Sow Asian greens in Montana twice a year: in spring, a couple of weeks before the last freeze, and again in early to mid August, about eight weeks before the first fall freeze. On the Billings reference station, the last spring freeze falls between April 29 and May 10, and the first fall freeze between October 1 and October 14. Colder valleys and higher ground will not match those dates.
The short answer for Montana
Those frost dates come from the NOAA NCEI 1991-2020 Climate Normals for Billings, and they measure a probability. Each date marks where the chance of a 32°F freeze sits in a 30 to 70 percent band. April 29 is not the day frost stops. It is the point where, in a given year, a freeze is still close to a coin flip. By May 10, the odds have mostly (not entirely) swung your way.
Billings is one of the milder valleys in the state. Mountain valleys elsewhere in Montana can frost in any month, so treat every date below as a starting point for the Billings area and a ceiling on optimism everywhere colder.
Spring and fall windows
Most Asian greens (bok choy, tatsoi, mizuna, komatsuna) shrug off a light frost once they are up. That lets you sow before the last-freeze band closes. Count back two to three weeks from April 29 and you land in the first half of April, assuming the soil is thawed enough to work. The tradeoff is that early sowings face cold soil and a freeze that may still arrive, so a sheet of row cover earns its keep.
Fall is the stronger season. Eight weeks back from October 1 is early August. Six weeks back from the same date is around mid-August. Days shorten and nights cool as the crop grows, which is the opposite of the spring squeeze, so the plants hold their leaves instead of throwing up flower stalks.
| Season | Sow | Anchored to | Best fit |
|---|---|---|---|
| Spring | First half of April to mid-May | Last freeze, April 29 to May 10 | Baby leaves, fast mustards, tatsoi |
| Fall | Early to mid-August (baby leaves into late August) | First freeze, October 1 to October 14 | Bok choy heads, komatsuna, mizuna, tatsoi |
Packet days-to-maturity vary by variety, so check the one on your seed. Baby-leaf harvests generally arrive in about a month, while full-size heads take noticeably longer. That difference decides how late in August you can still sow. For the broader picture of how these crops fit a Montana garden, see Growing Asian Greens in Montana.
What will not work here
Do the arithmetic on the season. From the late edge of the spring band (May 10) to the early edge of the fall band (October 1) is 144 days. From the early edge (April 29) to the late edge (October 14) it is 168 days. That is roughly 20 to 24 weeks, and it assumes Billings. Anything slower than that, or anything that wants a long run of warm nights, is playing against a short clock.
Slow heading types
Full-size napa cabbage is the classic casualty. Many varieties want well over two months from sowing, and they dislike being rushed by a hard freeze right as the heart forms. Sow too early and cold spring weather stalls them. Sow too late and October catches the heads half formed. Large daikon-type radishes have the same problem in a milder form. If you want either, pick the short-season versions sold specifically for 50-to-60-day harvests.
Summer sowings and warm-season greens
Sowing bok choy in June feels logical, since the soil is warm and the garden is busy. But June days in Montana are long, and long days plus heat push these plants to bolt before they size up. The rows that look best in late July are usually the ones sown in August.
Truly heat-loving Asian vegetables, such as water spinach, have no realistic path in a season this short without a greenhouse or hoop house. They ask for a long, steady warm stretch that the numbers above do not provide.
Overwintering outdoors
The mean daily minimum of the coldest month at Billings is 17.9°F. That is the average low on an ordinary winter night, not a record, and it already puts most Asian greens out of reach for planned overwintering in open ground. Billings is the mild end of the state. Treat these crops as annuals unless you grow under cover.
Sowing by the calendar alone
The last mistake is trusting a printed date over what the ground is doing. A year when April 29 arrives with frozen topsoil and a May snowstorm in the forecast is not a year to follow the table. A band that spans 30 to 70 percent will, in some years, go against you.
