Ontario gardeners sow Asian greens twice a year. The spring window opens a few weeks before the average last frost of April 30. The fall window runs from late July through August, so the crop is nearly done by the average first frost of October 16. Midsummer sowings tend to bolt, so the gap between the two windows is deliberate. Both dates come from the Toronto airport station, and gardens farther north run later.
The short answer for Ontario
Sow hardy Asian greens such as bok choy, tatsoi, mizuna and komatsuna from early April, as soon as the soil can be worked, and keep sowing into May. Then stop for the hottest weeks and start again in late July for a fall crop. Environment and Climate Change Canada puts the average last spring frost at April 30 and the average first fall frost at October 16, based on the 1981-2010 normals for Toronto Lester B. Pearson International Airport. The reference station has a frost-free period between those two dates.
Both dates are averages. In one year out of two, the last frost falls later than April 30. In one year out of two, the first frost arrives earlier than October 16. Treat each date as the middle of a window, not a safe line. Most Asian greens shrug off a light frost, but a hard freeze after seedlings are up can push some of them to bolt early. Cover rows with a sheet or row cover on nights that look clear and cold.
When do you sow Asian greens in spring in Ontario?
Early April through May works for the fast, cool-season types. Seeds germinate in cold soil, just slowly, and plants grown through April and May get the short days and cool nights they like. The goal is to harvest before the long days of late June, which are the main trigger for bolting.
When do you sow them for fall?
Count back from October 16. Take the days to maturity on the seed packet, which count from sowing for a direct-sown crop, and add a couple of weeks, because growth slows as days shorten and nights cool. Fast baby greens can go in well into August. Napa cabbage and other heading types need the earliest dates, late July or very early August. Fall is the better season for most of these crops in Ontario, since the plants mature as the weather cools and are not racing the heat.
What will not work here
Sowing in the heat of June and July is the most common failure. Bok choy, napa cabbage and most Chinese mustards respond to long days and warm nights by sending up a flower stalk before they make a useful plant. Ontario summers are also humid, so what does grow in July often faces leaf spots and rot. Close spacing makes that worse, because wet leaves stay wet. Give full-size heads room for air to move, and thin baby greens early rather than letting them crowd.
Late sowing of slow crops fails just as reliably. A napa cabbage sown at the end of August will not form a firm head before an average October 16 frost, and the frost comes sooner in one year out of two. If you want to grow slower types, the choice of variety matters more than the sowing date. The Best Asian Greens for Ontario goes through which types fit a season of this length.
Spring brings its own problem. Flea beetles chew small holes in young mustard-family seedlings from the first warm days of May, and a seedling with a few leaves has little to spare. Row cover laid right after sowing keeps them off. It also takes the edge off a late frost, which makes it the cheapest insurance in the garden.
Outdoor overwintering is another mistake. The mean daily minimum of the coldest month at the reference station is -10.2 °C. That is an average, so individual nights run colder. A few hardy types like tatsoi can survive under cover with some leaf damage, but most Asian greens will not come through an Ontario winter in open ground. Plan to harvest them in fall, and treat anything that survives as a bonus.
What changes across Ontario
The zone shown for Toronto, 7a, is the Canadian plant hardiness zone of the reference city from Natural Resources Canada, based on 1991-2020 normals. It is not a USDA zone, and the two systems do not convert. Ontario stretches from the Lake Erie shore to the far north, and the zone number falls by several full zones along the way, so there is no single answer for the province.
For a gardener north of the Greater Toronto Area, or on higher ground inland, the practical shift is simple. Spring sowing opens weeks later than early April, so wait until the soil is workable and no longer waterlogged. The fall window closes earlier, which means sowing for fall moves up toward the start of the July-to-August range. Stick to fast, hardy types, and treat the heading types as a gamble. Row cover goes from nice-to-have to standard equipment.
Near the Great Lakes, the water holds heat into fall and can delay the first frost, though it also keeps spring cooler for longer. In a low spot or a frost pocket, cold air settles and the season shortens by days even within one town. If you want to know the zone for your own municipality, look it up on the Canada’s Plant Hardiness Site map instead of guessing from the Toronto number. For a broader look at crop choices and growing methods, see Growing Asian Greens in Ontario.
Where to check this for your own county
A recommendation written for your county beats one written for the province, and this article is the provincial one. Start with the Government of Ontario’s agriculture and food pages, which are the province’s official source for growing information. Look there for any local planting calendar, and for notes on which varieties have been trialled in your area. A calendar built from nearby frost records will shift the dates above by a week or two in either direction.
What to look for matters. First, the average last spring frost and first fall frost for the station nearest you, not for Toronto. Second, any list of leafy brassica varieties trialled near you, with notes on bolting and disease. Third, guidance on pests such as flea beetles and cabbage root maggot, because their timing varies by region and decides when row cover has to go on.
If you cannot find a county-level answer, ask at a local garden centre or a gardening club. People who sow in your soil every year know when the ground warms up in your neighbourhood. The frost normals used here also cover 1981-2010, so recent warm springs may have moved the dates a little. Local experience will show you whether that has happened.
Checking this against your own garden
Your own observations outrank any date in this article. Three things are worth watching, and none needs equipment.
- Soil by feel. Squeeze a handful from a few inches down. If it clumps into a cold, sodden ball, wait. If it crumbles and feels cool rather than icy, seeds will germinate, if slowly. Test beds on different sides of the house, because a south-facing wall may be ready a week before the open lawn.
- First true leaves. The first pair of leaves a seedling shows are seed leaves. The real, shaped leaves come next. If seedlings are slow to reach that stage, the bed is cooler than you thought. Slow seedlings are the signal to use row cover or hold off on the next sowing.
- The first frost in your lowest corner. Look at which plants blacken first in fall. That shows where cold air pools in your yard, and where fall crops will be hit earliest.
Write the dates down each year. After two or three seasons you will have a calendar that fits your own yard better than a station average from another city.
Common questions
Can I sow Asian greens in Ontario before the last frost?
Yes, for hardy types. Bok choy, tatsoi, mizuna and komatsuna germinate in cool soil and tolerate light frost, so sowing starts in early April once the ground is workable. Because the April 30 average is only the middle of the window, keep row cover handy for late cold snaps.
What is the last date to sow Asian greens in Ontario for fall?
Count back from the average first fall frost of October 16 using the days to maturity on the packet, then add a couple of weeks for slowing growth. Fast baby greens can go in during August, but heading types such as napa cabbage need late July or very early August. Gardens farther north need earlier dates.
Will Asian greens survive an Ontario winter outside?
Most will not. The mean daily minimum of the coldest month is -10.2 °C at the reference station, and individual nights go colder. A few hardy types may survive under cover with some damage, but plan on harvesting in fall.
Does the Toronto frost date apply to my town?
Only roughly. The April 30 and October 16 dates come from the Toronto airport station, and northern or higher-elevation gardens run weeks behind. Use the nearest local frost data you can find, and adjust with your own soil and seedling observations.