In southern Ontario, Asian greens go in twice: a spring sowing once the soil has warmed, and a second one in late summer that finishes before the first hard frost. The Toronto Pearson station puts the average last spring frost on April 30 and the average first fall frost on October 16. Fall is the stronger of the two seasons, because bok choy, mizuna and their relatives grow best in cool, shortening days.
When the soil is warm enough in Ontario
What does the April 30 frost date tell you?
Environment and Climate Change Canada’s 1981-2010 normals for the Toronto Lester B. Pearson International station give April 30 as the average last spring frost. It is a midpoint. In about one year out of two the last frost lands later than that, and districts north of the lake or on higher ground can run weeks behind the airport. Check what your own area does before you trust the date.
Frost date and soil temperature are two different clocks. Air can stay above freezing while the ground is still cold and sodden from snowmelt, and the soil warms after the frost risk ends, not at the same moment. Seed sown into cold, wet ground sits there. It germinates unevenly, or it rots before it sprouts.
How can you tell the soil is ready?
A soil thermometer is the surest tool, but a few simple signs work too. Dig a handful from the top few inches. If it crumbles instead of smearing into a ball, the bed has drained enough to sow. If weeds are already up and growing steadily, the ground is awake. A test row sown a week ahead of the main planting will tell you whether seed is germinating in about a week or sulking.
Asian greens tolerate a light frost better than tomatoes ever will, so a sowing in early to mid spring is reasonable once the bed is workable. The real spring risk is different. A run of cold nights after the seedlings are up can push bok choy and napa-type cabbage toward bolting, meaning they throw up a flower stalk instead of leaves. Then longer days and warmer weather finish the job. Plan on spring crops being fast and small, harvested young.
These crops are brassicas, in the same family as cabbage, kale and mustard, and so are turnips and rutabagas. They share the same pests and diseases, so a bed that held any of them last year is not a fresh bed. Follow the crop rotation guide on this site and move them to ground that has been out of the cabbage family for a few seasons.
For named varieties that have been trialled in your part of the province, the Government of Ontario agriculture and food service is the right place to ask. A seed catalog can describe a variety, but only local trials show how it behaves in your soil and weather.
How long the season lasts here
How many frost-free days do you get?
The reference station has about 169 frost-free days between the mean last spring frost on April 30 and the mean first fall frost on October 16. Those are averages from the 1981-2010 normals, so a given year can start later or end earlier. A gardener in a cooler pocket should shorten the window accordingly.
What does a window that long allow?
It allows two plantings in the same bed. The first goes in as soon as the soil is workable, and it is cleared by early summer, before the heat of July and August makes these crops bolt and turn bitter. The second goes in from midsummer into early fall, when the days shorten and the nights cool. That second sowing is the more reliable one, since the plants mature as conditions improve instead of deteriorate.
To time it, count back from October 16. Take the days to maturity on the seed packet, which run from sowing for direct-sown crops, and add a couple of weeks because growth slows as the light fades in September and October. A sowing that cannot finish by then is better spent on baby leaves, which need only a few weeks. The succession sowing guide on this site lays out the spacing between plantings.
The length of the window also settles whether transplanting is worth the trouble. With this much season, direct sowing works for nearly everything, and seedlings that sit too long in a tray and get root-bound are more likely to bolt after planting. Transplants earn their place for napa-type cabbage started indoors for an early crop, or when a gap opens in a bed in midsummer. Check the roots before planting. They should be white and loosely holding the soil, not circling the cell.
Cooler growing regions face a similar calendar. Growing Asian Greens in Maine covers a place where the first sowing is often the only comfortable one and where fall timing is tighter still.
Heat and water in Ontario
Does summer heat matter for Asian greens?
