USDA Hardiness Zones for Missoula and the Bitterroot Valley

What hardiness zone is Missoula in?

Missoula is in zone 5b on the 2023 USDA Plant Hardiness Zone Map. That holds for every Missoula ZIP code we checked (59801, 59802, 59803, 59804 and 59808) in the ZIP code listing that the PRISM Climate Group at Oregon State University publishes as part of the official map data. Zone 5b means the average coldest night of the year falls between minus 15 and minus 10 degrees Fahrenheit.

The same listing puts Lolo (59847), Clinton (59825), Florence (59833), Stevensville (59870), Corvallis (59828), Victor (59875), Hamilton (59840) and Darby (59829) in zone 5b as well. In other words, the whole Missoula and Bitterroot valley floor shares one zone on the current map. That is a change from the 2012 map, and it is the reason older books, plant tags and websites disagree.

What does a hardiness zone actually measure?

A zone measures one thing: the average annual extreme minimum winter temperature, meaning the coldest temperature of each winter, averaged over a run of years. The USDA describes the map as the standard gardeners and growers use to judge which perennial plants are most likely to thrive at a location. Zones are 10 degree Fahrenheit bands, split into a and b half zones of 5 degrees each.

For the 2023 edition, the USDA says the map was built from 30 year averages covering 1991 to 2020, using data from 13,625 weather stations, and drawn on a grid of cells about half a mile on a side. The 2012 map used 1976 to 2005. So a zone number is a statement about how cold the worst night of a typical winter gets, not about summer heat, rainfall, snow cover or the length of the season.

How did the zones change from the 2012 map?

Nationally, the USDA says the 2023 map is generally about a quarter zone warmer than the 2012 map across much of the country, and that some places shifted cooler instead. In our area, comparing the 2012 and 2023 ZIP listings from PRISM shows a mixed picture. Missoula, Florence, Lolo and Hamilton were 5b in 2012 and are 5b now. Stevensville and Darby moved from 5a to 5b. Clinton moved the furthest, from 4b on the 2012 map to 5b on the 2023 map.

Two cautions from the USDA itself are worth repeating. First, the agency states that changes in zones are not reliable evidence of whether there has been global warming, because the map tracks only the extreme minimum and the newer map also uses more stations and better mapping methods. Second, a move from 5a to 5b does not mean a plant rated only to zone 6 is now safe. It means the coldest night in an average recent winter has been a few degrees milder than the older record showed.

Which zone should I use when I buy plants?

Use 5b as the starting point for the valley floor, then shop one zone colder whenever the plant is expensive, slow growing, or going into an exposed spot. In practice, that means preferring trees and shrubs rated to zone 4 or colder for the backbone of a landscape, and treating zone 5 plants as fine for most sheltered beds. Zone 6 plants are a gamble here, and a single hard winter can undo years of growth.

The reason for the margin is that a zone is an average. About half of all winters are colder than the average extreme, and the plants in your yard have to survive the cold winters too, not just the typical ones. Our guide to the best trees to plant in western Montana and our guides to shrubs for western Montana and hardy perennials lean on cold-proven species for that reason.

Why can my yard be colder than the zone map says?

Because a ZIP code gets one zone and a ZIP code in this part of Montana can cover a valley bottom, a bench and a mountain slope. The PRISM listing assigns one value per ZIP code, while the USDA notes that cities tend to hold more heat and higher elevations tend to be colder. Cold air also drains downhill at night and pools in low ground, so a creek bottom or the lowest corner of a lot can run colder than a house a few hundred feet up the slope.

Wind and sun matter too, even though the map ignores them. A south or west wall soaks up winter sun and can push buds out early, and an open lot facing the wind dries evergreens out on cold, bright days. Our guides to windy, exposed yards and winter burn on evergreens cover those effects. When you walk your yard, note the frost pockets, the windward side and the warm walls, and treat each as its own small zone.

How does the Bitterroot compare with the rest of Montana?

It is on the mild end of the state. Of the 399 Montana ZIP codes in the 2023 PRISM listing, only 63 are zone 5b or warmer, and the rest run from 3b to 5a. Going by ZIP code, Bozeman (59715), Butte (59701) and Billings (59101) are all one half zone colder than Missoula at 5a, while Kalispell (59901) matches it at 5b. The mildest part of western Montana lies downriver: Superior (59872) and Thompson Falls (59873) are 6a.

The valley's edges are colder too. Seeley Lake (59868), Philipsburg (59858) and Sula (59871), at the top of the Bitterroot, are all 5a. So a plant recommendation written for Missoula may be a stretch at the far end of a drive up the valley, and advice written for Billings or Bozeman will be a little conservative for Hamilton.

Is a hardiness zone the same as a frost date?

No, and mixing them up is an easy mistake. The zone tells you whether a perennial is likely to survive the winter. It says nothing about when the last spring frost happens or how many frost-free days you get, which is what decides when to plant annuals and vegetables and whether a late blooming shrub ever flowers. Two places in the same zone can have very different seasons.

The Bitterroot shows this clearly: the whole valley floor is one zone, yet weather station records show the median last spring frost six days later in Stevensville than in Hamilton. Sula is the extreme case: at 5a it is only a half zone colder than the valley floor and shares a zone with Billings, but its station record has a median frost-free season of just 28 days. Our companion guide to last and first frost dates in Missoula and the Bitterroot has the station numbers, and planning around a short growing season explains how to use both pieces of information together.

Does a warmer zone mean I can grow more things now?

A little, at the margin. Plants that were borderline at 5a, and that need a reliably mild winter to come through, have slightly better odds on the valley floor than the 2012 map suggested. But hardiness is only one test. A plant also has to tolerate the soil, which on many Bitterroot benches is calcareous and alkaline (see our guide to alkaline soil), the dry summers, the wind and the deer. Zone number alone has never been a good reason to plant something.

A good rule is to let the zone rule plants out, not in. If a plant is rated colder than your zone, it passes the first test, and you move on to soil, water, sun and size. If it is rated warmer, think hard about whether you have a genuinely sheltered spot for it, and whether you would mind replacing it.

How do I look up the zone for my own address?

The USDA runs an interactive map at the Plant Hardiness Zone Map website, where you can enter a ZIP code or zoom to your property. Because the underlying grid is about half a mile across, zooming in will sometimes show a different half zone on a hillside or in a canyon than the ZIP code lookup gives for the town. The PRISM Climate Group at Oregon State University also publishes the full data set, including the ZIP code table we used here, for anyone who wants the raw numbers.

If you live up a side drainage such as Rattlesnake, Grant Creek or one of the canyons on the west side of the Bitterroot, or on a high bench, zoom in on the interactive map rather than trusting the ZIP code answer. Then compare it with what you see in your own yard over a few winters.

How we use the zone when we design a landscape

When we lay out a planting plan, the zone is the first filter, not the last. We start from cold-proven trees and shrubs for the structure of the yard, put the borderline plants in the warmest protected pockets, and match everything to the soil and the irrigation zones. That approach is part of our landscape design and installation work across Missoula and Ravalli counties, and it fits with the rest of the Montana Landscaping, Lawn & Irrigation Guide.

If you want a planting plan built around your actual site, call HJ Property Care & Tree Service at (406) 215-7319 or request a free estimate. We will walk the yard with you, point out the cold spots and warm walls, and recommend plants that will still be there in ten years.

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