Wyoming gardeners get two windows for Asian greens: spring, before the heat arrives, and late summer, ahead of the first hard freeze. Sowings made in the middle of summer tend to run to seed before they make a crop. The dates below come from the Cheyenne reference station at roughly 6,000 feet, and they slide with every foot of elevation you gain or lose.
The short answer for Wyoming
Sow hardy Asian greens such as mizuna, tatsoi, bok choy, komatsuna and mustard greens outdoors from April into early May. Sow them again from late July into mid-August for a fall crop. Both windows hang on two frost dates: a last spring frost of May 7 to May 18 and a first fall frost of September 24 to October 5.
Those dates are probabilities, and the source matters. They come from the NOAA NCEI 1991-2020 Climate Normals and mark the 30-70% band of the 32°F freeze probabilities. Within that band, the odds of a freeze are neither remote nor certain. A freeze can still arrive after May 18 in some years, and it can show up before September 24 in others. The reference station has about 130 frost-free days between the mean last spring frost and the mean first fall frost, and that is a short stretch for any crop.
When should you sow Asian greens in spring in Wyoming?
Start as soon as the bed can be worked without turning to mud, usually April. Seedlings of the leafy types shrug off light frost. The sowing window runs through early May, which is roughly when the frost band opens. A row sown on May 1 is not at risk from the dates themselves. It is at risk from the one cold night that falls outside the average, so keep a sheet of row cover handy.
Spring sowings have a clock running against them. Long days and rising heat push these plants to bolt, meaning they send up a flower stalk and the leaves turn bitter. Sowing early gives them weeks of cool growth before that happens.
When should you sow Asian greens for a fall crop?
Count backward from the first fall frost. The packet’s days to maturity are counted from sowing for direct-sown greens, so a type that needs six weeks to reach harvest size must go in at least six weeks before the frost band starts on September 24. Add a couple of weeks, because growth slows as nights cool and days shorten. That arithmetic lands most leafy types in late July through mid-August.
| Type | Spring sowing | Fall sowing |
|---|---|---|
| Quick leafy greens (mizuna, tatsoi, komatsuna) | April to mid-May | Late July to mid-August |
| Bok choy and pac choi | April to early May, bolts as days lengthen | Late July to early August |
| Mustard greens | April to early May | Late July to mid-August |
| Heading napa cabbage, large daikon | Rarely worth the risk | Early to mid-July at the latest, with protection |
For variety choices within each type, see The Best Asian Greens for Wyoming. For the wider picture of how the crop fits a Wyoming garden, start with Growing Asian Greens in Wyoming.
What will not work here
A 130-day frost-free season rules out anything that needs a long, steady stretch of warmth. The slowest Asian greens are the first casualties. Heading napa cabbage and big daikon radishes want a long run of cool-but-mild weather to size up. Sown in spring, they hit summer heat before they finish. Sown in July, they race a frost band that opens on September 24, and at higher elevation it can open sooner. Gardeners who pick these types should plan on a cold frame or row cover, or pick a different crop.
Can you sow Asian greens in midsummer in Wyoming?
Rarely with good results. A seedbed in full sun dries out in a day or two under strong sun and wind, and seed that sprouts and then dries stops growing. Midsummer heat also triggers bolting in the young plants that do survive. If you want a late-July sowing to work, shade the bed with cloth or plant on the east side of a fence or tall crop, and keep the soil surface damp until the seedlings are up.
Does weekly watering work for Asian greens in Wyoming?
No. Wyoming is a dry, windy state, and Asian greens have shallow roots that sit in the top few inches of soil. A deep soak once a week leaves that layer dry for most of the week. The result is slow, stressed growth, tough leaves and early bolting. Short, frequent watering works better, with mulch over the soil to hold the moisture in. Check the top inch by hand every day or two in dry spells.
Do Asian greens survive a Wyoming winter?
Not unprotected. The mean daily minimum of the coldest month at the reference station is 18.1°F. That is the mean daily minimum of the coldest month, so many nights run colder. Tender heading types will not come through that in an open bed, and even the hardy leafy types need a cold frame or low tunnel to hold on. Treat these greens as a spring and fall crop, and count any winter harvest from a covered frame as a bonus.
