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USDA Zones, Frost Dates & Calendar FoundationsLast Frost vs. First Frost: Building Your Calendar Anchor
Every planting calendar hangs on two dates, the last spring frost and the first fall frost. Neither is a promise, so the real skill is choosing how much risk you will carry.
Key points
- Choose a threshold that matches how tender your crop is.
- Choose a risk level you can afford, then plant to that date.
- Write the spring and fall anchors at the top of your calendar.
- Keep a record of actual frosts to refine the published numbers.
What do last frost and first frost mean on a calendar?
The last spring frost is the final day in spring when temperatures fall to a chosen threshold. The first fall frost is the first day in autumn that reaches that threshold. The stretch between them is your frost-free period, and most warm-season crops have to fit inside it.
The dates you see published are averages or probabilities from past records. They describe typical years. They do not forecast this year.
How do frost probabilities work?
Weather agencies publish dates at different probability levels. NC State Extension, explaining the NOAA tables, notes that a 90 percent spring date is earlier, with a frost after it still likely, while the 10 percent spring date is later, with a frost after it unlikely. In fall the order flips: the 10 percent date is earlier and the 90 percent date is later.
The tables also come with temperature thresholds, commonly 36, 32 and 28 degrees Fahrenheit, standing for frost, freeze and hard freeze. Tender crops such as tomatoes and beans suffer at the warmer thresholds, while hardy crops like kale and spinach tolerate colder ones.
Remember that the dates in any table are measured at a weather station, often at an airport or a town, so your own site may run warmer or colder.
How do you choose a risk level?
The Arizona example makes the tradeoff clear. In a Prescott table, a 32 degree freeze was about a coin flip in mid May and fell to roughly 10 percent by the start of June. A conservative grower plants after the low probability date. A moderate grower plants near the 50 percent date. An aggressive grower plants earlier and relies on row cover or low tunnels, or is ready to replant.
Choose by asking what a loss costs. Ten cheap tomato transplants can be replaced. A whole field of transplants cannot.
- Conservative: plant after the 10 percent spring date.
- Moderate: plant near the 50 percent date.
- Aggressive: plant early with cover and a replant plan.
How do you set the anchor dates?
Pick the nearest weather station, choose the threshold that matches your crop, and write down both a spring and a fall date at your chosen risk level. Put them at the top of your calendar. Every other date is counted from these two.
For example, if your tomato transplants need to go out after the chance of a 36 degree night is low, you use the 36 degree row of the table. If you are protecting kale, you may be fine using the 28 degree row.
Write a second pair of dates for your hardy crops using a colder threshold, so you can plant spinach, peas and onions earlier than your tender crops without confusing the two sets.
What mistakes make an anchor date unreliable?
Using a regional or state-wide average is the first one. Frost timing varies with elevation, slope and nearby water, so a number for a city 40 miles away can be off by a week or more. Mixing thresholds is another, since a 32 degree date and a 28 degree date can be weeks apart.
The last one is treating the date as a deadline for planting. A frost date is a risk marker. Soil temperature also has to be right, and a warm spell in early spring can tempt you to plant too early.
How do you adjust the anchor each year?
Check the forecast as the date nears and watch nighttime lows, not just daytime highs. Keep a notebook of the actual last frost you see each spring and the first in fall. After five or six seasons, you will have a local record that may differ from the published numbers.
Keep covers ready until a week or two past your chosen date. Frost cloth, buckets or old sheets are enough for small plantings.
How does the fall anchor guide your harvest?
The fall anchor sets your harvest deadline. Count backward from it using days to maturity, and add a margin for slower growth as days shorten and nights cool. Tender crops should be ripe or picked before the first frost at their threshold.
Keep row cover and bed sheets handy, because a single frost can arrive well before the season is truly over. A light freeze followed by a warm spell may give you several more weeks of harvest from covered crops.
- Count back from the fall anchor to set the last sowing date.
- Allow extra days for slower fall growth.
- Pick tender crops or cover them when a frost threshold is forecast.
Timing at a glance
| Item | Window | Caveat |
|---|---|---|
| Frederick, Maryland, last spring 32 degree freeze | Probability table in the source lists April 19 at 10 percent, April 9 at 50 percent and March 29 at 90 percent, from 1991 to 2020 normals. | Example for one Maryland station. Your station will differ; use your own frost probability table and the current forecast. |
| Prescott, Arizona, spring 32 degree freeze | The source describes a freeze as about a coin flip on May 12, with about a 10 percent chance left by June 1. | Based on 1922 to 2022 records for one station. Check a station near you. |
Sources and further reading
Last reviewed: November 2026. Educational information, ranges only. Confirm timing with your county extension office.