How to read a spring frost
Median dates, 1-in-10 late dates, radiational frost, and why a warm April week is not permission to plant tomatoes.
A frost date is a description of the past at one instrument. It is not a forecast, and it is not a property line. Used that way, it is the most useful number in a Western garden.
Used as “the last frost is May 1,” it is how tender plants get set out on a warm Saturday and die on a clear Wednesday. This page is the method. The station numbers live on the calendar pages, where each table names its source.
What the three spring dates mean
NOAA’s freeze-occurrence tables for Colorado, and the Montana tables hosted by Montana State University Extension, print three dates for each threshold. The columns are a 90 percent, a 50 percent, and a 10 percent probability. For a last spring freeze, read them in plain language.
The middle date is the median. Half of the years in the record were earlier, half were later. This is the date people mean when they say “average last frost,” and it is already a simplification.
The right-hand date is the late case. Only about one year in ten is later than this date. If you plant a tender crop on the median and you will not cover it, you are accepting that a substantial share of years still freeze after you planted. If you wait for the right-hand date, you give up some growing days and you avoid most of those years.
The left-hand date is the early case. Most years still freeze after it. It is not a planting date for tomatoes. It is context.
Denver’s Stapleton station, in the Colorado table, shows the pattern for a 32°F threshold: April 16, April 30, and May 13. The median is April 30. A last freeze after May 13 is the uncommon year.
A last freeze after April 16 is the usual year. Anyone who plants peppers on April 16 because “it has been warm” is planting before the median, not on it. The same table puts Castle Rock at May 3, May 22, and June 11.
A gardener who moves from central Denver to the Palmer Divide and keeps the April 30 habit is now planting before the early date of the new station. That is not a small error.
These Colorado and Montana PDFs are the detailed station comparisons widely published for gardeners, and the Colorado file sits in NCEI’s deprecated-normals archive. For a current 30-year normal, the Colorado Climate Center’s tool is the check. Do not average a deprecated median with a newer one and invent a third date. Pick a source, name it, and keep your own record.
A 32°F threshold is the usual “frost” line in these tables. A 28°F line is a harder freeze. A 36°F line is a light chill that still ruins some tender crops.
Know which row you are reading. Seedlings in a low spot can be killed when the airport, on a rise, reports 34°F.
Radiational nights and windy freezes
A radiational frost happens on a clear, calm night. The ground loses heat to the sky, and the coldest air collects in the lowest few feet and in the lowest parts of the yard. A thermometer on the house wall, six feet up, in the stored heat of the siding, will read several degrees warmer than a leaf at the bottom of the garden. Put the thermometer at plant height, in the open, if you are deciding whether to cover.
An advective freeze is a mass of cold air with wind. Covers help less because the wind strips the warm air out of a loose blanket, and the whole neighborhood gets cold, not just the hollow.
Pin covers down. A row cover that is tight to the ground can still save a bed. A draped sheet that flaps will not.
Cold-air drainage is why two thermometers on the same lot disagree. University of Idaho’s short-season bulletins and USU’s orchard guidance say the same thing: slopes shed cold air, hollows collect it. The median for the airport is not the median for your creek bottom. If your hollow freezes two weeks later than the station in spring, write that down and plant as if your station were the colder one.
Matching crops to the dates
CSU’s planting guide is the crop side of this method. Hardy crops, including peas, spinach, lettuce, and the brassicas on that list, can go in as soon as the soil can be worked, often two to four weeks before the median last spring frost. They survive a light frost.
Tender crops, including beans, corn, and summer squash, are timed around the median, when days are above about 55°F, and they do not survive a freeze. Very tender crops wait until the weather is summer-like, typically more than two weeks after the average last frost, because a week of cool days can stunt them even if the night never hits 32°F. The warm-season page names tomatoes and peppers as that later group in mountain practice.
A cover moves a plant across one of these lines for a night.
It does not erase the line. If you will be home to manage a cold frame or a blanket, you can plant a little closer to the median.
If you will be gone, wait for the late date. The plants you set out on the median without a cover are a bet. Make it on purpose.
Fall has a mirror image. The Colorado Climate Center’s writing on first fall freezes shows that the timing follows elevation, that high country freezes in September, and that nearly everywhere in the state can still freeze in September in an early year, with the Grand Valley as a notable exception.
Do not assume October because last year was October. Watch the forecast in September, and keep the cover handy. Where this site quotes a fall date, it quotes a publication. It does not subtract a spring median from a guessed season length.
A week of warm weather
Seventy degrees in April on the Front Range is ordinary. It does not mean the median has been repealed.
Soil is still cold. Transplants that go into 50°F clay sit and sulk, and a radiational night is still likely. Harden plants.
Warm the soil with a cover if you are early. Or wait. The seed does not care that you have a three-day weekend.
The same patience applies in reverse in a cold May. A late date that is “usually” May 13 can be May 20 in the year you are in. The table’s 1-in-10 column was trying to tell you that such years exist. Cover the tender plants or do not put them out. Hardy greens can go in either way.
Keep the record that replaces the table
Write down the date of the last 32°F morning at plant height in your own garden, and the first one in fall. Note whether it was calm or windy. After five years you will know your offset from the airport.
That notebook is worth more than a refined average from a station five miles away and two hundred feet higher. Until you have it, use the station, use the late date for anything you cannot bear to lose, and use a cover for the nights in between.
Checklist
- Read the median and the 1-in-10 late date, and know which threshold you are looking at.
- Name the station and the publication. Do not borrow a city an hour away without looking at the elevation and the landform.
- Put the thermometer at plant height.
- Plant hardy crops by soil condition, often before the median.
- Plant tender crops around the median only if you can cover them.
- Hold very tender crops for the late date or for summer-like days.
- Treat a warm April week as weather, not as a canceled frost date.
- Write down what your yard actually does.
The month-by-month work that uses this method is on the Front Range, mountain town, Utah, Wyoming and Montana, Idaho, and northern New Mexico calendars.
Sources
- NOAA NCEI, Freeze/Frost Occurrence Data for Colorado.
- Montana State University Extension, Frost Freeze Data for Montana.
- Colorado Climate Center, 1991–2020 freeze normals and the first fall freezes discussion.
- CSU Extension, Vegetable Planting Guide.
- University of Idaho Extension, BUL 936, on frost dates as estimates and on microclimates.
- USU Extension, on cold-air drainage and bench sites.