The four dates that matter
| Date | When | What it means |
|---|---|---|
| Last spring frost (50%) | — | Half of all years see a freeze after this date. It is an average, not a guarantee. |
| Last spring frost (10%) | — | Only one year in ten freezes after this. Use it for anything you cannot afford to replant. |
| First fall frost (50%) | — | Half of all years freeze before this date. Fall crops are counted back from here. |
| First fall frost (10%) | — | An early-freeze year. If a crop must finish, aim to harvest before this. |
Where these numbers come from
There is no long-record weather station inside Maryvale itself, so these frost dates are modelled, not observed. They come from a half-degree climate grid built by fitting the freeze dates of 6,808 NOAA stations (1991–2020 Normals) against terrain and the USDA 2023 hardiness raster, then reading the cell your city sits in. Checked against the stations themselves, that model lands within about a week on average — and its zone matches the station's exactly 67% of the time and within one half-zone 97% of the time. A cell covers roughly 35 by 28 miles, so this is a neighborhood answer, not a backyard one. If there is a station-backed location near you, prefer it.
How precise is any of this?
Extension recommendations for the same crop routinely disagree by two to four weeks, and occasionally more, so these offsets should be presented as windows and starting points rather than deadlines. Three things drive the spread. (1) Regional risk tolerance: for warm-season crops, Colorado State and Utah State put tomatoes, peppers and melons out about 14 days after the last frost, while University of Missouri's calendar works out to roughly 25-35 days after the last frost and University of Maryland's window runs from the frost date itself out to 45 days past it. That is a real 6-week spread for the single most popular garden crop, and it exists because a short-season mountain state is optimizing against a short season while a humid continental state is optimizing against cold, wet spring soil. (2) Soil temperature versus calendar date: most Extension services now say the honest trigger is soil temperature at 4 inches, not the frost date, and the frost-relative number is a convenience translation. A cold, wet spring can push the real date two or three weeks past the calendar recommendation. (3) Days-to-maturity is variety-specific and is measured differently for direct-seeded versus transplanted crops - seed catalogs count from transplant date for tomatoes, peppers, eggplant and cole crops, but from sowing for roots and greens - so a 'days to maturity' range of 60-85 for tomato is genuinely 60-85 depending on which packet you bought. Additionally, the average last frost is a 50% probability, meaning half of all years have a frost after that date; Extension services commonly advise waiting 1-2 weeks past it for anything frost-tender, or being prepared to cover. Treat every number here as plus or minus about 10-14 days.
On the station itself: a weather station is one point. Cities hold their heat, valleys pool cold air, and a garden on a north slope can run two weeks behind one a mile away. Treat this page as your starting point and let your own garden correct it — that is exactly what the journal in the app is for.
Nearby locations
Planting reminders for Maryvale
A short note at each new and full moon telling you what is in its window right now for Maryvale — worked out from the frost dates on this page. Free, one click to stop.
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Want this on your phone, in your garden? Moon Phase Garden keeps your frost dates, your planting windows and tonight's moon phase in one place, works offline, and lets you override these dates with what your own garden actually does. Free and ad-free.