Sow Asian greens in Michigan twice a year: in early spring, a few weeks before the last frost, and again from late July through August for a fall harvest. Summer sowings mostly fail. The frost dates frame both windows, and the Detroit area and the rest of the Lower Peninsula sit a couple of weeks apart.
The short answer for Michigan
Two numbers anchor the calendar. MSU Extension puts the last spring frost around April 30 in the Detroit area, and early to mid May across most of the Lower Peninsula. For the first fall frost, MSU Extension’s Michigan Garden Calendar gives October 6 to October 21 for zone 6a. Treat these as probabilities of frost, not promises. They describe when frost typically stops or starts, so a cold night after the spring date, or before the fall one, still happens in some years.
Asian greens such as bok choy, tatsoi, mizuna and komatsuna are cool-season crops, and that works in your favor at both ends. Seedlings take a light frost better than a tomato transplant does. So the spring window opens before the last frost date, not after it, and the fall window closes against the first frost, with the sowing date set by how long each green needs to size up.
- Spring sowing: from early April, once the soil is workable, through the last-frost dates above (around April 30 near Detroit, early to mid May elsewhere in the Lower Peninsula). Aim to harvest before summer heat arrives.
- Fall sowing for full-size heads: late July to mid August, so slower crops like napa cabbage and bok choy have time before October 6 to 21.
- Fall sowing for baby leaves: through August, because mizuna, tatsoi and baby mustards can be cut young, often within about a month.
The spring and fall windows are not equal. Fall is the better season. Soil is warm for germination, days are shortening (which discourages bolting, the rush to flower that ruins leaf quality), and cooling nights sweeten the leaves. Gardeners who only try spring often decide Asian greens are fussy. Fall tends to change their mind.
For variety choices that suit these windows, see The Best Asian Greens for Michigan. For the full growing picture, start with Growing Asian Greens in Michigan.
What will not work here
Sowing bok choy in late May or June for a summer harvest is the classic miss. The plants hit the long days of June and the first hot spells, then send up flower stalks before the heads fatten. Spring sowings have a finish line, and it arrives sooner than most people expect. If your seed is still in the packet on Memorial Day, wait for fall.
The opposite mistake happens in September. With the first fall frost as early as October 6, a September sowing of anything that forms a head is a gamble. Napa cabbage often needs eight to ten weeks to form a tight head, which means counting back from October 6 lands you in late July. Growth also slows in October, when days are short and nights are cold, so the calendar days on the seed packet understate how long a fall crop really takes. A packet that says 50 days can mean closer to 60 in a cool October.
Overwintering outdoors is another plan that sounds better than it is. The reference station for Detroit shows a mean daily minimum of 19.2°F in the coldest month. That is an average of the nightly lows, not the record cold, so individual nights run lower. Tatsoi and some mustards can ride out a hard frost, but a winter of nights in the teens will finish most unprotected Asian greens. A low tunnel or cold frame can change that, though treat any winter harvest from it as a bonus, not a plan.
Last, trusting the average date too literally. If you sow tender transplants on April 30 because the Detroit-area average says so, you are betting that this year is typical. Seeds can wait under the soil through a cold snap, but seedlings under a clear sky at 28°F need a sheet or row cover. The lesson of a frost-date average is that some years fall outside it, so keep a cover handy through mid May.
One more caution about the cold figures. The 19.2°F number and the hardiness zone both describe winter survival. They do not decide when to sow an annual leafy green. Frost dates and day length do that.
What changes across Michigan
Michigan is not one garden. The USDA Plant Hardiness Zone Map (2023, built on 1991-2020 normals) spans 4a to 6b across the state, a spread of nine steps if you count half zones. Zone is built on a single variable, the average annual extreme minimum temperature, so it measures the winter cold a plant must survive. There is no single answer for the whole state, and this page does not pretend otherwise.
The frost dates here come with limits too. The April 30 date is for the Detroit area, and the early to mid May range covers most of the Lower Peninsula. The October 6 to 21 window is for zone 6a. This page does not have frost dates for the Upper Peninsula or the far northern Lower Peninsula, and it will not borrow a neighbor’s numbers to fill the gap.
