Your Last Frost Date Is a 50/50 Coin Flip

Every February the same question comes round: "what is my last frost date?" It sounds like a question with an answer, the way "what is my elevation?" has an answer. It is not. A last frost date is a statistical estimate with a probability attached to it, and if you do not know which probability your source is quoting, you do not actually know what you have been told.

Nearly every seed-starting article on the internet is built on a single instruction: look up your last frost date, count backward, sow. The counting is fine. The lookup is where the whole structure gets shaky.

What a Last Frost Date Actually Is

Behind every frost-date lookup is a pile of daily minimum temperature records from a weather station, usually thirty years of them. Someone sorts through those records, finds the date of the last spring day at or below some threshold temperature in each year, and then asks a statistical question of that distribution.

The question is not "what is the last frost date." The question is "by what date has the probability of a later frost dropped to X percent?"

You can ask that question at different values of X. The standard climatological products publish at least three: the 10 percent level, the 50 percent level, and the 90 percent level. You can also ask it at different temperature thresholds, since 32°F, 28°F, and 24°F are three different events with three different dates.

So for one weather station there is not one last frost date. There are nine, and they can span five or six weeks.

Here is the part that trips people up, and it is worth stating very plainly:

A 50 percent last frost date means there is a 50 percent chance of a frost after that date.

The median date — the one most tools publish, the one called "average last frost" — is beaten by a later frost in roughly half of all years. Set out tomatoes on your average last frost date every spring and you should expect to lose them about one year in two. That is not bad luck. That is the definition of the number.

The Garden Professors have a clear walkthrough of the statistics behind last frost and planting dates that makes the same point with the underlying math, and UC Master Gardeners cover it from the practical end in ask a Master Gardener: last frost date.

Sources Disagree About Which Threshold to Show

If every lookup tool published the 50 percent date and labeled it as such, this would be a minor footnote. They do not.

Some publish the 50 percent date. Some publish a more conservative 10 percent date, on the view that gardeners are better served by a date they can trust. Some interpolate between stations to fill in a ZIP code with no station of its own. Some still run on an older period of record rather than current normals. Some never say which temperature threshold they used.

The practical result is that two reputable-looking tools will hand you dates two or three weeks apart, and neither is wrong. They are answering different questions without telling you which.

Add to that the state of frost-date search results generally. This is a high-traffic seasonal query, and it has attracted a large volume of thin lookup pages that exist to serve ads — pages that scraped a dataset once, never documented it, and never updated it. Some of them are wrong by a month. Treat any single date from any tool with suspicion until you know its threshold and its period of record.

How to Find Out Which Threshold You Have

Look for the documentation, not the number. A source that knows what it is doing will say so somewhere on the page or in a footnote. You are looking for three things:

1. The probability level. Words like "50 percent probability," "median," "average," or "exceeded in half of years" all point at the median. "10 percent probability" or "90 percent chance of being frost-free after" points at the conservative date.
2. The temperature threshold. 32°F, 28°F, and 24°F are all in common use. A 32°F date is the one that matters for basil and cucurbits. A 28°F date is a hard freeze and is the relevant number for hardened brassicas.
3. The period of record. Current thirty-year normals, or something older.

University of Maine Extension's frost and freeze dates publication is a good example of the format you want: probability levels and temperature thresholds laid out in a table, with the station and period of record stated. Your own state extension almost certainly publishes an equivalent table. That table, not a ZIP code widget, is the thing to bookmark.

The Conservative Date and the Gambler's Date

Once you have the full set of probabilities, the decision becomes clear.

The 10 percent date is the conservative planting date. By that date, only one year in ten sees a later frost. This is the date for crops you cannot replace — the peppers you started in February, the one flat of heirloom tomatoes, anything where losing the planting means losing the season.

The 50 percent date is a coin flip. It is useful as a center of gravity for planning, and it is a reasonable date for crops you can resow cheaply or for a first succession you are willing to gamble.

The 90 percent date is the gambler's date. Nine years in ten will bring frost after it. Nobody should plant frost-tender crops on that date without cover standing by.

There is a fourth factor the tables cannot give you: your own site. Cold air is dense and drains downhill, so a garden in a low spot can run days to a couple of weeks behind a nearby hilltop served by the same weather station. Keep a min-max thermometer in the garden, and after two or three seasons you will know your offset from the published number better than any dataset does.

How This Interacts With Counting Back to Sow

Indoor sowing dates are usually set by counting backward from the last frost date — eight weeks for peppers, six for tomatoes, and so on. The uncertainty in the frost date propagates straight into that count.

If you count back from a 50 percent date and the spring runs cold, your transplants sit in their pots for an extra three weeks getting rootbound while you wait for weather.

Two things help. Count back from a date you have chosen deliberately at a stated probability, not from whatever a widget produced. And build slack into the transplant itself: pot up rather than let plants stall, and give hardening off its full week or more so a delayed set-out does less damage. There is more on that stage in hardening off seedlings.

Air Temperature Is Not Soil Temperature

This is the failure that catches experienced gardeners, because it survives a correct frost-date reading.

Frost dates are computed from air temperature at a weather station. They say nothing about whether the soil in your beds is warm enough for the crop to do anything once it is in the ground. Soil warms slowly, and it lags air temperature by weeks — more in heavy clay, more under mulch, more on a north slope.

Peppers and eggplant set out into 55°F soil will sit and sulk for three weeks and never fully recover. Beans and cucurbits sown into cold wet soil rot instead of germinating. You can clear your frost date by a comfortable margin and still lose the planting, and the frost date will have told you nothing about it. Use a soil thermometer and the crop-by-crop minimums in soil temperature and planting as the second gate that every warm-season planting has to pass.

How to Hedge

You cannot remove the uncertainty. You can make it cost less.

• Plant in successions rather than all at once. Split tomatoes into two plantings ten days apart. A late frost takes half the crop instead of all of it, and in a warm year the early planting gives you fruit sooner.
• Keep row cover on standby, not in storage. Lightweight floating row cover buys a few degrees; a heavier cover over hoops buys more. The point is that it is folded at the end of the bed in April, not in the shed under the hoses. A frost warning at 9 p.m. is not the time to go looking for it.
• Water the soil before a frost night. Moist soil holds and releases more heat than dry soil, often a degree or two at plant height.
• Watch the forecast, not the calendar, for the final call. The date sets the window. The ten-day forecast picks the day inside it.

The broader question of how to stretch the season at both ends, including covers, tunnels, and fall protection, is covered in seed starting and season extension.

The Short Version

Find your state extension's frost table. Read the 10 percent date, the 50 percent date, and the threshold temperature they were computed at. Plant the irreplaceable things on the conservative date, gamble deliberately on the median, check soil temperature before any warm-season crop goes in, and keep the row cover where you can reach it in the dark. That is the whole method, and it beats a single date from a page that will not say where the number came from.