When to Sow Asian Greens in Iowa

In Iowa, sow Asian greens twice: in early spring, as soon as the soil can be worked, and again in late summer so the crop matures before the first fall freeze. Skip midsummer. Bok choy, mizuna, tatsoi and their cousins bolt in long, hot days, so the two cool shoulders of the year are the whole game. For variety picks, see The Best Asian Greens for Iowa.

The short answer for Iowa

Spring sowing starts around the last spring frost, which NOAA places between April 12 and April 24. Fall sowing is timed backward from the first fall frost, October 14 to October 25. Both ranges come from the NOAA NCEI 1991-2020 Climate Normals, and both are probabilities, not promises. Specifically, they are the band where the chance of a 32°F freeze falls between 30 and 70 percent. In spring, April 12 is a roughly 70 percent chance that a freeze is still ahead, and April 24 is roughly 30 percent. The fall dates read the same way in reverse.

That band matters because Asian greens are not equally fussy about cold. Most of the leafy types shrug off a light frost once they are up, so you do not need to wait for the last freeze to pass. The seedlings are tougher than tomato transplants, and an early sowing that gets hit by a 30°F night usually just sits there and waits. The real deadline in spring is the other end, when warmth and lengthening days push the plants to flower.

Which dates to trust

The NOAA numbers come from an airport station, and airports run warm. Iowa State Extension county figures (1950-2021) run about 4 days later in spring and 10 days earlier in fall. Treat the airport numbers as the optimistic end of the range.

For fall, work backward. Take the days to maturity on your seed packet, subtract them from the earlier, county-style fall date, and add a buffer, because growth slows as days shorten. A packet that says 40 days, counted back from October 4, lands you in early September at the latest. Baby-leaf harvests need less time than full heads. This calendar rests on frost dates and packet arithmetic. It is not a tested sowing schedule for each crop.

What will not work here

Overwintering in open ground fails. The mean daily minimum of Iowa’s coldest month is 13.8°F at the Des Moines reference station, and that is an average night, not a record low. Nights well below that happen every winter. Tender bok choy and similar types will not come through that, and a fall sowing does not become a spring harvest unless it sits under protection. Even hardy-leaning types like tatsoi and mizuna are better treated as a crop you finish before the hard freezes.

Late spring sowing is the second failure. Iowa’s frost-free stretch is long enough for warm-season crops, which is the problem. By the time the soil warms enough for tomatoes, long days and heat have turned a spring-sown bok choy into a flower stalk. Anything sown after the first warm spell in late spring tends to bolt before it fills out. The slow, large heading types, like napa cabbage, suffer most because they need the longest stretch of mild weather to form a head.

The third failure is the fall sowing that starts too late. With a first frost as early as October 4 on county-style dates, a seed sown in mid-September has a few short weeks to do what a spring sowing does in a longer, brighter season. Plants that are still small at the first hard freeze stall, and they rarely recover. You will see this most with full-size bok choy and heading cabbages, while quick baby-leaf mixes forgive more.

Finally, there is the trap of copying a calendar written for somewhere else. A Gulf Coast or California schedule, which has you sowing greens through winter, does not translate to a state where the coldest-month night averages under 14°F. Iowa gives you two real windows, not a year-round one, and gardeners who treat it as three or four seasons usually lose a crop to bolting in July or a freeze in November.

For crop-by-crop background, Growing Asian Greens in Iowa covers the basics that sit behind these dates.

What changes across Iowa

The USDA Plant Hardiness Zone Map (2023 edition, built on 1991-2020 normals) puts Iowa at 4b to 6a. That is four half-zone steps, so there is no single answer for the state. A gardener near the Minnesota line and one near the Missouri line are working with winters several steps apart.

The zone rating has a narrow job. It is built on one variable, the average annual extreme minimum temperature, so it tells you what survives winter, not when to sow a crop that lives eight weeks. For a dated recommendation, the Des Moines reference figures on this page are the anchor, and other parts of the state differ from them.

The practical shift follows the same direction as the zone. In the colder northern part of the state, expect the spring window to open later and the fall one to close earlier, which squeezes both. Lean on the county-style dates in the table rather than the airport ones, and favor quick types you can harvest young. In the milder south, the spring window opens a bit sooner and the fall window runs a bit longer, though summer heat still ends spring crops on roughly the same logic.

Local conditions add noise on top of the north-south gradient. River valleys collect cold air on still nights, a south-facing wall holds warmth, and a garden in a town often runs ahead of one in open farmland a few miles away. To find where you fall, look up your location on the USDA Plant Hardiness Zone Map itself. This page cannot tell you which zone your garden is in.

Where to check this for your own county

The best next stop is Iowa State University Extension. Look for two things. The first is the local planting calendar for your county, which will use frost dates built for where you live instead of one airport’s. The second is the list of varieties trialed in your county or a neighboring one, since a variety that held up in a northern Iowa trial tells you more than a catalog description.

A county recommendation beats a state one. This page is the state one, built from NOAA normals and the hardiness map, and it is meant to get you to a sensible first guess. It does not cite an Iowa State planting calendar written specifically for Asian greens, so check whether your county office has one, or ask whether it can point you to the nearest comparable cool-season vegetable schedule.

When you contact the county office, bring specifics: the crop, whether you garden in raised beds or open ground, and whether you want a spring or a fall harvest. Those details let staff or Master Gardener volunteers answer in one reply rather than several.

Checking this against your own garden

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

• Soil warmth by feel. In early spring, push your bare hand into the bed. Soil that feels raw and clammy is too cold for good germination, and soil that crumbles and feels neutral is ready. A bed that reaches that point a week before your neighbor’s tells you it drains well and catches sun.
• The first male flowers on squash or cucumbers. Those blooms open ahead of the female ones, and when they show, summer heat has arrived. Count that as your cue to stop sowing greens for spring and begin planning the late-summer round.
• The first true leaves. The first pair after the seed leaves marks real growth. In fall, seedlings that reach this stage well ahead of frost are on schedule. If they are still slow to show it as nights cool, your site is colder than the calendar assumes, so sow earlier next year.

Write down the dates you see. A single season of notes gives you a calendar that fits your corner of Iowa better than any published range.

Common questions

Can I sow Asian greens in Iowa in the middle of summer?

You can, but most will bolt or struggle in the heat. The dependable windows are early spring and late summer into early fall, timed so harvest finishes before the first fall freeze.

How early can I sow in spring?

As soon as the soil can be worked. The last spring frost runs April 12 to April 24 by NOAA’s airport figures, and county figures run about 4 days later, but young greens tolerate light frost.

When should I stop sowing in fall?

Count back from the first fall frost, October 14 to October 25 at the airport station, or about 10 days earlier by Iowa State Extension county figures. Subtract the days to maturity on the packet and add a buffer.

Will Asian greens survive an Iowa winter outdoors?

Not reliably. The mean daily minimum of the coldest month is 13.8°F at the Des Moines reference station, and tender types will not make it through without protection.