Thin Air Harvest Practical gardening for the high, dry West

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Planting calendars for the high West

How to use station frost tables, then open a month-by-month calendar for the Front Range, mountain towns, Utah, Wyoming, Montana, Idaho, or northern New Mexico.

By the Thin Air Harvest editorial team. Updated November 1, 2026.

A planting calendar is a station plus a habit. The station supplies the historical freeze dates. The habit is what you do in each month given those dates, the soil, and the crop’s temperament. This section publishes both, and it refuses to average them into one “Mountain West last frost.”

The spring dates quoted from NOAA’s older Colorado freeze-occurrence tables, and the matching Montana tables hosted by Montana State University Extension, are printed as three values. In those tables the columns run 90 percent, 50 percent, and 10 percent. Read the middle date as the median last spring freeze at that threshold, usually 32°F. Read the right-hand date as the late case: only about one year in ten is later. The left-hand date is the early case. A full explanation, including why a warm April week does not retire the late date, is on reading a spring frost. Fall dates are quoted only where a publication actually gives them. This site does not subtract a spring median from an unrelated fall median and call the result a growing season.

The tables also have a vintage. The Colorado and Montana occurrence PDFs in wide circulation are the NCEI freeze/frost supplements, and the copy linked from these pages sits in NCEI’s deprecated-normals archive. They remain the most complete station-by-station public tables for this kind of comparison, and they are not a substitute for the current 30-year normals. For Colorado, the Colorado Climate Center’s freeze tool serves 1991–2020 normals and is the right place to confirm a station before you rely on it for a commercial planting. For a backyard, the older table plus your own notebook is already more honest than a date printed on a national seed packet.

Pick the page whose stations sit near your elevation and your landform, not the page for the largest city in your state.

The Front Range calendar covers Denver, Boulder, Fort Collins, Colorado Springs, Longmont, Greeley, Castle Rock, Pueblo, and Cañon City, with Grand Junction included only as a contrast. Castle Rock’s median in the NOAA table is May 22. Denver Stapleton’s is April 30. Those towns are an hour apart by car and three weeks apart by frost. The Palmer Divide is not “the Denver date plus a little.”

The mountain-town calendar uses Alamosa, Buena Vista, Salida, Gunnison, Steamboat Springs, Dillon, Crested Butte, and the high station at Climax, plus the Gilpin County extension figures at 9,300 feet. Several of those medians fall in June, and Climax’s median last 32°F date falls on July 7. The month-by-month work on that page treats June as spring.

The Utah calendar follows Utah State University Extension’s suggested planting-date table, which prints an average last spring frost by city, from St. George and Moab to Logan, Heber, Coalville, and Woodruff. A 2019 USU note adds Park City and a set of frost-free day counts, and it shows why a bench and a valley floor in the same county should not share a weekend. Where the two USU pieces disagree, both numbers are shown.

The Wyoming and Montana calendar uses the University of Wyoming Extension spring probability table for Cheyenne, Casper, Laramie, and Jackson, National Weather Service fall statistics for Cheyenne and Laramie, and the MSU-hosted freeze table for Billings, Bozeman, Helena, Missoula, Great Falls, Butte, Livingston, Miles City, and Hamilton. Bozeman’s campus station and the Belgrade airport are both listed, because they are not the same date.

The Idaho calendar stays inside University of Idaho publications: frost-free day figures from the short-season bulletin for towns such as McCall, Cascade, Salmon, and Fairfield, the northern Idaho climate summary for Coeur d’Alene, and the Teton County extension note that puts the frost-free stretch from mid-June into early September with summer frost still possible. It does not borrow an unsourced Boise date. The Treasure Valley is a longer, lower, warmer season than those mountain towns, and the page says how to confirm it rather than inventing the day.

The northern New Mexico calendar uses NMSU Circular 457B, which ties average last-frost windows and frost-free day ranges to USDA zones from PRISM data for 1994–2023, and it labels those windows as statewide zone ranges. A 2005 NMSU column in which county agents talk about Santa Fe, Taos, and Los Alamos is quoted as historical local practice, not as a current normal.

Across all six, the monthly tasks rhyme and the dates do not. Hardy sowings still track two to four weeks before the median last frost when the soil is workable, following CSU’s planting guide. Very tender transplants still wait until after the late date, or they go under cover. Garlic is still a fall planting. The rhyme is the method. The date is local. Keep a pencil record of the nights you actually freeze. After five years your notebook beats every table on this site, and the tables will have done their job.

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