Manitoba gardeners get two good windows for Asian greens, not one: a spring sowing timed around the average last frost of May 23, and a late-summer sowing counted back from the average first frost of September 22. The hot, long-day stretch between them is when bok choy and its relatives bolt. Both dates come from Winnipeg, so they mark the middle of a window, not a guarantee.
The short answer for Manitoba
Sow the hardy, fast types outdoors as soon as the soil can be worked, and keep frost cloth handy until well after May 23. Sow the second round from late July to about mid-August, so the plants mature before and just after the average first frost on September 22. Those two rounds fit most of what the Growing Asian Greens in Manitoba guide covers.
The two dates deserve a careful read. Environment and Climate Change Canada puts the last spring frost at May 23 on average and the first fall frost at September 22 on average, from the 1981-2010 normals at the Winnipeg Richardson International Airport station. A mean date is not a safe date. In one year out of two, the last frost falls later than May 23, and in about half of years the first fall frost arrives earlier than September 22. Treat each as the midpoint of a spread, not a line on the calendar. Also note that Canadian normals cover 1981-2010, while the US figures elsewhere on this site cover 1991-2020, so the two aren’t a clean side-by-side.
Between the two dates sits a frost-free stretch of roughly four months. That sounds generous until you subtract the weeks when cold soil stalls germination in spring and the weeks when shortening days slow growth in fall. Asian greens need only a fraction of it, which is why they suit this climate better than tomatoes or peppers.
| Round | Sowing window | Anchored to | Suits |
|---|---|---|---|
| Spring | Once soil is workable, through early June | Average last frost, May 23 (half of years see frost after it) | Mizuna, tatsoi, komatsuna, baby bok choy |
| Midsummer gap | Mid-June to mid-July | Long days and heat | Mostly skip; shade and daily watering only |
| Fall | Late July to mid-August | Average first frost, September 22 | Full bok choy, napa (early), mizuna, tatsoi, radishes |
The fall dates come from simple counting. Full-size bok choy takes roughly 45 to 60 days, so working back from September 22 lands you in late July or early August. Baby leaves for cutting need about 25 to 30 days, which stretches the window into mid-August. Light frost doesn’t kill most of these plants, so a few days past the average date is fine, but don’t build a plan around it. Variety choices are in The Best Asian Greens for Manitoba.
What will not work here
Napa cabbage sown in spring is the classic Manitoba failure. It needs a long run to form a dense head, and a May sowing puts the heading stage right into the longest days of the year. The plant reads that light as a signal to flower and sends up a seed stalk instead. The same goes for most heading Chinese cabbages, and for any mustard green that has to sit through late June.
Sowing on May 23 and assuming you’re clear is the second trap. Since the date is a mean, frost after it is a coin flip. A bok choy seedling can handle a light frost but not a long cold snap; extended chilly weather during early growth can push it to bolt later, even if the plant survives. Spring sowings that came up fine and then produced flower stalks by midsummer usually went through exactly that. Row cover is cheap insurance, and it doubles as protection against flea beetles, which chew shot-holes in crucifer seedlings across the Prairies in spring.
Slow, big-root crops run into the short season from the other direction. Long daikon types want two months or more of steady growth. Sown in mid-August, they run out of warmth and light before they size up, and the first frost on a Winnipeg-style calendar can arrive close to September 22 with a hard freeze not far behind. Anything that needs well over 60 days from sowing should go in by mid-July at the latest, and even then it’s a gamble.
Midsummer sowings in the gap are the quiet disappointment. Seed germinates fast in warm soil, then the plants sulk. Heat and very long days push them toward bolting while the soil dries out faster than a weekly soaking can keep up with. If you must sow in July, pick bolt-resistant, fast types, shade the bed with cloth in the afternoon, and water lightly every day or two instead of heavily once a week.