Less than for most vegetables, because this family belongs to the second half of the year. Southern Ontario does get hot, humid spells in July and August, and those are the weeks when a spring-sown bed bolts. The answer is to schedule around them, not to fight them. Clear the spring crop before the hottest weeks, and sow the next one when the nights start to cool. Where a hot stretch lands on a growing crop, shade cloth and a layer of mulch keep the roots cooler.
The opposite problem is the more common one here. A cool, damp summer brings mildew on the leaves, slow ripening, and rot at the base of the plant, especially where leaves stay wet and plants are packed tight. Give each plant room for air to move, water at the soil line in the morning, and thin seedlings early. Slugs and flea beetles also come with wet, mild weather. Row cover over the bed from the day of sowing is the standard practice against flea beetles, and it works only if the edges are sealed to the ground.
Gardeners at the other extreme, in a tropical climate where heat is the main limit, face a different calendar, covered in Growing Asian Greens in Hawaii.
What does the first frost and the winter low mean for the crop?
The first fall frost, October 16 on average at the Pearson station, sets how late a sowing can still finish. Many Asian greens shrug off a light frost, and some taste better after one. A hard freeze is another matter. A killing freeze can end the season for the tender types the same week it arrives, so keep a sheet of row cover ready to throw over the bed.
The winter low decides what stands through the cold. The mean daily minimum of the coldest month at the Toronto reference station is -10.2 °C (13.6 °F). Natural Resources Canada places Toronto in Canadian hardiness zone 7a, a system built on several climate variables that does not convert to a USDA zone. At those temperatures even the hardiest types need a cover, such as a low tunnel or thick row cover, to carry on through a Toronto winter. Without one they end with the first hard freeze. A cover buys a few degrees, which is enough for leaves to survive a cold night in November, but it will not hold a crop through a deep January.
Which Asian greens suit Ontario
Which types fit a short, variable season?
Pick by type first. The earliest, fastest crops, such as baby bok choy, mizuna, tatsoi and other loose-leaf greens, suit a window that is long by northern standards but cut short at both ends by cold nights. They reach harvest quickly and still give a crop if a bad week knocks a sowing back. Heading types, like napa cabbage, need more time and are better placed in the second sowing, when the cool weather helps them form tight heads.
What helps in a humid or hot spell?
Where summers run hot, the useful traits are slow bolting and the ability to grow under airflow and shade at the root. Look for catalog descriptions that say bolt-resistant or slow to bolt, and plant them with space between rows. Where summers run damp, disease resistance is the first thing to check. Mildew and leaf spots spread fast in a crowded bed, and a resistant type saves more trouble than any spray.
The general guide to Asian greens covers how the family bolts and which sowing dates avoid it. For the specific varieties that have done well in Ontario trials, the Government of Ontario agriculture and food service is the source worth trusting, and no variety is named here for exactly that reason. Locally trialled recommendations carry real information, and a name copied from a list for another climate does not. Ask for the crops tested in your own region, since southern Ontario, the Ottawa Valley and the north behave like three different gardens.
Common questions
Can you sow Asian greens before the last frost?
You can, once the soil is workable, because the seedlings tolerate a light frost. Sow a small test row first. If a late cold snap arrives after the seedlings are up, expect some to bolt.
What is the best time for a fall crop?
Sow from midsummer into early fall, counting back from the mean first frost on October 16 at Toronto Pearson. Quick baby leaves can go in later than heading types. Keep the seedbed moist in warm weather so germination stays even.
Can Asian greens stay in the garden over winter?
Only the hardiest types, and only under a low tunnel or thick row cover. With a mean daily minimum of -10.2 °C (13.6 °F) in the coldest month at the Toronto reference station, uncovered plants usually end at the first hard freeze.
How do you check napa cabbage or other heads before storing them?
Look over the whole head and the cut stem. Firm, dry outer leaves and a clean base are the minimum. A soft patch, a weeping spot or a moldy crown ends the question, and that head should go to the compost, not the fridge. Do not trust a head just because it looks sound on the outside, and use stored heads promptly.