What changes across Wyoming
Wyoming is not one garden. On the USDA Plant Hardiness Zone Map (2023, built on 1991-2020 normals), the state runs from zone 3a to zone 6a. That system rests on a single variable, the average annual extreme minimum temperature, so it describes winter cold only. A spread of that size means there is no single answer for the whole state. The dates above fit a garden at about 6,000 feet, near Cheyenne. They are a starting point for everyone else.
If your garden is in the north or at altitude, shift both windows toward the middle of summer. Start spring sowings later, since the soil warms more slowly and the last frost comes later. Finish fall sowings sooner, because the first freeze arrives earlier. The shorter your season, the more you should lean on the fast leafy types and on row cover, and the less you should lean on anything that heads up or forms a large root. Gardens in the high basins and mountain valleys can see frost in any month, so a frost cloth stays within arm’s reach all summer.
Where to check this for your own county
Your county extension office has the best local information. University of Wyoming Extension runs the state’s land-grant extension program and has offices that serve local growers. Contact the one for your county, or browse its garden resources, and look for two things.
- A local planting calendar with spring and fall dates for cool-season vegetables in your area.
- Variety trials from your county or the nearest one at similar elevation, showing which Asian greens finished before frost and which bolted.
A county recommendation beats a state one. The dates in this article are a statewide average built from one reference station, and Wyoming covers a huge range of elevations and microclimates. An extension agent who works with growers a few miles from you knows how late the frost lingers in your valley and which varieties did well in local plots.
When extension calendars list a crop by species, use the one that matches what you are actually growing. Asian greens are cool-season crops, so follow the cool-season entries for spring and fall, not warm-season dates meant for tomatoes or squash. If the office has no Asian greens entry, borrow the calendar for spinach or lettuce as a rough guide, then trim a week or two off the end of each window to allow for how quickly these greens bolt.
Checking this against your own garden
Your own observations outrank every date here. Two things are worth watching, and neither needs equipment.
Soil warmth by feel. Push your hand into the bed a couple of inches deep in the morning. If the soil feels cold and clammy, seed will sit and sprout slowly or rot. If it feels cool but not shockingly so, hardy greens will germinate. A bed that warms faster than your neighbor’s, say one against a south-facing wall, can go in a week or two earlier. A low, shaded bed is the opposite.
First true leaves. Seedlings first put out a pair of rounded seed leaves, then the first true leaves with the shape of the mature plant. If that takes much longer than the packet suggests, the bed is cold or dry. If seedlings jump to true leaves quickly and then try to flower early, the bed is hot and the plants are heading toward bolting. Either way, shift your next sowing accordingly.
Common questions
What is the last frost date for Asian greens in Wyoming?
At the Cheyenne reference station, the last spring frost falls between May 7 and May 18, based on the 30-70% band of 32°F freeze probabilities in the NOAA 1991-2020 Climate Normals. Hardy Asian greens go in before that date. Higher or colder gardens see later frosts.
Can I sow Asian greens in April in Wyoming?
Yes, once the soil can be worked and is no longer waterlogged. Quick leafy types such as mizuna, tatsoi and komatsuna tolerate cool soil and light frost. Cover the row on nights that dip below freezing, and expect later germination in cold soil.
When is the last day to sow Asian greens for fall in Wyoming?
Count back from the first fall frost, which at the reference station falls between September 24 and October 5. Take the packet’s days to maturity, add a couple of weeks for slowing growth, and subtract the total from September 24. For most leafy types that points to late July through mid-August, earlier at higher elevation.
Do I need row cover or a cold frame in Wyoming?
Plan on it. About 130 frost-free days at 6,000 feet leaves little margin, and frost is possible in any month at higher elevations. Row cover protects spring seedlings and extends the fall harvest, and a cold frame is the only realistic way to keep greens going into winter, when the coldest month averages a daily low of 18.1°F.