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

East and west of the Divide

The Continental Divide runs north to south through western Montana. The Western Regional Climate Center describes the mountainous southwest half of the state as having a modified north Pacific climate: milder winters, cooler summers, more cloud, lighter wind and rain spread more evenly through the year. East of the Divide the climate is continental, with more wind, hotter summer spells and colder winter outbreaks.

You might expect the warmer plains to have the shorter season. In fact the western valleys have the shorter growing season. The middle Yellowstone Valley around Miles City has some of the longest freeze-free stretches in the state. Some high western mountain valleys have no dependable freeze-free period at all. Elevation also changes things within a single county. Half of Montana lies above 4,000 feet, so check the frost dates for your city above rather than assuming a regional average.

What limits Montana gardens

A short, unpredictable season. MSU Extension’s Successful Home Vegetable Gardening MontGuide warns that the real season in a given spot can run about two weeks longer or shorter than the average. Days-to-maturity figures on seed packets are useful for comparing varieties, but they are not precise for Montana conditions. Because season length varies so much across the state, gardens in different towns end up looking very different.

Dry air and strong sun. Transplants grown indoors have a thin protective cuticle. They need gradual hardening before they face Montana’s sun and low humidity. Cut back water and temperature, raise light levels, and set plants outside in good weather before planting. Allow one to two weeks for cool-season crops and two to three weeks for warm-season crops.

Alkaline soil. Most Montana soils are alkaline and strongly buffered, so their pH is hard to shift. Work with the soil rather than against it. Add amendments in fall, and be careful with bought compost. MSU Extension notes that compost carrying residual herbicides has ruined many gardens.

Hail and wind. Hailstorms are Montana’s most troublesome severe weather. They come mainly in July and August. Wind is a bigger factor east of the Divide, where strong chinook winds sweep the belt from the Browning–Shelby area toward Billings in winter. MSU Extension’s transplant table suggests direct-seeding cucumbers and muskmelons in windy locations instead of setting out transplants.

What MSU Extension recommends

MSU Extension’s planting calendar is the MontGuide Can I Grow That Here? Vegetable Seed and Transplant Schedules for Garden or Container (MT199308AG). It is not a fixed list of dates. For 34 vegetables it gives planting windows counted from your average last spring frost and first fall frost. You fill in your local dates and work out when to sow seeds and when to start transplants. It also flags crop quirks. Chinese cabbage tends to bolt in the first hot days, so it is best planted in summer for a fall crop. Cold nights early in the season can make celery bolt. Brussels sprouts taste sweeter after moderate freezes.

Other points from Successful Home Vegetable Gardening:

• Give vegetables 6 to 8 hours of sun and run rows north to south.
• Water once or twice a week, when the top 1 to 3 inches of soil is dry. Wet the top 5 to 6 inches, which usually works out to 1 to 2 inches of water a week.
• Use sheet mulch to warm the soil for cucumbers, tomatoes, peppers and eggplant.
• Start cucumbers, squash and melons in peat pots, because they don’t regrow damaged roots well.
• Plant only Montana certified seed potatoes.
• Plant garlic from mid-September to mid-October.

Tomatoes. Growing Tomatoes in Montana (MT199217AG) recommends very early to early-midseason cultivars that ripen in 60 to 70 days. Gardeners in mountain valleys should choose the very earliest types. Lower-elevation gardens with longer seasons can grow early to early-midseason types. Very early cultivars on its list include Early Girl, Stupice, Sub Arctic Plenty, Siberian, Oregon Spring, Gem State and Prairie Fire. Flowers drop without setting fruit below 55°F or above 90°F, or when winds are dry.

Season extenders. MSU Extension’s MontGuide on hotbeds, cold frames and low tunnels presents them as a practical stand-in for a greenhouse. They let you plant earlier in spring and harvest later in fall. Low tunnels over raised beds suit early leafy greens. Hardy crops such as cabbage need only simple protection. Peppers and eggplant need more elaborate setups in Montana’s colder areas.

Test before you amend

MSU Extension does not ask you to guess at fertilizer. Its soil testing guide recommends collecting at least 10 samples from the top 6 inches of a garden up to 1,000 square feet. Mix them, then dry a pint-sized subsample right away so nutrient levels don’t change. Send it to one of the regional labs listed by MSU’s Department of Land Resources and Environmental Sciences, or ask your county Extension agent which lab to use. At a minimum, ask for nitrate-N, Olsen phosphorus, potassium, organic matter, soluble salts and pH.