The last frost date in Missoula typically falls around May 19 on average. In mild years it can arrive a week earlier, and in colder years it can push into late May. That one date 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.
I put this page together as a reference for Missoula gardeners who want a clear answer on timing. I’ll update it each season as conditions warrant.
Missoula’s Frost Dates at a Glance
Here are the dates you need to know:
- Last spring frost: around May 19 (average)
- Safe planting date for warm-season crops: Memorial Day weekend (late May)
- First fall frost: around September 22
- Growing season length: approximately 120 days
These are historical averages for Missoula, which sits at about 3,200 feet in the Clark Fork River valley. If you garden at a higher elevation nearby or in a low spot that collects cold air, your actual frost dates could shift a week or more in either direction. A yard on the south-facing bench above the valley will behave differently than one near the river bottom.
The 50% probability date means there’s roughly a real chance of frost on or after May 19 in any given year. Most experienced Missoula gardeners wait until Memorial Day weekend before putting warm-season transplants in the ground.
What Missoula’s Climate Means for Your Garden
Montana’s growing season is shorter than most of the country but longer than people expect. 120 days 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 stresses plants that need consistent warmth, like peppers and basil, which is why soil temperature matters as much as air temperature when you’re deciding when to plant.
Missoula also tends to be drier than people plan for. The valley averages around 13 inches of rain a year, so irrigation is not optional for most vegetable gardens. 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. Working backward from a May 19 last frost date, here is a general timeline:
- Tomatoes and peppers: Start 6 to 8 weeks before last frost. That puts you at late March to early April.
- Broccoli, cabbage, and cauliflower: Start 4 to 6 weeks before last frost. Early to mid April.
- Squash and cucumbers: Start 2 to 4 weeks before last frost. Mid to late April. These grow fast and get leggy if started too early.
- Basil: Start 4 to 6 weeks before last frost. Early to mid April.
Cool-season crops like lettuce, spinach, kale, chard, and peas do not need to be started indoors. They tolerate light frost and can be direct-seeded in the garden as early as late March or early April when the soil is workable.
One thing I see a lot with Missoula gardeners: starting seeds too early. A tomato started in February looks impressive in April but it is root-bound and stressed by the time it goes in the ground. Bigger is not always better with transplants. A healthy six-week-old tomato will outperform a stressed twelve-week-old one every time.
The mix you start seeds in matters just as much as the timing. Using garden soil or a dense potting mix for seedlings is one of the most common early mistakes. For what to look for and what to avoid, see why your seed starting mix matters for Montana gardeners.
When to Plant Outside
Cool-season crops go in first. Peas, lettuce, spinach, kale, chard, radishes, and carrots can all handle a light frost. You can plant these as early as late March or early April, as soon as the soil is workable to a few inches deep. Soil that is still cold and wet will rot seeds rather than germinate them, so use your judgment on timing.
Warm-season crops wait until after last frost. Tomatoes, peppers, squash, cucumbers, beans, and corn go in no earlier than Memorial Day weekend. This is the most common mistake I see in Missoula gardens. People plant warm-season crops too early because we get a warm stretch in late April. Montana will almost always send one more cold night before it is truly done with frost, and one hard freeze on a tomato you planted in early May means starting over.
If you want to push the season a little, row covers and wall-o-waters help. A row cover can protect plants from a few degrees of frost, which buys you a week or two on both ends of the season. Wall-o-waters are popular in Missoula because they let you get tomatoes in the ground in late April and give them a real head start on summer.
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 first fall frost lands around September 22 on average. Working backward from that date, you can plan a second round of cool-season crops. Kale, spinach, lettuce, and radishes planted in late July or early August will mature before frost and often taste better in cool fall weather than they did in the heat of summer.
Tomatoes planted in May should be producing well before September. If a hard frost is forecast before you have finished harvesting, pick any tomatoes that have started to turn color and let them ripen indoors on the counter. Do not put them in the refrigerator – cold kills the flavor. Green tomatoes can also be harvested and used as-is, or left on the counter for a few weeks where some will slowly ripen.
Row covers over pepper plants can often buy you an extra two to three weeks of harvest in September, which matters when peppers take all 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.
If you want help planning a garden that fits your specific yard, soil, and schedule, that is what I do. Garden City Plant Care offers vegetable garden consulting for Missoula homeowners. Whether you are starting from scratch, dealing with a problem that keeps coming back, or just want a plan you can actually follow. Learn more about vegetable garden consulting.
Still have questions? The Plant Care FAQ covers frost protection, watering, and more. Or reach out and ask us directly.