How we rate cold hardiness, and how to read our zone ratings

We rate every plant we sell by the coldest temperature it has actually come through, not by an average. A bare zone number like 3 means hardy through the whole zone, down to -40F, and a 3b means the warmer half only, down to -35F. The numbers come from real winters at our two sites in Missoula and Florence, Montana, and from growers we trust between Fairbanks, Alaska and Homedale, Idaho.

What a USDA zone measures

A USDA zone is the average of the coldest night of the year over 30 years, drawn at a scale far bigger than your yard. A tree doesn't die to an average, though. It dies to the one bad night in thirty.

Across the country the gap between that average and the one-in-thirty-year low runs 15 to 34F, always in the wrong direction. Your USDA zone makes winters look easier than they are. Missoula is technically USDA zone 5, and we still see temperatures close to -30F.

Bob Purvis told Minnesota homeowners the same thing: trees rated only to zone 5 may get through mild winters, then get badly hurt in a really cold one.

We zone each plant to the coldest winter it has survived

Every plant on our site is zoned to the minimum extreme temperature it has actually survived. Not an average, and not a number borrowed off somebody's plant tag. Some examples:

• Mantet came through two separate -47F winters at Ed Schultz's farm in Gallatin Gateway, Montana with no winter injury. We list it as a 2.
• Pink Pearl is commonly listed as zone 4. At the late Rod McIver's, in a cold spot near the Flathead River in Kalispell, it took about -35F on January 13, 2024 without injury. We list it as 3b.

A rating isn't the plant's floor. It's as far as anybody has tested it. Trader mulberry has lived long term through -40F, but nobody knows yet what it does at -50F. If a variety's known extreme is a few degrees warmer than your winters, it just hasn't been proven there yet.

How to read our zone ratings

A bare number means hardy through the whole zone, including its colder half. A "b" means the warmer half only. We never print an "a". Each zone spans 10F, and each half 5F.

A range like zones 3 to 8 means hardy from zone 3 up through zone 8. The warm end is where the plant still gets enough winter chill to bloom and fruit well. A plant rated 3b to 8 isn't rated for the colder half of zone 3.

Don't shop our numbers against another catalog's. Most ratings out there are built on averages, and mixing the two scales gets you trees that aren't hardy enough.

Where our ratings come from

Mostly from real winters at our two growing sites, in Missoula and in Florence in the Bitterroot Valley, and from fruit-growing friends we trust across the North: the late Rod McIver (Kalispell, MT), Bob Purvis (Homedale, ID), Les McCartney (Bozeman, MT), Ed Schultz (Gallatin Gateway, MT), Steve Masterman (Fairbanks, AK), Roger Joy (Corvallis, MT), Tom McCamant (Paradise, MT), Bernie Nikolai (Edmonton, AB), Old Blue Moon Orchard (Stevensville, MT), Fred Stewart (Missoula, MT) and James Sapp (Missoula, MT), plus our customers.

Where we have no local record, we lean on published nursery and university numbers. Purvis and Steve Masterman of Alaska Fruit Trees rate hardiness the same way we do. Plenty of folks don't, which is usually why a variety shows up warmer somewhere else than on our zone hardiness guide, a living document that new winters keep changing.

Why your ground can sit a half zone or a full zone off the map

Your ground can easily sit a half zone, sometimes a full zone, off the USDA map in either direction. What moves it:

• Cold air drainage. Cold air flows downhill and pools, so low spots frost first.
• Slope and aspect. South and east slopes shed cold air and warm early. North slopes stay colder and bloom later.
• Elevation. Roughly 3 to 5F colder per 1,000 feet.
• Big water. A lake or big river moderates both extremes.
• Snow cover. Reliable snow insulates roots and low graft unions.
• Wind. Open sites dry out wood and lose their snow.
• Buildings and pavement. South walls and town heat can buy you a half zone.
• Windbreaks. Shelterbelts and overstory trees soften the worst nights.
• Soil and drainage. Wet feet going into winter kills more trees than cold does.
• Rootstock. Roots never get as hardy as the top, and some rootstocks are far less hardy than the variety on them. Cornell found the Geneva stocks G.935 and G.890 lose hardiness early in spring.
• Age. A new whip is more tender than an established tree.

