Sow hardy Asian greens such as bok choy, tatsoi and mizuna in New Brunswick from early May, then again from late July into mid-August for the fall crop. Both windows hang on one pair of numbers: an average last spring frost of May 23 and an average first fall frost of October 2, both measured at Moncton. They are averages, not promises. Roughly half of all years break them.
The short answer for New Brunswick
The spring window opens about two weeks before May 23 and closes before the long days of mid-June. The fall window opens at the end of July and runs to about the second week of August for most crops. Between them lies a stretch where many Asian greens bolt, the term for sending up a flower stalk before the leaves are worth eating.
What those dates measure matters. May 23 is the mean date of the last spring frost at the Moncton station, according to Environment and Climate Change Canada. It is the midpoint of a spread, not a safe date. In about one year out of two, frost arrives after it. The October 2 fall date works the other way: in about one year out of two, the first frost comes before it. Between the two averages sit roughly 19 weeks, and that is the whole frost-free budget a Moncton-area gardener starts with. Note too that the Canadian normals cover 1981-2010, while the US figures elsewhere on this site cover 1991-2020.
The windows below are rules of thumb I derived by counting from those two dates and from typical days to harvest. They are not figures measured in New Brunswick trials.
| Type | Spring sowing | Fall sowing |
|---|---|---|
| Baby-leaf mizuna, tatsoi, komatsuna (about 3 to 4 weeks to cut) | Early May to early June | Early August to early September |
| Bok choy and other pak choi (about 6 to 7 weeks) | Early to late May, bolt-resistant types only | Late July to mid-August |
| Napa cabbage (about 2 months) | Skip it | Late July to early August |
Napa sets the earliest fall deadline. Count back about two months from October 2 and you land in the first days of August. Baby leaves are more forgiving because you can cut them young, and they shrug off a light frost.
For variety choices within each type, see The Best Asian Greens for New Brunswick. The broader picture is in Growing Asian Greens in New Brunswick.
What will not work here
A June sowing of bok choy or napa is the classic failure. These plants read lengthening days and warming nights as a signal to flower, and in New Brunswick the longest days arrive just as seedlings from a late-May sowing are getting going. You get a plant that bolts at half size. The fix is to sow the same crop in August, when days are shortening and the plant has no reason to bolt.
Waiting for a “safe” spring date is the second mistake. If you hold off until May 23 passes, you have already waited too long for the cool-weather types, and you have not actually bought safety, since frost lands after that date in about half of years. Hardy seedlings take a light frost. Tender crops planted on the strength of that average do not.
Slow, long-season types also run out of road. Anything that needs well over two months from sowing to harvest has to be sown early enough to finish before October 2, and the plants have to cope with the colder nights that come with September. A heading napa started in mid-August will often be a sprawl of loose leaves when the first hard frost arrives.
Overwintering outdoors is off the table too. The mean daily minimum of the coldest month at Moncton is -13.9 °C (6.9 °F). That is the average, so individual nights run colder. Bok choy and its relatives will not sit through a New Brunswick winter in an open bed. A low tunnel or cold frame can stretch the harvest into November, but plan on it ending, not on spring regrowth.
Finally, fall sowings can fail in a different way: dry soil. Late July and August seed beds dry out quickly, and brassica seed that sprouts and then dries once will stall. Keep the surface evenly moist for the first week, shade the bed with a board or floating row cover until germination, and flea beetles (small jumping insects that riddle seedling leaves with holes) will be the next thing to manage. A light row cover handles both problems.
What changes across New Brunswick
Moncton is one reference point. NRCan lists its Canadian hardiness zone as 5a (1991-2020 normals). That is the Canadian system, built on seven climate variables, and it does not convert to a USDA zone. NRCan says plainly that there are no simple conversions between the two. It also covers only the reference city, not the province.
This page does not have zone figures for other New Brunswick districts, so it cannot hand you a province-wide spread. What it can say is that there is no single answer. The Moncton frost dates are the central case, and the caution attached to them is blunt: northern and upland districts run weeks behind that station. A garden in the north or on higher ground should shift the whole calendar later in spring and earlier in fall.
