When to Sow Asian Greens in Washington

In western Washington, Asian greens go in during two windows: late March through April, and again from mid-July to early September. The second one is the better bet. East of the Cascades the season is much shorter, so the dates below need adjusting.

The short answer for Washington

The frost dates frame both windows. At the Seattle reference station, the last spring frost falls between February 27 and March 18, and the first fall frost between November 13 and November 30. Those dates come from NOAA’s 1991-2020 climate normals and mark the span where the chance of a 32°F freeze sits between 30 and 70 percent. A date inside that span means roughly even odds of a freeze, so treat the whole range as uncertain ground.

Winters are mild on the maritime side. The mean daily minimum of the coldest month is 37.1°F, which is above freezing on a typical night. That explains why hardy Asian greens can sit in the ground well into winter west of the mountains. The planting dates, though, are set by soil and light more than by frost.

When can you sow Asian greens outdoors in spring in Washington?

Late March through April is the working window in the western lowlands, even though the last freeze can come as early as the end of February. Cold, wet soil and low light hold spring crops back long after the freeze danger ends. Seed sown into soggy ground in February tends to rot or sit idle. Wait until a handful of soil crumbles in your fingers instead of smearing, and until the bed has had a few mild, drier days in a row. Cloches or row cover let you start a couple of weeks sooner on a well-drained bed.

Spring sowings are a race against lengthening days. Greens that reach harvest size quickly, such as mizuna, tatsoi and baby bok choy, beat the bolting that comes with the long days of late spring.

When should you sow Asian greens for fall in Washington?

Mid-July through August is the sweet spot for most types, with the quickest leaf greens still workable into early September. Seedlings get falling day length and cooling nights, which suits these crops. With the first fall frost not arriving until November 13 to November 30, plants have months to size up. Plants that reach harvest size before December can then stand through winter, ideally under row cover, because light is too weak by then for much new growth.

For more on matching types to the season, see The Best Asian Greens for Washington.

What will not work here

Sowing in February because the last frost is near fails most often. A mild frost date is easy to misread as a green light. The soil is cold and saturated, daylight is short, and the seed has little reason to germinate. Whatever does come up is often eaten by slugs, which stay active in a cool, wet spring and fall. Treat the frost date as the earliest sensible start for covered beds and the beginning of the real window for open ground.

Why do late spring sowings bolt in Washington?

Bok choy, napa cabbage and many mustards respond to lengthening days by sending up a flower stalk. Sown in May, they run to seed before they make a decent head or a full rosette of leaves. Western Washington has very long days near the summer solstice, so the damage starts earlier than many gardeners expect. A June sowing of napa cabbage is mostly wasted seed. If you want greens in late spring, sow fast baby-leaf types and cut them young.

What goes wrong with midsummer and late-season sowing?

Western Washington summers are dry. A bed watered once a week will not keep a July seedling alive, because the top inch of soil, where the seed sits, dries out in a day or two. Seedlings need light watering daily until they have true leaves, plus shade cloth or the shadow of taller plants on the hottest afternoons. East of the Cascades, the high desert is hotter and drier still, and a July sowing in open sun bolts or burns.

Going the other way, sowings made after early September in the west fail for a different reason. The plants stay small as daylight shrinks, then wait through winter without reaching a size worth cutting. Close spacing hurts in this season too. In cool, damp fall air, crowded leaves stay wet, which invites gray mold and gives slugs cover. Give heading types room, and thin seedlings early.

Heat-loving greens such as water spinach need warm nights that the maritime side rarely delivers. They are better left to a greenhouse or a hot corner than counted on in an open bed. For a broader view of what to grow, start with Growing Asian Greens in Washington.

What changes across Washington

The state does not have one garden. The USDA Plant Hardiness Zone Map (2023, built on 1991-2020 normals) puts Washington across zones 4a to 9a. That is a wide range, from the mild coast and Puget Sound lowlands up to the cold mountain valleys and northeastern basins. No single answer fits that spread, so the Seattle dates above serve as a reference for the western lowlands and nothing more.

A hardiness zone rests on one variable, the average annual extreme minimum temperature. It measures how cold the worst winter night gets and says nothing about season length, summer heat or soil wetness. Two places in the same zone can still have very different planting dates. A reader east of the Cascades, in the north, or at altitude should shift in three ways:

• Start spring sowings later, since soil stays frozen or cold longer.
• Sow for fall earlier, counting back from the local first frost, which comes sooner than November.
• Use row cover or a cold frame to stretch both ends of the season.

Do not assume the zone map answers the sowing question for you. Look up your own location on the USDA map, using your ZIP code, then treat the result as one piece of information. Frost dates, elevation and local drainage matter just as much for a crop that grows in weeks.

Where to check this for your own county

Your state extension service is the best next stop. Washington State University Extension runs county offices and publishes regionally specific gardening guidance. Look for two things on the site or at the county office: a local planting calendar for vegetables, and any leafy greens varieties that have been trialled in your county.

What should you look for on the extension site?

A county calendar gives sowing windows tied to local frost dates, which is more useful than a statewide range. Check which dates it lists for cool-season leafy crops, since bok choy, mizuna and mustards fall under that heading. Then look for variety notes. A trial result showing which bok choy held up in your county’s heat or resisted bolting is worth more than any general ranking. If your county has a Master Gardener program, its volunteers can usually tell you which local microclimates run colder or wetter than the official numbers.

A county recommendation beats a state one. Any statewide window, like the one above, averages over very different ground. A county office knows the local frost pockets, the typical dry spells and the soils.

Checking this against your own garden

Your own observations outrank every date here. Three things are worth watching, and none needs equipment.

What can you see and feel in your own yard?

First, check soil warmth by feel. Push your hand into the bed in the morning. If it feels clammy and cold, or the soil clumps into a muddy ball, wait. Soil that crumbles and feels cool rather than icy is ready for hardy greens. Compare a sunny, raised bed with a shady, low one. The difference can be a couple of weeks and tells you which spots to use early and late.

Second, watch for the first true leaves on a test sowing. After the two round seed leaves open, watch for the first true leaves to follow. If seedlings sit at the seed-leaf stage for weeks, the spot is too cold, too dark or too dry for that date. Quick true leaves say you chose well.

Third, watch for the first sign of bolting in spring rows. A thickening center, a lengthening stem or a flower bud means the days or the heat have turned against that crop. Note the date and move next year’s sowing earlier, or switch to a quicker type. A frost on the lawn a week before your neighbor’s tells you about cold pockets, which matters when you decide how far into November you can leave plants uncovered.

Common questions

Can I sow Asian greens in January or February in western Washington?

Only indoors or under cover. Outdoors, the soil is cold and wet and daylight is short, so seed sits or rots. Starting seedlings in a bright window or a cold frame gives a head start for late March transplanting.

Is fall better than spring for Asian greens in Washington?

For most types, yes. Falling day length and cooling nights suit them, and the first fall frost does not arrive until November 13 to November 30 at the Seattle reference station. Spring sowings have to finish before the long days of late spring trigger bolting.

Do Asian greens survive Washington winters?

Hardy types often do on the maritime side. The mean daily minimum of the coldest month is 37.1°F at the Seattle reference station, so ordinary nights stay above freezing. Row cover protects plants through the occasional cold snap, though growth nearly stops once days are short.

Do the same dates work east of the Cascades?

No. The high desert has a far shorter season, so start spring sowings later and fall sowings earlier than the western dates. Use your county extension office to get the local frost dates, then count back from them.