Missoula’s last spring frost usually comes in mid to late May, but it moves around from year to year, and in some years it comes in June. That timing shapes everything about vegetable gardening here. Get it wrong and a late frost wipes out weeks of work. Plan around it and you can grow a productive garden even in Montana’s short season.
This page is a reference for Missoula gardeners who want a clear answer on timing. It is built on NOAA climate data and Montana State University Extension planting schedules, with sources listed at the bottom.
Missoula’s Frost Dates at a Glance
Here are the numbers worth knowing, from NOAA’s 1991 to 2020 climate normals for the Missoula International Airport weather station:
- Median last spring frost (32°F): about May 16
- Late last frost, about 1 year in 10: after June 1
- Median first fall frost (32°F): about September 26
- Early first frost, about 1 year in 10: by September 12
- Frost-free season: about 130 days in a typical year, and under 110 in about 1 year in 10
Missoula is USDA plant hardiness zone 5b on the 2023 USDA map. The zone rates how cold winters get, so it matters for perennials, shrubs, and fall-planted garlic that stay in the ground over winter. It does not tell you when to plant vegetables. Frost dates do that.
The airport station sits at about 3,200 feet in the Missoula valley. Older numbers in MSU Extension’s Montana frost table, based on 1971 to 2000 records, put the airport’s median dates later in spring and earlier in fall (about May 26 and September 18), which is a good reminder that these dates are estimates, not fixed points. Your own yard can also differ. Low spots where cold air settles often frost earlier in fall and later in spring, and a yard higher on a bench can behave differently than one near the river bottom.
A median date is a coin flip. In about half of years, the last spring frost comes after May 16. That is why many Missoula gardeners hold tomatoes, peppers, and other warm-season transplants until late May or early June and check the 10-day forecast before planting. Memorial Day weekend is a popular local rule of thumb, not a guarantee.
What Missoula’s Climate Means for Your Garden
Missoula’s growing season is shorter than most of the country’s but longer than people expect. About 130 frost-free days in a typical year, and fewer in a cold year or a cold spot, is enough to grow tomatoes, peppers, winter squash, and most vegetables if you start at the right time and choose varieties suited to a shorter season.
The challenge here isn’t the season length. The temperature swings are the real challenge. Spring nights stay cold well into May, and Missoula can see frost again in early fall before you’ve finished harvesting. Summer afternoons get hot, but the nights cool down fast. That combination slows plants that like steady warmth, like peppers and basil, which is why night temperatures and soil warmth matter alongside the frost date when you’re deciding when to plant.
Missoula also tends to be drier than people plan for. NOAA’s current normal for the airport is about 14 inches of precipitation a year, and summers are usually dry, so most vegetable gardens need regular watering. Our soil varies a lot across the valley. Some yards are clay-heavy, others rocky and fast-draining. Both affect how you water and what amendments your soil needs before planting.
When to Start Seeds Indoors
Starting seeds indoors is how you make the most of a short Montana season. MSU Extension lists how many weeks each crop needs to reach transplant size, so count back from the date you actually plan to set plants out, not just from the frost date:
- Tomatoes: 6 to 8 weeks, per MSU’s tomato guide. For a late May transplant, that means sowing from about the end of March to mid April.
- Peppers: 6 to 8 weeks on MSU’s schedule. Peppers grow slowly, so many gardeners sow toward the 8-week end, around late March to early April.
- Broccoli and cabbage: 6 to 8 weeks. Hardened transplants handle light frost, so they can go out weeks before tomatoes. Sowing in March lines up with an April or early May planting.
- Cauliflower: 4 to 5 weeks. Sow in late March or early April for a late April or early May planting.
- Cucumbers and summer squash: about 4 weeks. Winter squash: 3 to 4 weeks. Use peat or paper pots, since these crops do not like root disturbance, and sow in late April or early May for planting after frost danger.
- Basil: about 6 weeks, per Penn State Extension. Basil is very cold sensitive, so wait for warm nights before planting it out.
Cool-season crops like lettuce, spinach, kale, chard, and peas are usually direct seeded instead of started indoors. They tolerate light frost, and MSU’s schedule puts spinach in about 6 weeks before the last frost, peas 4 to 6 weeks before, and lettuce about 4 weeks before. In Missoula that works out to early to mid April, once the soil can be worked.
A common mistake is starting seeds too early. A tomato started in February looks impressive in April, but by planting time it is often root-bound and stressed. MSU’s tomato guide makes the same point and warns against the temptation to start tomatoes too early. A sturdy six- to eight-week-old transplant usually settles in faster than an overgrown one.
The mix you start seeds in matters too. Garden soil and dense potting mixes hold too much water and air too poorly for young seedlings. For what to look for and what to avoid, see why your seed starting mix matters for Montana gardeners.