Cold isn't the only thing that kills trees in winter

Our number only holds when winter plays fair. Bob Purvis sorted winter injury into three situations:

1. Early winter. After a dry summer and a wet fall, wood hardens off slowly and the base of the trunk can be badly hurt. Trees harden from the twig tips down, so the trunk base is last. It's why we taper water from mid-August in zone 4 and colder.
2. Midwinter. Extreme cold, or a mild spell that dehardens trees followed by a sudden drop below zero. Purvis put that at a little injury around 0F and fatal injury by -12F.
3. Late winter. Southwest injury, when sun heats the trunk by day and the bark and cambium freeze at night. It's worst on young apple and pear trees.

We've seen the early kind. In late October 2020, Missoula went from mild fall weather to -10 to -20F almost overnight and wiped out trees rated plenty hardy on paper, because they hadn't gone dormant yet. Varieties with a long chill requirement tend to sleep through midwinter warm spells better.

Flower buds, young trees and the three-winter rule

A few more of Purvis's lessons change how you read any rating:

• On stone fruit, leaf buds are usually several degrees hardier than flower buds, so the crop can be gone when the tree is fine.
• In extreme cold, very young and very old trees are the most vulnerable.
• Trees "learn" to handle harsh climates, as he saw in Alaska and Minnesota, especially northern varieties propagated up north.
• Early-ripening varieties usually survive cold places better than late ones, with more time after harvest to harden off and store starch.
• If a fruit tree survives three winters, the odds are very good it'll keep going.
• A well-nourished tree in a sunny spot with well-drained soil can sometimes take a half to a full zone colder than it's rated.

Ontario's agriculture ministry and University of Maine Extension add that overcropping, drought and leaf diseases like scab all lower hardiness. So pick varieties rated a zone or two colder than you need. They come through bad winters more often, even when they take some damage.

Spring frost is a separate problem

Our ratings are about midwinter. Frost at bloom is its own fight, and spring 2026 brought a severe one across Montana. A grower friend in Gallatin Gateway saw 15F at bloom and got no fruit at all; even his chokecherry blossoms froze. Two growers we know in Missoula were hit hard, one of them low in the valley. Growers higher on the valley floor and up on the benches fared much better, and a Bitterroot orchard lost about 30% of its blossoms.

That 30% isn't a disaster. Our rule of thumb: unless the fruit comes in clusters, like cherries and grapes, most fruit can lose half its blossoms or more with little effect on the crop. The frost just thins it for you.

How to use our cold hardiness calculator

Our cold hardiness calculator gives you the coldest night you ought to plan for, then shows everything we grow that has come through it.

1. Enter where the trees are going. A street address works best.
2. Pick your ground: a low spot where cold air settles, typical open ground, or a slope or bench above the valley floor. If you're not sure, pick typical.
3. Go by the temperature, not the zone label. Our zone is built on extremes, the USDA zone beside it on averages.
4. Browse the list: plants that have come through your number first, then ones within 5F of it as worth a try.

It works from the coldest night of every winter back to 1995-96 at nearby long-record weather stations, fits a one-in-thirty-year low, then reads the lay of your land off a 100-foot elevation model. Tested against about 1,650 held-back stations, it lands within 3.5F half the time. It's a beta, so if your ground tells you something different, go with your ground and let us know.

Related guides

• Rootstocks for cold country, and why we use ours
• Winter protection for young fruit trees
• Growing apricots where winters hit -30F

Where this comes from

Our zone hardiness guide, cold hardiness calculator, planting guide and product records, plus our observations in Missoula and Florence. Reports from the growers named above and our customers. Bob Purvis, Fruit Growing for Homeowners lecture notes. Ontario Ministry of Agriculture, Food and Agribusiness, The effects of winter on tree fruit. University of Maine Cooperative Extension, Preparing Trees for Winter. Calculator data: NOAA Regional Climate Centers (ACIS), the USDA 2023 zone map, the US Census geocoder and USGS/Mapzen elevation data.

Updated September 2026.