worth keeping on hand for a week or two after it. The calculator is honest about how often the tail hits.

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