What Are Frost Dates?
A frost date marks when freezing temperatures (32°F / 0°C) typically begin or end in your area. The last spring frost is the date after which a hard freeze becomes unlikely — the green light to set out tender plants. The first fall frost is the date in autumn when freezing returns and the growing season effectively ends for frost-sensitive crops.
These two dates bookend your growing season, and almost every planting decision keys off them. Sow too early and a late freeze can wipe out seedlings; sow too late and a crop may not mature before the first fall frost. Because frost timing varies enormously from one location to the next, a date that works in one town can be weeks off in another — which is why these numbers are pulled from the weather station nearest your ZIP rather than a national average.
Understanding the Probability Bands
Frost dates aren't a single guaranteed day — they're probabilities. Gardenable reports three bands from NOAA's Climate Normals. The typical (p50, or median) date is the 50/50 point: in half of years frost falls before it and half after. The conservative (p90) date is the safe bet — frost is gone by then in 90% of years. The aggressive (p10) date is a gamble — only 10% of years stay frosty that late.
For most planning, use the typical date. Use the conservative date for tender crops that a single freeze will kill — tomatoes, peppers, basil, and squash — where losing the planting costs you weeks. Reach for the aggressive date only if you're willing to gamble for an earlier harvest and have row cover or cloches ready to protect plants if a late frost threatens.
USDA Hardiness Zones Explained
Your USDA Hardiness Zone measures something different from frost dates: the average annual extreme minimum temperature — how cold the coldest night of a typical winter gets. The map runs from Zone 3 (coldest) to Zone 13 (warmest), in half-zone steps like "8b." It's most useful for choosing perennials, shrubs, and fruit that must survive winter, and less relevant to annual vegetables, which are governed by frost timing and season length instead. Read the two together: the zone tells you what survives the winter, the frost dates tell you when to plant what doesn't.
How to Use Your Frost Dates
Once you know your dates, the timing math is straightforward. To find indoor seed-starting dates, count backward from your last spring frost — most warm-season crops are started four to eight weeks ahead. The seed starting calculator does this counting for you per crop.
For warm-season direct sowing, count forward from the last spring frost: beans, corn, and squash go in once the soil has warmed and frost danger has passed. To find the last viable sow date for any crop, count backward from your first fall frost by the crop's days to maturity plus a short safety buffer — sow after that and the crop likely won't finish. The succession planting scheduler applies all of this automatically to build a full season of staggered sowings.
Frequently Asked Questions
What is a frost date?
A frost date is the average date of the last freezing temperature in spring (last frost) or the first freezing temperature in fall (first frost). Gardeners use these dates to time planting so crops aren't killed by unexpected cold.
What is the USDA Plant Hardiness Zone?
The USDA Plant Hardiness Zone Map divides the US into zones based on average annual extreme minimum temperatures. Zone 3 is coldest; Zone 13 is warmest. It's primarily used for perennial plants, not annual vegetables.
What is the difference between p10, p50, and p90 frost dates?
These are probability thresholds from NOAA's Climate Normals. The p50 date means frost occurs after this date in 50% of years — the median. The p90 date is safer: frost is gone by this date in 90% of years. The p10 date is aggressive: only 10% of years see frost this late.
Where does Gardenable's frost date data come from?
Frost dates come from NOAA's 1991–2020 Climate Normals, the most recent 30-year climatological baseline. Hardiness zones come from the USDA 2023 Plant Hardiness Zone Map.