What you can say is the direction of the shift. A colder zone means a later last spring frost and an earlier first fall frost, which squeezes the season from both sides. A gardener in the north should expect to push spring sowing later than the Detroit dates, and to move the fall sowings earlier, so heads have time to size up. The fall window likely closes first, so count back from your own first frost, not the 6a dates above. How many days to shift depends on your location, which is why a local source matters (see below).
Gardeners in the colder zones may find fast, small crops easier to rely on than the slowest ones. Baby leaf mizuna and tatsoi, or a quick pak choi, fit a short season better than a full napa head. Row cover can buy extra days at either end.
Not sure where you fall on that 4a to 6b range? Look up your location on the USDA Plant Hardiness Zone Map and check it there rather than guessing from this page.
Where to check this for your own county
The state dates are a starting point. The next step is Michigan State University Extension, Michigan’s land-grant extension service, which keeps the frost-free date information used in this article and offers local offices that know your county’s soil and weather.
When you reach it, look for two things. First, the local planting calendar or frost-free dates for your county, which are more precise than a statewide range. Second, the vegetable varieties that have been trialled in your county or nearby, since a variety that performed in the Detroit area may not suit a colder part of the state. Extension offices can also answer questions about local pests such as flea beetles, which chew small holes in young brassica leaves.
A county recommendation beats a state one. This page is the state one: it gives a defensible window across a large and varied place, and it cannot see your valley, your lakeshore or your low spot where frost settles. If MSU Extension’s county advice and this page disagree, go with the county.
Extension offices publish calendars by crop and by region, and the guidance is not interchangeable. A calendar written for a different crop group, or a different state, will not match what Asian greens need in Michigan. Read the one written for cool-season vegetables in your area.
Checking this against your own garden
Your own observation outranks every date above. Three things are worth watching, and none needs an instrument.
Soil warmth by feel. Push your hand into the bed a few inches deep. If it feels cold and clammy, germination will be slow and seeds may rot. If it feels cool but comfortable, spring greens will sprout. A bed that warms early tells you that your garden runs ahead of the county average, while a shaded or low bed that stays cold tells you the opposite.
The first true leaves. After the seed leaves open, the next pair is the first true leaves. Note the date you sowed and the date they appear. A quick appearance means your site is warm enough for the crop. A long delay in spring points to cold soil or shade. In fall, a slow one warns that the season is running out faster than the packet suggests.
The first flower stalk. When one plant in a spring sowing starts to bolt, write down the date. That is your garden’s own cutoff for spring sowing, and next year’s sowing can come earlier or move to a cooler spot. Do the same in fall with the first frost-damaged leaf: that is your garden’s real first frost, which may differ from the 6a window.
A notebook with a few dates for two seasons will do more for your calendar than any statewide table.
Common questions
Can I sow Asian greens in March in Michigan?
Only under cover. The soil is usually too cold and wet outdoors, and the last frost is still weeks away (around April 30 in the Detroit area, early to mid May across most of the Lower Peninsula). A cold frame or low tunnel gives hardy greens a head start, but start seeds indoors for anything tender.
What is the last date to sow bok choy in fall?
Count back from your first frost. The first fall frost for zone 6a runs October 6 to October 21 in the Michigan Garden Calendar. Bok choy sized for full heads needs roughly six to eight weeks, so mid August is a safe limit in most places. Baby leaves can go in later.
Is the hardiness zone enough to plan my sowing?
No. The USDA zone describes the average annual extreme minimum temperature, which decides whether a perennial survives winter. Annual greens depend on frost dates and day length instead. Michigan spans zones 4a to 6b, so check yours on the USDA map and then use your local frost dates.
Do I need row cover?
It is a cheap insurance policy. A light fabric cover protects spring seedlings from a late frost, keeps flea beetles off, and can stretch a fall crop past the first frost. It will not save plants from a hard freeze, though. Michigan’s coldest-month average minimum of 19.2°F in Detroit shows how quickly the season ends.