Idaho planting calendar: valleys and high country
University of Idaho frost-free figures for McCall, Cascade, Salmon, and Coeur d'Alene, and how to time a garden when summer frost is possible.
Idaho’s garden problem is that the state contains a long, irrigated valley and a set of high towns where the frost-free season is measured in weeks. University of Idaho Extension’s short-season bulletin, BUL 857, lists frost-free days and a hardiness zone for many of those high towns. The zones in that bulletin are the map vintage of the publication.
Recheck the 2023 USDA map for your address, and remember that a zone is not a season length. Teton County’s extension office states the local version directly: zones from about 5a in the valley to 3a higher up, a frost-free stretch that runs from mid-June into early September, and summer frost that is not unusual.
This page quotes the UI figures it actually has. It does not fill in a Boise last-frost day from a non-extension roundup. The Treasure Valley is lower, warmer, and longer than McCall or Fairfield. Confirm its current normals with the National Weather Service and with UI’s vegetable guidance for the valley before you copy a mountain schedule downhill or a valley schedule uphill.
High-country figures from UI bulletins
Frost-free days below are as printed in BUL 857, with the elevation and zone column from that bulletin.
| Place | Elevation in the bulletin | Zone in the bulletin | Frost-free days |
|---|---|---|---|
| Sandpoint | 2,100 ft | 4 | 121 |
| Garden Valley | 3,143 ft | 5 | 104 |
| Salmon | 3,949 ft | 4 | 118 |
| Elk City | 3,980 ft | 4 | 44 |
| Idaho City | 3,965 ft | 4 | 64 |
| New Meadows | 3,868 ft | 3 | 57 |
| McCall | 5,025 ft | 4, with a colder note | 63 |
| Cascade | 4,865 ft | 4 | 68 |
| Challis | 5,175 ft | 4 | 114 |
| Fairfield | 5,065 ft | 3 | 59 |
Forty-four days at Elk City and 63 at McCall are not seasons for corn and peppers. They are seasons for greens, roots, peas, and a potato if you are quick, plus covers. Challis and Salmon, both over 100 frost-free days in the bulletin, can attempt more, and they still are not the Treasure Valley. Sandpoint’s 121 days are a northern-lake season, cooler and cloudier than a Magic Valley summer, with its own last-frost behavior.
Coeur d’Alene, from the northern Idaho climate summary
UI’s northern Idaho weather compilation, using the Coeur d’Alene record at about 2,160 feet, prints probabilities rather than a single date. For a 32°F threshold, five years in ten the last spring freeze is later than May 16, and one year in ten it is later than June 3.
In fall, five years in ten the first 32°F freeze is earlier than September 26. The median length of the season above 32°F in that compilation is 133 days. That is a real season and still a season that punishes an early tomato. Use May 16 as a median-style mark, not as a guarantee, and keep cover available into early June.
Teton County and the high valleys
UI Extension in Teton County describes a frost-free period from mid-June to early September, with frost possible in midsummer, and zones from 5a in the valley to 3a above it. Planting before mid-June in the open, for tender crops, fights that description.
Hardy crops can go in earlier under a cover or when a soil thermometer says the ground is ready, following the same 40°F-at-6-inches idea CSU uses in its mountain sheet. Summer frost means a row cover stays in the shed all season, not just in May.
Bulletin 936, UI’s introduction to season extension for high-altitude short-season gardens, is written for this pattern. It treats frost dates as estimates and tells you to watch microclimates, because a county can span several hardiness zones. Valley County is the bulletin’s example, from roughly zone 3 to zone 6.
April and May
In Coeur d’Alene and other northern lake towns, April is for soil preparation when the ground is dry enough, and May is for hardy sowings, with tender crops waiting until after the mid-May median and preferably toward the early-June late case if you will not cover them. Start brassicas indoors in April for a May set-out. Do not start them in February.
In McCall, Cascade, Fairfield, and Teton County, April and May are indoor and cold-frame months.
Open-ground hardy sowings belong in late May or June, when snow is gone and soil crumbles. Starting a tomato in March for a mid-June set-out can work if you have the lights and you will not let it starve in a pot. Starting it so it is huge and flowering in a window in May is how you plant a stressed plant into cold ground.
Treasure Valley gardeners should follow valley guidance and a local NWS station, and they can plant hardy crops much earlier than McCall. The mistake to avoid is the opposite one: a Boise schedule used in a 63-day town.
June
High country: this is planting time. Peas, spinach, lettuce, radish, potatoes pre-sprouted, brassica transplants. A bush bean only if you are inside a season of at least a couple of months and you can cover the start.
Tomatoes only under protection, short-season, and only where the frost-free figure is long enough that an August harvest is plausible. In a 60-day town, skip them. Eat greens.
Coeur d’Alene: finish tender plantings in early June if May was cautious. Mulch after the soil warms. Put up the hail or wind cover if your site needs it.
Northern Idaho’s problem is as often a cool, wet spell as a dry wind. Do not water on a schedule copied from a Utah desert. Probe the soil.
July
Everything that is going to be planted is planted. Harvest lettuce before it bolts if a warm spell arrives. In the high valleys a warm spell may never come, which is why lettuce works and peppers do not.
Water new plantings. Watch for a midsummer frost in Teton County and similar valleys, and cover the beds when the forecast says so.
A frost in July is not a failed season if your crops are hardy. It is the season the extension office described.
August and September
Harvest potatoes after vines die, or before a hard freeze. Coeur d’Alene’s median first freeze near the end of September allows a little more finish than a Teton County garden that may be done in early September. Do not sow a long fall crop in a 60-day town in August.
Do sow a quick radish or a spinach if you have the cover and the dates. Pull irrigation before the lines freeze. Plant garlic in September in the high towns, in October only where the ground stays open and you know it.
Winter
UI’s woody-plant bulletin, BUL 859, is the winter reading: fruit cultivars should mature early and bloom late, south walls help vegetables and can hurt evergreens, and raised beds warm faster in spring. Use the winter to choose next year’s short list. A garden that tried twelve warm-season crops in a 63-day climate can try four cool-season crops and a frame instead.
Checklist
- Read the frost-free days for your town in BUL 857 if it is listed, and do not borrow a neighbor’s.
- Use Coeur d’Alene’s probabilities as probabilities. May 16 is the 5-in-10 spring mark in that compilation, not a law.
- In Teton County and similar valleys, treat mid-June as the start and early September as the close, and expect a summer frost.
- Confirm Treasure Valley dates from NWS and UI rather than from this mountain table.
- Keep covers available all summer in the high country.
- Match crops to the day count before you match them to a catalog.
Sources
- University of Idaho Extension, BUL 857, short-season, high-altitude gardening, town table of elevation, zone, and frost-free days.
- University of Idaho Extension, BUL 936, An Introduction to Season Extension for High-Altitude, Short-Season Gardens.
- University of Idaho Extension, BUL 859, short-season woody plants and microclimates.
- University of Idaho Extension, northern Idaho weather records compilation, Coeur d’Alene frost probabilities at about 2,160 feet.
- University of Idaho Extension, Teton County horticulture.
- USDA Plant Hardiness Zone Map, planthardiness.ars.usda.gov.