Last, don’t count on winter to do anything. The mean daily minimum of the coldest month at the Winnipeg reference station is -21.4 °C (about -6.6 °F), far too cold for any Asian green to overwinter in the open. Everything here is an annual crop, and the soil thaws late in spring, which is part of why that first window stays short.
What changes across Manitoba
Manitoba is not one garden. The hardiness number this page has is for the Winnipeg reference city: zone 3b in the Canadian system, from Natural Resources Canada’s plant hardiness site, based on 1991-2020 normals. It is the zone of that city, not of the whole province, and this page does not hold figures for other districts. So there is no single answer for Manitoba, and the real spread is wider than one number can show.
That zone also can’t be read as a USDA zone. The Canadian system draws on seven climate variables, and the USDA on one. Natural Resources Canada itself says there are no simple conversions, and that Canadian zone 4a overlaps nine USDA zones, from 3a to 7a. If you’re comparing against a US seed packet or calendar, treat the zone as a rough hint only.
What matters for sowing is that northern and upland districts run weeks behind the Winnipeg dates. For gardeners there, the practical shifts are straightforward:
- Start the spring sowing later and lean harder on frost cover.
- Move the fall sowing earlier, because the first frost arrives sooner than September 22.
- Choose the fastest types, such as baby leaves and mizuna, over full heads.
Don’t assume which zone applies to your own garden. Look up your municipality on the map. A garden near the Red River Valley floor can behave differently from one on higher ground only a short drive away, and a south-facing wall or a shelterbelt can move your frost dates by more than a week in either direction.
Where to check this for your own county
The best local source is Manitoba Agriculture, the province’s agriculture department. Look there for two things. The first is a planting calendar for your region of the province, ideally one written for vegetables and not field crops. The second is any list of varieties that have been trialled near you, since a bok choy that performs in one district can bolt in another.
A caution: this page does not have a published Manitoba calendar for Asian greens, and it won’t pretend otherwise. If the provincial site doesn’t carry one, ask a regional agricultural office or a local garden club that keeps frost records, which are often better than any website.
In Manitoba, read “county” as your rural municipality or district. A recommendation from that level beats anything written for the whole province, because it reflects local frost pockets, soil type and the varieties that have actually been grown nearby. This page is the provincial view, built from a single reference station and one set of normals. Use it to plan the shape of your year, then let the local source set the actual dates.
Checking this against your own garden
Your own observations outrank every date above. Three are worth watching, and none needs equipment.
Soil by feel. Push a bare hand into the bed in spring. If it still aches with cold after a few seconds, or the soil clumps into a wet ball, wait. Beds that warm and crumble early are your sheltered, well-drained spots, and you can sow there first.
The first true leaves. Seedlings open their seed leaves first, then the true leaves. If they stall at the seed-leaf stage for more than a week or so, your soil or air is running colder than the calendar assumed. Note the date the true leaves appear, and next year you’ll know how long germination takes in your own beds.
Frost marks in the fall. After the first light frost, walk the garden. Blackened leaves in the low corner while plants by the house wall stay green show you cold air pooling, which tells you where fall greens will last longest and where they’ll go first. A few notes each year beat any average.
Common questions
Can I sow Asian greens before May 23?
Yes, for the hardy types, once the soil is workable. Because the date is only a mean, frost can come after it in about half of years, so cover seedlings when a cold night is forecast.
Is September 22 the day to stop?
No. It is the average first frost at the Winnipeg station, so it falls earlier in roughly half of years. Most Asian greens tolerate a light frost, but count your sowing back from it rather than hoping to beat it.
Can I sow in midsummer?
You can, but results are mixed. Long days and heat push many types to bolt, so choose fast, bolt-resistant varieties, shade them, and water often. The late July to mid-August sowing is the more dependable second round.
Does zone 3b tell me what to sow?
Not by itself. It is the Canadian zone of the Winnipeg reference city, it doesn’t convert to a USDA zone, and your own district may differ. Use the Natural Resources Canada map and your local frost experience instead.