When to Sow Asian Greens in Maine

In Maine, sow Asian greens twice: outdoors from mid-April into May, then again from late July through August for a fall crop that finishes ahead of the first hard frost. The fall sowing is the better bet. Spring plantings race lengthening days toward bolting, while late-summer ones grow into cooler weather. The dates here are built on Portland’s frost data, the mild end of the state, so inland gardeners should treat them as a starting point.

The short answer for Maine

Sow hardy Asian greens such as mizuna, tatsoi, komatsuna and bok choy outdoors from mid-April to mid-May, then again from late July to late August. Both windows come from one set of numbers. At Portland, the NOAA 1991-2020 Climate Normals put the last spring frost between April 22 and May 1 and the first fall frost between October 9 and October 21.

Those are not guarantees. Each range is the 30-70% probability band for a 32°F freeze: on those dates, the odds of another freeze (spring) or of the first one arriving (fall) sit somewhere between roughly one in three and seven in ten. Think of them as a coin-flip zone, not a deadline. Asian greens tolerate light frost, so you can sow into that zone in spring. In fall you simply count backward from it.

The counting works like this. Baby leaves of mizuna or tatsoi are ready in about a month, while full heads of bok choy need roughly six to eight weeks, and napa cabbage longer. Fall growth also slows as days shorten, so add a cushion. Working back from an October 9 to 21 frost window lands you in late July through mid-August for heading types, with leafy types stretching into early September under row cover.

Treat the table as a planning draft built from the frost dates and typical days to maturity, not a tested Maine schedule. Your seed packet’s days-to-maturity number should override it. For help choosing which cultivars suit these windows, see The Best Asian Greens for Maine.

What will not work here

Overwintering outdoors fails. Portland’s mean daily minimum in the coldest month is 15.6°F, and that is an average of the nightly lows, so individual nights run well below it. Bok choy and pak choi are not built for that. Even the toughest mustards and tatsoi will not make it through a Maine January in an open bed. A cold frame or unheated hoop house stretches the harvest into early winter, but it is a season extender, not a winter crop.

Late-spring sowing of heading types fails too. Sow bok choy or napa in late May or June and the plants meet the longest days of the year, well over 15 hours of light, just as they size up. Many bolt before forming a decent head. Spring is for leaves you cut young, which is why the table above keeps napa out of that column.

The slow cultivars are the other casualty. Portland gets a little under six months between its last spring frost and first fall frost, and that is the generous end of the state. A napa cabbage that wants 70-plus days, planted in September because summer got away from you, will not head before the freeze. The same goes for any variety whose packet quotes a long maturity and assumes a warm autumn. Maine’s autumn does not stay warm.

A third trap is timing the spring sowing to the calendar instead of the soil. Seed sown into cold, waterlogged ground in early April may sit for weeks, and the seedlings that do emerge are exactly what flea beetles go after. These small beetles pit the leaves of every brassica, and Asian greens are brassicas. Row cover laid at sowing is the cheapest answer, and it also buys a few degrees of frost protection.

Finally, the hardiness zone is a poor guide to sowing dates for these crops. The USDA system measures only the average annual extreme minimum temperature. It tells you what survives winter, not how many frost-free weeks you have, and these are short-season annuals. Frost dates matter far more than the zone number.

What changes across Maine

There is no single Maine answer. The USDA Plant Hardiness Zone Map (2023 edition, based on 1991-2020 data) runs from zone 3b to 6a across the state. That spread covers the coast near Portland, the inland river valleys, the western mountains and the far north, and the planting calendar shifts along with it.

The Portland dates carry a warning label for good reason: the coastal city is the mild end. Inland and northern Maine run substantially later in spring and earlier in fall. For a gardener in the north or at elevation, that squeezes both windows from both ends. Spring sowing slides toward the middle or end of May, since the soil thaws late. The fall window moves earlier, with sowings in late July or the first days of August, because the frost date you are counting back from arrives sooner than October 9.

That squeeze favors some choices over others. Quick leafy greens such as mizuna, tatsoi and komatsuna fit short seasons far better than napa cabbage, which needs the long runway. Low tunnels or row cover, put on before the first frost rather than after, can extend a northern fall crop by a few weeks.

I will not tell you which zone you are in, because that depends on your exact address. The USDA map lets you enter a ZIP code and read off your own zone. A word of caution, though: use that result to judge perennials and overwintering, and use your local frost dates for sowing. If you are new to the whole group, Growing Asian Greens in Maine covers the basics that apply anywhere in the state.

Where to check this for your own county

A county recommendation beats a state one, and this page is the state one. Maine’s range from coastal to northern conditions is too wide for any single calendar to fit a particular garden. The place to narrow it is University of Maine Cooperative Extension, the state’s land-grant extension service.

When you get there, look for two things. First, a local planting calendar or frost-date guidance for your county, ideally one that names cool-season vegetables and brassicas. Second, any variety trial results for your area, which show which cultivars were actually grown and judged in Maine soil and weather rather than in a seed company’s test garden. County offices also field questions from gardeners, so a phone call or email about your specific site is often faster than searching.

This page does not have a Maine extension calendar written specifically for Asian greens, and I would rather say so than borrow one from another state. Extension calendars elsewhere are usually published per crop or per species, so a document for a different plant or region will not match Maine’s dates. Stick to Maine sources.

Checking this against your own garden

Your own observations outrank every date above, and three of them cost nothing.

• Soil warmth by feel. In spring, push your hand a few inches into the bed. If it feels bitterly cold or the soil clumps into a sodden ball, wait. A bed that crumbles and feels merely cool is ready for hardy greens. South-facing, raised or sandy spots will pass this test days or weeks before shaded, clay-heavy ones.
• The first true leaves. After the seed leaves come the first true leaves. If they appear quickly, the soil is warm enough and your timing is right. If seedlings stall for weeks, you sowed early or the spot runs cold.
• Where frost lands first. After the first frost of fall, walk the yard at dawn and note which areas are white and which are green. Low spots collect cold air and frost first. Slopes and spots near walls hold out longer, and that is where your last fall sowing belongs.

Keep notes for a year or two. A garden’s real frost dates often sit a week or more away from the nearest published normal, and a notebook will show you that faster than any map.

Common questions

Can I sow Asian greens indoors in March?

You can start them indoors a few weeks before the April 22 to May 1 window at Portland, but direct sowing usually works as well for leafy types. Seedlings started too early get leggy and can bolt after transplanting.

Is it too late to sow in September?

Not for baby leaves in much of southern and coastal Maine, especially under row cover. Heading bok choy and napa are better sown by mid-August. In northern or high-elevation gardens, the cutoff comes earlier.

Will Asian greens survive a Maine frost?

Light frost, yes. Many turn sweeter after it. A hard freeze is another matter, and with Portland’s first fall frost range at October 9 to October 21 (and earlier inland), cover or harvest before one is forecast.

Why did my spring bok choy go to seed?

Long days and warm spells trigger bolting. Sowing earlier in spring and harvesting young leaves usually avoids it, and the fall crop is far less prone to the problem.