What changes across Montana
The USDA Plant Hardiness Zone Map (2023 edition, built on 1991-2020 normals) places Montana across zones 3a to 6a. That is three full zones. There is no single answer for the state, and anyone offering one is averaging away the real differences between a high valley and the Yellowstone River corridor.
The hardiness zone is built on one variable, the average annual extreme minimum temperature. It tells you about the coldest night of the winter, which matters for perennials. For a crop you sow in April or August, frost dates and the length of the season matter more. Still, the zone is a useful signal of how harsh a location runs overall. A garden at the 3a end will almost always have a shorter, less forgiving growing season than one at the 6a end.
The Billings dates on this page are a reference point, not a statewide rule. If you garden in the north, in a high mountain valley, or at altitude, shift the calendar this way:
- Push spring sowings later, and expect the last freeze to land after the Billings band.
- Pull fall sowings earlier, since the first freeze will likely arrive before October 1.
- Favor fast baby-leaf types over full heads, because the usable window is shorter.
- Plan on row cover or a cold frame as standard equipment, not an extra.
To find your own zone, use the USDA Plant Hardiness Zone Map and look up your ZIP code. Do not guess from a neighbor’s garden or a nearby town. Zones can change over a few miles of elevation. The variety side of this decision is covered in The Best Asian Greens for Montana.
Where to check this for your own county
This page is the state-level view, built on one reference station. Your county is more specific, and a county recommendation beats a state one every time. Start with Montana State University Extension, the land-grant extension service for the state, and find your county office from there.
Two things to look for once you are there:
- A local planting calendar. Ask whether one exists for your county or your nearest town, and what frost dates it uses. If it gives different dates from the Billings ones above, follow it.
- Varieties trialled nearby. A variety that did well in a trial in your county has already survived your wind, your elevation and your late freezes. That counts for more than a catalog description.
If your county has no published calendar for leafy greens, call or email the office. Extension staff and Master Gardener volunteers often know the local last-freeze pattern from years of questions. Calendars from other states’ extension services, written for other climates, will not transfer, so skip them.
Checking this against your own garden
Every date on this page is an average from one station. Your own observations outrank all of them. Three are worth watching, and none needs equipment.
Soil warmth by feel. Push your hand into the top few inches of bed soil in the morning. If it feels numbingly cold and sticks in clumps, wait. Soil that crumbles and feels merely cool is ready for hardy greens. Compare beds. The one against a south-facing wall will warm first, and a low corner where cold air pools will lag.
The first flush of male flowers. Willow and aspen catkins are male flowers, and they open on a schedule set by local weather. Note the date they appear in your yard each year. Over a few seasons, you build a personal marker that tracks your microclimate better than a station fifty miles away.
The first true leaves. After the seed leaves, the first true leaves signal that a seedling is established. If a spring sowing sits at the seed-leaf stage for weeks, the soil was too cold or the spot too shaded. If a fall sowing reaches true leaves quickly, you have a warm spot worth reusing next August.
Jot the dates in a notebook. After two seasons, your garden’s own calendar will do better than any statewide table.
Common questions
Can I sow Asian greens in July in Montana?
You can, but August is safer. July sowings face summer heat and long days, which push bok choy and similar crops to bolt. Counting back from the October 1 edge of the Billings fall band, early to mid August gives the plants cooling weather as they grow.
Do I have to wait until after the last frost in spring?
No. Many Asian greens tolerate a light frost, so you can sow before the April 29 to May 10 band ends. Remember that the band is a 30 to 70 percent chance of a 32°F freeze, so cover seedlings if a hard freeze is forecast.
Can Asian greens overwinter in a Montana garden?
Not reliably in open ground. At Billings, the mean daily minimum of the coldest month is 17.9°F, and that is the milder end of the state. Treat them as annuals, or grow them under a hoop house or in a cold frame.
Which varieties should I choose?
Favor quick-maturing types over long-season heading ones, because the season between freezes is short. The pick-by-type breakdown is in The Best Asian Greens for Montana.