Last and First Frost Dates in Missoula and the Bitterroot Valley

When is the last frost in Missoula?

At the Missoula International Airport station, the median date of the last spring freeze (a low of 32 degrees or colder) is May 25, based on the Western Regional Climate Center's freeze probability table for the station's period of record, which runs from 1948 to 2016. Median means that in half of the years on record the last freeze came later than May 25, and in half it came earlier.

The same table shows a 10 percent chance of a 32 degree freeze on or after June 15, and in 9 years out of 10 the last freeze came after May 6. The latest spring freeze in the record was July 7. So Memorial Day is a reasonable rule of thumb for Missoula, but it is a coin flip, not a guarantee, and tender plants set out in the second week of May are taking a real risk.

When is the last frost in Hamilton, Stevensville and Darby?

The Bitterroot stations tell different stories, which is why one date for the whole valley does not work. From the WRCC tables for each station's period of record:

Hamilton: median last spring freeze May 24, with a 10 percent chance of one on or after June 11. Stevensville: median May 30, with a 10 percent chance on or after June 21, and a latest recorded spring freeze of July 19. Darby: median May 29, with a 10 percent chance on or after June 28. Hamilton's record runs from 1895 to 2016, Stevensville's from 1911 to 2016, and Darby's from 1948 to 2005.

Stevensville is the surprise. It sits north of Hamilton, yet its station records later spring freezes and a shorter frost-free season. A station reflects the exact spot where the thermometer sits, and cold air settles into low ground on clear nights, which is the likely reason for differences like this one. Your yard can behave like either station depending on where it sits.

When is the first fall frost?

At Missoula airport the median first fall freeze is September 21, with a 10 percent chance of one by September 8. Hamilton's median is also September 21, with a 10 percent chance by September 7. Stevensville's median is September 10, with a 10 percent chance as early as August 27, and Darby's is September 12, with a 10 percent chance by September 1.

That is why tomatoes and squash in the Bitterroot are often finished by early September, and why fall planting of trees and shrubs is timed by soil temperature and rain rather than by the first frost. A light freeze in September rarely hurts established hardy plants; it ends the season for tender annuals.

How long is the growing season in Missoula and the Bitterroot?

The WRCC tables also give the length of the freeze-free season (the days between the last spring and first fall 32 degree freeze). The medians are 119 days at Missoula airport, 120 days at Hamilton, 104 days at Darby and 101 days at Stevensville. At Missoula airport, about one year in ten had a season shorter than 97 days, and the shortest on record there was 69 days.

Roughly 100 to 120 frost-free days is a short season by national standards, and it shapes almost every decision in a landscape, from which perennials flower before fall to when a new lawn can be seeded. Our guide to planning a landscape around a short growing season goes through those decisions one at a time.

What is the difference between a frost and a hard freeze?

The WRCC tables report several temperature thresholds, not just 32 degrees, and the gap between them matters. A 32 degree night is the one that matters for frost-tender plants such as the snap beans, cucumbers, peppers, summer squash and tomatoes that MSU Extension lists as having no frost tolerance. A colder night, around 28 degrees, is the one gardeners usually call a hard freeze, and it is the better marker for when the hardy part of the garden and the landscape shuts down.

At Missoula airport, the median last spring 28 degree freeze is May 5 and the median first fall 28 degree freeze is October 3. At Hamilton the medians are May 3 and October 4. Stevensville's are May 16 and September 22, and Darby's are May 11 and September 25. In practice, the window for frost-hardy crops opens close to three weeks before the window for tender ones at most of these stations, and closes about two weeks after the first light freeze.

Can it frost in June in western Montana?

Yes. A June freeze is not common on the valley floor, but every station we looked at has one in the record. The tables also track a 36 degree threshold, a reading that is not a freeze at the thermometer but is close enough that plants near the ground on a clear, calm night can run colder. The median last spring 36 degree night is June 26 at Stevensville and June 19 at Missoula airport, so cool nights near freezing are normal well into June.

The practical response is to keep row cover or old sheets handy until the end of June, to set out tender plants after the median date for your area and not before, and to check the National Weather Service forecast from the Missoula office before planting anything tender. Covering plants overnight and uncovering them in the morning is the simplest protection a home landscape has.

How should I use frost dates to plan planting?

Montana State University Extension's guidance on cold-hardy vegetables starts from the same two numbers: the average date of the last frost in spring and the first frost in fall, with the days in between as your window. MSU's tables then sort crops by frost tolerance. Peas are listed as very hardy; beets, broccoli, carrots, kale, lettuce and onions tolerate frost; and snap beans, cucumbers, peppers, summer squash and tomatoes do not. MSU gives snap beans, for example, a planting window from one week before the last frost.

For a landscape rather than a vegetable garden, the frost dates mostly set the timing of annual color and container plantings, the start of irrigation, and the last safe window for seeding. Hardy trees, shrubs and perennials are not frost-tender in the way annuals are, so they are planted on soil conditions and nursery availability rather than on the median frost date. Our spring lawn and landscape startup guide covers the order of spring work, and the Montana landscape maintenance calendar lays out the year.

What about Sula, Seeley Lake and the side valleys?

Move away from the main valley floor and the season shrinks fast. The WRCC table for Potomac, east of Missoula, shows a median last 32 degree freeze of June 30, a median first fall freeze of August 13, and a median freeze-free season of just 42 days. Seeley Lake Ranger Station shows June 29, August 28 and 64 days. Sula, at the upper end of the Bitterroot, shows July 17, August 9 and 28 days. These three records end in 2005.

A freeze in any month of summer is possible at stations like these, and a home in a similar mountain valley or a cold canyon bottom should plan accordingly. No station sits at Clinton, Lolo, Florence, Victor or Corvallis in these tables, so if you live there, use the nearest station as a guide and watch whether your first and last frosts run earlier or later than its dates.

Why do frost date websites disagree?

Mostly because they use different stations, different periods of record and different probabilities. Some sites publish the date with a 50 percent chance of a later frost; others use 30 or 10 percent; some use a nearby airport station for a whole region. The station tables we used here are period of record summaries, so they include old years as well as recent ones, and they will not match a site built only on the 1991 to 2020 normals.

The fix is to pick the station that best matches your yard, decide how much risk you are willing to take with a given plant, and read the matching probability. For a flat of annuals, the median date is fine. For an expensive specimen or a whole vegetable garden, plan around the 10 percent date. And keep your own notes: a few seasons of first and last frosts in your own yard beat any website.

Frost dates and your irrigation system

Frost dates also set the irrigation calendar. We start sprinkler systems in spring once hard freezes are unlikely, and blow them out in fall before a hard freeze, because water left standing in a backflow preventer or shallow pipe can freeze and crack it. Our guides to spring sprinkler startup and winterizing an irrigation system cover the timing, and our irrigation team handles startups, repairs and fall blowouts across Missoula and Ravalli counties.

This guide is part of our Montana Landscaping, Lawn & Irrigation Guide. If you are planning new plantings and want them timed and placed for your yard's real season, call HJ Property Care & Tree Service at (406) 215-7319 or ask for a free landscaping estimate. Our landscape design work starts with a walk of the property and a plan that fits the site.

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