In the Boise valley, sow Asian greens twice: in early spring, then again in late summer for a fall crop that finishes before hard freezes. Higher and colder parts of Idaho shift both windows, and midsummer is the wrong time for most of these greens everywhere. The dates below come from Boise freeze data, so treat them as a lowland baseline.
The short answer for Idaho
Sow Asian greens outdoors starting a couple of weeks before the Boise last-frost window of April 23 to May 7, and again about 45 to 60 days ahead of a first fall frost that falls between October 7 and October 21. Those two windows bracket a summer you mostly skip.
The frost dates need a careful reading. They come from the NOAA NCEI 1991-2020 Climate Normals for the Boise station, and they mark the 30 to 70 percent band of probability for a 32°F freeze. On April 23, the chance of another freeze is still around 70 percent. By May 7 it has dropped to around 30. Neither date is a promise that frost is over or has arrived. They are odds, and the weather gets a vote.
Spring sowing
Bok choy, tatsoi, mizuna and komatsuna shrug off light frost, so there is no reason to wait for May 7. Sow as soon as the soil is workable and not sodden, which in a typical Boise year means around the start of the frost window or a bit before. Count from May 7 to October 7 and you get 153 days of likely frost-free weather. That sounds generous, but spring sowings bolt as days lengthen and heat arrives, so the useful harvest is a few weeks, not a few months. Sow small batches every ten days or so rather than one big row.
Fall sowing
Fall is the better season. Cool nights, shorter days and no bolting pressure suit these greens. Pick a variety, read the days to maturity on the packet, and count back from October 7. A packet that says 45 days points to about August 23. Add a week or two for slow germination in hot soil, and for the chance that fall arrives early. Fall sowing is also where the type of green matters most, which is covered in The Best Asian Greens for Idaho.
| Type | Spring sowing | Fall sowing (Boise baseline) |
|---|---|---|
| Baby-leaf greens (mizuna, tatsoi, komatsuna) | Soil workable through the frost window; repeat every ten days | Count back about 30 to 45 days from October 7 |
| Full-size bok choy and similar | Early, before the frost window closes | Count back 45 to 60 days from October 7 |
| Heading types (napa-style) | Risky; they bolt before they head | Count back from the packet, with cover ready |
For the broader picture of what grows here, start with Growing Asian Greens in Idaho.
What will not work here
Sowing heading Chinese cabbage in late spring fails more reliably than any other plan. The plant wants a long, cool run to form a tight head. Idaho gives it a short spring that turns hot and long-daylight fast, and the plant sends up a flower stalk instead.
Midsummer sowing is the next trap. Lettuce-style seed beds in July dry out in hours, and brassica seed that does germinate in hot soil produces weak seedlings that flea beetles chew through. If you want greens in August, sow in the shade of taller crops and water daily until emergence. Otherwise, wait until the last third of the month.
Weekly watering does not work either. Shallow-rooted greens in an Idaho summer need moisture that is steady, not a deep soak once a week. Skip a few days in heat and the leaves turn bitter and the plants bolt. A mulch layer of straw or grass clippings helps, but it does not replace watering at germination.
Then there is the winter side. The mean daily minimum of the coldest month at Boise is 25.4°F, and that is an average. Many nights run below it, so any plan to leave bok choy standing through winter without cover is a gamble. Tatsoi and mizuna tolerate more cold than most, but under row cover, not bare. Winter harvests in Idaho belong to a low tunnel or a cold frame, not an open bed.
Finally, anyone gardening outside the Boise valley should not copy these dates at all. The source notes are blunt on this point: Boise is a low valley, and most of Idaho is higher and colder with far shorter seasons. A gardener at a mountain elevation who counts back from October 7 will be sowing seed into a bed that is already freezing. For them, slow-maturing varieties fail outright, and only fast baby-leaf types are a safe bet.
What changes across Idaho
The USDA Plant Hardiness Zone Map (2023 edition, built on 1991-2020 normals) puts Idaho in zones 3b through 7b. That is a very wide spread, and it means there is no single answer for the state. The zone system rests on one variable, the average annual extreme minimum temperature, so it describes how cold winter gets, not how long summer lasts.
That distinction matters for sowing. A zone number tells you whether a perennial survives the winter. It does not tell you the date of your last spring frost. Two gardens in the same zone can have frost dates a month apart if one sits in a cold-air basin and the other on a slope.
Still, the direction of the shift is predictable. In the north and at altitude, spring sowing moves later and fall sowing moves earlier, which shrinks the gap between them. If you garden up high, count back from a first frost that arrives sooner than the October 7 to 21 Boise window, and lean on baby-leaf types that mature in a month. The Boise numbers will mislead you more the higher you go.
Do not guess your own zone from this page. The USDA map has a search by ZIP code, and it takes under a minute. Look up your location, then compare it with the Boise baseline and shift your calendar accordingly, earlier or later, by the size of the gap between your local frost dates and Boise’s.
Where to check this for your own county
Your state extension service is the next stop. In Idaho that is University of Idaho Extension, which has county offices staffed by people who know local frost pockets, soil and water.
This page does not carry a county-level Asian greens calendar, and it does not carry any extension-published planting chart for Idaho. That is a gap, not an oversight, and it is the reason to go to the source. Two things to look for there:
- The local planting calendar for your county, especially fall dates for cool-season vegetables, since those are the dates that matter most for these greens.
- Any varieties trialled in your county, and which ones bolted, rotted or held up.
A county recommendation beats a state one. This page is the state one, built on a single valley station, and it can only point the direction. A county office can tell you that your area’s last frost runs three weeks later, or that a particular bok choy performed well in the local trial. When the two disagree, go with the county.
Checking this against your own garden
Your own observations outrank every date above. Three are worth tracking, and none needs equipment.
First, soil warmth by feel. Push your bare hand into the bed in the morning. If the soil feels cold enough to make your fingers ache within a minute, it is still early for anything but the hardiest seed. If it feels merely cool, you are in range. Beds that warm sooner than your neighbor’s tell you about drainage and sun exposure.
Second, the first true leaves. Sow a small test row of mizuna or tatsoi and note how many days pass before the first true leaves, the ones after the round seed leaves. Fast, steady growth means the season is cooperating. Slow growth in a cold spring, or a stalled row in August heat, tells you your microclimate runs cooler or hotter than the baseline.
Third, the first frost marks. Watch for blackened leaves on squash, basil or tomatoes in fall, and note the date. If those leaves go before October 7, your garden is colder than the Boise valley and your fall sowing should come forward. If it holds past October 21, you have a bit of room.
Write the dates down. Two seasons of notes will tell you more about your own bed than any state calendar.
Common questions
Can I sow Asian greens in Idaho in July?
You can, but it is a poor bet. Soil is hot, seed beds dry fast, and seedlings struggle. Late August works far better for most Idaho gardens.
What is the last frost date in Idaho?
There is no single date. For the Boise station, the 30 to 70 percent band of 32°F freeze probability runs April 23 to May 7. Higher areas of the state are later.
When is the first fall frost in Idaho?
At Boise, the same probability band runs October 7 to October 21. Most of Idaho is higher and colder, so expect earlier frosts there.
Do Asian greens survive an Idaho winter?
Rarely in the open. The mean daily minimum of the coldest month at Boise is 25.4°F, with many nights colder. Row cover or a cold frame gives hardy types a real chance.