Sow hardy Asian greens like bok choy, mizuna and tatsoi outdoors in Minnesota as soon as the soil can be worked in spring, then again from midsummer so they mature before the first fall freeze. In the Twin Cities, the last spring frost falls between April 17 and April 30, and the first fall frost between October 11 and October 23. Northern gardens shift later in spring and earlier in fall.
The short answer for Minnesota
Those date ranges are probabilities, not promises. They come from NOAA’s 1991-2020 Climate Normals for the Minneapolis station, and they mark the span where the odds of a 32°F freeze sit between 30 and 70 percent. In spring, that means a freeze is still a real possibility on April 17 and growing unlikely by April 30. In fall, it means a freeze has become roughly a coin flip somewhere between October 11 and October 23. Some years the frost comes earlier or later than either end.
Spring sowing
Mizuna, tatsoi, komatsuna and bok choy are cool-season crops. Once up, they shrug off a light frost, so you do not need to wait for the last-frost window to close. Sow when the soil is workable, which in the Twin Cities lands in the same weeks as that April 17 to April 30 range. Then sow a small batch every week or two. Spring is a race against lengthening days and warming weather, and the early rows are the ones that reach harvest before the plants decide to bolt.
Fall sowing
Fall is where the calendar needs arithmetic. Take the days to maturity printed on the seed packet and count back from October 11, then add a cushion, because shrinking days slow growth. A variety with a 45-day packet count lands on August 27 before any cushion. That is why the fall sowing season starts in the dog days of summer, not after Labor Day. For a fuller look at which types suit this schedule, see The Best Asian Greens for Minnesota.
What will not work here
Winter survival
Overwintering in open ground is out. The mean daily minimum of Minneapolis’s coldest month is 8.8°F, and that is an average, not the worst night. Minnesota spans USDA hardiness zones 3a to 5a, a system built on the average annual extreme minimum temperature. Asian greens are grown as annuals here, and even the toughest ones end their season in a hard freeze. A late-fall sowing meant to “sit through winter” and wake up in March will simply be gone.
Calendars and varieties that do not fit
Southern planting calendars are the next trap. Advice from states where fall sowing runs through September or October assumes a freeze that arrives months later than Minnesota’s October 11 to October 23 window. Follow it here and the seedlings will still be small when the ground freezes.
Slow, heading types are the other casualty. A napa cabbage with a long packet count sown in late spring has to form a head before long days push it to bolt. Sown late in summer, it has to finish before the freeze with shortening days working against it. Either way, the window is narrow. Fast, loose-leaf greens are far safer bets, and baby-leaf harvests forgive a short season in ways that full heads do not.
Finally, sowing by the date alone fails. The April 17 start of the frost window says nothing about whether the ground is thawed and drained. Seed pressed into cold, saturated soil can rot before it sprouts. And midsummer sowings for fall go into warm, drying ground, so a seedbed watered only now and then will leave patchy rows. For the bigger picture of what suits the state, see Growing Asian Greens in Minnesota.
What changes across Minnesota
Minnesota has no single answer. The USDA Plant Hardiness Zone Map (2023 edition, built on 1991-2020 normals) places the state between zones 3a and 5a, a spread of several zones inside one set of borders. The dates on this page use the Twin Cities as the reference city, and the Twin Cities are the mild south. Northern Minnesota runs two to three weeks tighter at both ends of the season.
Applied to the calendar, that means pushing spring sowing back by two to three weeks, to roughly early or mid May, and pulling the fall deadline forward to late September or early October. Those are shifts from the Twin Cities numbers, not published dates for any northern town. Because the frost-free window is shorter, the weight moves toward fast crops. Mizuna, tatsoi and baby bok choy fit; heading napa mostly does not.
Keep in mind what the zone does and does not measure. It reflects winter cold, the single coldest-night variable, not the length of your growing season. Two gardens in the same zone can still differ by weeks in spring thaw. To see where your own place falls, use the USDA map itself and enter your ZIP code. This page cannot place you on it.
Where to check this for your own county
The best next stop is University of Minnesota Extension, the state’s land-grant extension service. It has horticulture staff and educators who work with growers in your region, and its guidance is written for Minnesota soils and seasons rather than for the country as a whole.
Look for two things there. First, a local planting calendar for your county or region, with frost dates tied to a nearby weather station instead of the Twin Cities. Second, any list of vegetable varieties trialled in your county, since a variety that performs in the south of the state can bolt or stall in the north. This page does not have a University of Minnesota calendar for Asian greens specifically, so I am not quoting one. If extension has published one, it outranks anything here.
A county recommendation beats a state one, and this page is the state one. It rests on a single reference station, Minneapolis, and a probability band, not on your lot. Treat it as a way to decide when to start looking, then let the local source and your own observations set the actual date.
Checking this against your own garden
Your own observations outrank every date above. Three are worth watching, and none needs a gadget.
- Soil warmth by feel. Dig down a few inches in the bed you plan to sow. Soil that crumbles in your hand and feels cool but not icy is ready for hardy greens. A bed that stays soggy and cold while a neighbor’s is workable tells you your site is slower, perhaps shaded or low and wet. Adjust your start date accordingly.
- The first male flowers on nearby trees. Catkins on birch, hazel or aspen, or the first maple blossoms, signal that spring has turned in your area. If they open later than in your neighbor’s yard, your microclimate runs behind the regional average.
- The first true leaves on a test row. Sow a short row early. The seed leaves come first; the true leaves that follow show real growth. Slow true leaves mean a cool spot. Fast ones mean a sheltered bed that may bolt sooner in warm weather.
Write down the dates for each. After a season or two, you will have a calendar tuned to your yard, which no state page can supply.
Common questions
Can I sow Asian greens before the last frost?
Hardy types, yes. Mizuna, tatsoi, komatsuna and bok choy tolerate light frost once up. The Twin Cities last-frost window is April 17 to April 30, but sow only when the soil is workable, not simply when the date arrives.
Is it too late to sow in September?
In the Twin Cities, the first fall frost falls between October 11 and October 23. A September sowing leaves little time, so only the fastest baby-leaf types are worth trying. In northern Minnesota, where fall comes two to three weeks sooner, it is likely too late.
Will Asian greens survive a Minnesota winter?
No. Minneapolis’s coldest-month mean daily minimum is 8.8°F, and the state sits in USDA zones 3a to 5a. Treat Asian greens as spring and fall annuals.
Should I start seeds indoors or sow them outdoors?
Most of the fast leafy types do well sown directly in the garden. Indoor starts can help with slower, heading types, but they still face the same short window. Check your county’s extension guidance for local recommendations.