In practice that means two moves. In spring, push the first sowing back by a week or two and rely on the fastest baby-leaf types. In fall, pull every deadline forward, so napa goes in at the very start of the window, or earlier, and the leafy, quick types take over the later sowings. Coastal gardens sometimes lag in spring and hold on longer into fall, but this page has no figures to quantify that, so treat it as a local question.
Look up your own zone rather than guessing it. NRCan’s Canada’s Plant Hardiness Site lets you search by municipality. Read the zone it gives for your town and treat the Moncton dates as the baseline to adjust from.
Where to check this for your own county
A county recommendation beats a state one, and this page is the provincial one. It rests on a single station’s normals. Your own district may differ by weeks.
New Brunswick does not run county extension offices the way a US state does, and the provincial department’s site blocked our automated checks. We link the federal department, Agriculture and Agri-Food Canada, because that link was confirmed working. From there, look for provincial agriculture and horticulture contacts and any regional crop publications.
Once you reach a local source, you want two things. First, a planting calendar for your own district, with its own last-spring and first-fall frost dates rather than Moncton’s. Second, a list of the Asian greens actually trialled near you, since a variety that thrives in a milder valley can bolt on a cold plateau. If a published trial lists days to harvest, compare it against your fall deadline rather than the seed packet.
This page also does not have a New Brunswick-specific Asian greens calendar from an extension service to cite. The extension calendar we hold is a Clemson lawn guide for tall fescue, a different crop in a different climate, so we leave it out rather than borrow it. If your local source and this page disagree, go with the local source.
Checking this against your own garden
Your own observations outrank every date above. Three are worth watching, and none needs equipment.
Start with soil warmth by feel. In spring, push your bare hand into the top few inches of a bed. If it feels cold enough to numb your fingers after a minute, wait. Cool-season seed germinates in cool soil, but sodden, near-frozen ground rots it. A bed that warms to a mild, neutral feeling a week or more before the neighbor’s tells you your site is early, probably sheltered or well drained. A bed that stays cold in late May is telling you to shift everything later.
Next, watch the first true leaves. The seed leaves that appear first are rounded and plain. The true leaves that follow look like the mature plant. Note how many days your seedlings take to reach that stage in spring and again in August. Faster in August means warm soil is helping, and you can sow the fall crop a few days later. Slower means move your deadlines earlier.
Last, watch for the first flower bud, a tight cluster in the center of the plant. If your spring bok choy shows buds at half size, your site runs warm or your sowing ran too late into long days. Note the date and sow earlier next year, or switch to bolt-resistant types. Keep a few lines in a notebook each year. After two seasons, your own record will predict your garden better than any provincial average.
Common questions
Can I sow Asian greens before the last spring frost in New Brunswick?
Yes, for the hardy types. Mizuna, tatsoi, komatsuna and mustard greens tolerate light frost, so sowing a couple of weeks before the May 23 average at Moncton is reasonable once the soil can be worked. Sowing before that date carries some risk of a hard frost, so cover seedlings if a cold night is forecast.
What is the latest I can sow for a fall harvest?
It depends on the crop. Napa cabbage needs about two months, so the first days of August is the practical limit counting back from the October 2 average first frost. Bok choy can go in until mid-August. Baby-leaf greens can be sown into early September. These are rules of thumb, and northern or upland gardens should pull them earlier.
Does May 23 mean I am safe from frost after that date?
No. It is a mean date from Environment and Climate Change Canada for the Moncton station (1981-2010 normals). In about one year out of two, the last spring frost comes after it. Treat it as the middle of a window and protect tender seedlings accordingly.
Can I overwinter bok choy outside in New Brunswick?
Not in an open bed. The mean daily minimum of the coldest month at Moncton is -13.9 °C (6.9 °F), and individual nights run colder. A cold frame or low tunnel can extend the harvest into late fall, but do not expect plants to survive the winter and regrow in spring.