Gardening in Idaho: zones, frost dates and when to plant

Idaho’s growing regions

Elevation and mountain shadows decide most of what you can grow in Idaho. NOAA’s state climate summary explains that mountains to the east and west shelter the low valleys of southern Idaho, which keeps them dry. Some southwestern stations get less than 10 inches of precipitation a year. The higher country of northern and central Idaho can get up to four times as much rain and snow, and some northern mountain sites top 40 inches. Summers differ too: Boise averages a July high near 93°F, while Coeur d’Alene in the north averages about 83°F.

The Idaho Master Gardener Program Handbook, University of Idaho Extension’s main training text, sorts gardeners by season length:

• Southwestern and south-central Idaho. These lower, drier valleys of the southern high desert are the warmest and have the longest seasons. Most cantaloupe varieties will ripen here, and tomato variety choice is less critical.
• Northern Idaho and the Panhandle. This region is cooler and moister, with shorter seasons. Its soils tend to be more acidic, and the Palouse is known for wind-deposited silt soils.
• Southeastern Idaho. The handbook groups it with the north as a short-season area where only the earliest tomatoes are reliable.
• Central mountains. The growing season can be as short as 60 to 90 days.

What limits Idaho gardeners

The handbook sums up the state’s challenges as frost, cool nights, intense sunshine, low humidity and alkaline soils. It warns that seed packet descriptions and days-to-harvest figures are usually not enough to judge whether a variety will work here. Ask experienced local gardeners, Master Gardener volunteers, nurseries and county Extension educators instead.

Other limits to plan for:

• Late spring frost. Cold air is heavier than warm air and drains downhill, so a garden at the top of a slope gets less frost damage than one in a low spot. Fences, walls and dense hedges can trap cold air.
• Wind. The handbook advises avoiding windy sites, then admits that is hard to do in Idaho. If you can’t avoid wind, build or plant a windbreak. New transplants of tender crops such as peppers and melons need protection from wind, intense sun and near-freezing nights.
• Alkaline, low-organic soils. Dry climates let lime build up in the soil, which is why southern Idaho soils run alkaline, and some have a cemented lime layer called caliche that roots and water can’t get through. Extension says lowering the pH of these soils is nearly impossible. Low-pH soils in northern Idaho and some high mountain valleys can be limed. Most Idaho soils are low in organic matter, so add compost, leaves or aged manure every year or two.
• Disease. Low humidity and short seasons keep squash and pumpkin diseases fairly light. Verticillium wilt is very common in southern Idaho, especially on potatoes.

University of Idaho Extension planting advice

University of Idaho Extension’s Spring Vegetable Planting Guide for Idaho (BUL 965) starts from your hardiness zone, season length and frost-free days, then gives a chart of planting ranges for cool-season and warm-season crops. The Master Gardener Handbook’s chapter “Principles of Vegetable Culture” offers a simple schedule based on your average last spring frost (check the frost dates for your city above):

• Three to four weeks before: onions, peas, kale, and cabbage, broccoli and cauliflower, either seeded or transplanted.
• One to two weeks before: beets, carrots, spinach, Swiss chard, leeks and lettuce. Sweet corn can go in now if you accept some risk of frost damage.
• Within a week or two after: beans, cucumbers, eggplant, melons, squash, pumpkins, peppers and tomatoes, if the forecast stays mild. Transplants may need a few more days.

Check soil temperature before you trust the calendar. The handbook suggests a daytime soil temperature of at least 50°F for cold-hardy crops, 55°F for moderately hardy crops and 65°F for warm-season crops. It also admits that in Idaho, waiting for the ideal soil temperature often means planting too late, so plant somewhere between the minimum and the ideal.

Other Idaho-specific tips from the handbook:

• In short-season areas, grow onions from transplants or sets rather than seed.
• Hold back nitrogen on tomatoes, because heavy feeding delays ripening when the season is short.
• Use raised beds to improve drainage and warm the soil faster in spring.

Test the soil

The handbook recommends testing a new garden for nutrients and pH, then retesting at least every three years. University of Idaho and private labs both do the work. County offices such as UI Extension in Ada County provide University of Idaho Analytical Sciences Laboratory sample kits and will help you interpret the results.