Gardening in Montana

Hardiness zones in Montana

Montana spans USDA hardiness zones 3b through 6b. That range sets the coldest winter temperature a plant in that zone is expected to survive, based on the average lowest temperature each year. Zone 3b marks the coldest parts of the state, while zone 6b marks the mildest. A difference of several zones across one state means a shrub that survives winter in one town may die in another just a few hundred miles away. The zone number says nothing about summer heat, rainfall, humidity, or soil type. Two towns in the same zone can still have very different summers. Use the zone number only to judge winter cold survival, then check the frost dates below to plan planting and harvest. This page draws on data from 206 weather stations across the state, so the zone range reflects real local variation rather than one single statewide number.

• Zone 3b
• Zone 4a
• Zone 4b
• Zone 5a
• Zone 5b
• Zone 6a
• Zone 6b

Frost dates

Frost dates mark the start and end of Montana's growing season, and they vary widely across the state. In the mildest spots, the last spring frost falls as early as April 30, while in the coldest spots it can linger until June 18. Fall's first frost arrives just as unevenly: as early as September 2 in the coldest areas, or as late as October 15 where the season runs longest. Those two dates give you the window for frost-sensitive crops like tomatoes and squash. Check your own local last and first frost dates rather than relying on one statewide date. Nearly seven weeks separate the earliest and latest last-frost dates recorded here. Plant frost-sensitive seedlings only after your local last frost date passes. Harvest or cover tender plants before the first fall frost arrives. These dates come from long-term averages, so an individual year can still run earlier or later than its typical frost date.

Climate

Montana's climate data for this page comes from 206 weather stations spread across the state. That large number of stations matters because Montana's terrain ranges from high mountain valleys to open plains. A single statewide average would hide how different those places really are. The growing season length shows that spread clearly: some stations record as few as 22 frost-free days a year, while others record as many as 165. A gardener near the short end of that range needs to pick fast-maturing crops and may rely on row covers or a greenhouse to stretch the season. A gardener near the long end can grow crops that need a longer stretch of warm weather without added protection. Hardiness zones across the state run from 3b up to 6b, tracking the same pattern. Colder zones cluster where the season is shortest, and milder zones cluster where it runs longest. None of these figures describe summer rainfall, humidity, or how hot a Montana summer gets in any one spot. Check the zone and season length for your own location before choosing plants, rather than treating Montana as one climate.

Season length

Growing season length is the number of days between your last spring frost and first fall frost. It is the single most useful number for planning a garden. Across Montana's weather stations, that number ranges from as short as 22 days to as long as 165 days. A 22-day season leaves room only for the fastest-maturing crops, started indoors and moved out once the danger of frost passes. A 165-day season supports a much wider range of vegetables and flowers, including those that need a long stretch of warm weather to mature. Find your own station's season length rather than assuming the statewide range applies to your yard. The gap between the shortest and longest season recorded is more than four months.