Missoula Planting Calendar by Crop
This table pulls the timing together. Hardy crops follow MSU’s frost-relative planting rules. Warm-season crops with no frost tolerance follow MSU’s tomato guide and go in after the danger of frost has passed.
| Crop | Start indoors | Plant or sow outdoors in Missoula |
|---|---|---|
| Broccoli | 6 to 8 weeks | Early April to about June 20 |
| Cabbage | 6 to 8 weeks | Early April to late June |
| Cauliflower | 4 to 5 weeks | Late April to about June 20 |
| Lettuce | Optional, 4 to 6 weeks | Mid April to mid August, sowing every 2 weeks |
| Spinach | Direct sow | Early April for spring, early to mid August for fall |
| Peas | Direct sow | Early to mid April (up to early June) |
| Kale | Direct sow | Mid April for spring, early to mid August for fall |
| Carrots | Direct sow | Mid April to late June |
| Beets | Direct sow | Mid April to late July |
| Radishes | Direct sow | Early April for spring, early August for fall |
| Onions | 4 to 8 weeks (or use sets) | Late April to late May |
| Potatoes | Plant seed pieces | Mid to late May (shoots are frost tender) |
| Tomatoes | 6 to 8 weeks | After frost danger, usually late May to early June |
| Peppers | 6 to 8 weeks | After frost danger, usually late May to early June |
| Cucumbers | 4 weeks, in peat or paper pots | After frost danger, late May to early July |
| Summer squash and zucchini | 4 weeks, in peat or paper pots | After frost danger, late May to mid July |
| Winter squash and pumpkins | 3 to 4 weeks, in peat or paper pots | After frost danger, late May to mid June |
| Snap beans | Direct sow | After frost danger, through about July 4 |
| Sweet corn | Direct sow | Late May to early June |
When to Plant Outside
Cool-season crops go in first. Peas, lettuce, spinach, kale, chard, radishes, and carrots can all handle a light frost. In Missoula most of them go in from early to mid April, as soon as the soil can be worked a few inches deep. Seed sown into cold, soggy soil can rot before it sprouts, so use your judgment on timing.
Warm-season crops wait until the danger of frost has passed. Tomatoes, peppers, squash, cucumbers, beans, and corn have no frost tolerance, so in Missoula they usually go in from late May into early June, after a look at the 10-day forecast. Planting them during a warm stretch in late April is a common mistake. Cold nights often return in May, and one hard freeze on an unprotected tomato means starting over.
If you want to push the season a little, row covers and water-filled plant protectors help. MSU Extension suggests a row cover with about a 4°F frost rating as a good balance of warmth and light, and its tomato guide recommends covering new transplants with water walls, hot caps, or row covers. Protection like this can let you plant a little earlier, but how much it helps depends on the product, how it is set up, and the night. Setting tomatoes out in late April is a gamble even with protection.
For a complete guide to growing tomatoes in Missoula including variety selection, transplanting, and season extension, see Growing Tomatoes in Missoula: A Short-Season Guide.
Extending Into Fall
Missoula’s median first fall frost is about September 26 at the airport, and in about 1 year in 10 it arrives by mid September. Working backward from that, you can plan a second round of cool-season crops. MSU’s schedule puts fall spinach and kale in 6 to 8 weeks before the first frost and allows lettuce up to about 6 weeks before, so late July to mid August sowings of kale, spinach, lettuce, and radishes have time to size up. Some greens also turn sweeter after a few cool nights.
Tomatoes planted in May should be producing well before September. If a hard frost is forecast before you have finished harvesting, pick tomatoes that have started to turn color and ripen them indoors. MSU’s tomato guide suggests about 65°F. Keep ripening fruit out of the refrigerator for the best flavor. Full-size green tomatoes can also be cooked as they are, or left indoors where many will slowly ripen.
A row cover over peppers can carry them through an early light frost and keep the harvest going if the weather turns mild again, which matters when peppers take most of the summer to produce.
For crop-by-crop details including the best varieties for Missoula, pest management, and planting tips for each vegetable, see the Missoula Vegetable Growing Guide.
Still have questions? The Plant Care FAQ covers frost protection, watering, and more. Or reach out and ask us directly.
Sources
- NOAA NCEI, U.S. Climate Normals 1991 to 2020 (Missoula International Airport)
- Montana State University Extension, Can I Grow That Here? Vegetable Seed and Transplant Schedules
- Montana State University Extension, Growing Tomatoes in Montana
- Montana State University Extension, Hotbeds, Cold Frames, and Low Tunnels for Montana Gardeners
- Montana State University Extension, Frost/Freeze Occurrence Data for Montana (1971 to 2000)