In Hawaii’s lowlands you can sow Asian greens in nearly any month. No freeze occurs in inhabited areas, so no frost date sets the calendar. Rainfall, elevation, and whether your garden faces the trade winds or sits in their lee decide it instead. Choose the month by those three, then choose the variety by how the weather tends to run that month.
The short answer for Hawaii
A frost date is a probability, not a fixed day on the calendar. “Last spring frost, April 20” means that by that date, in a stated share of years, the final freeze has already passed. Seed packets and mainland guides count backward or forward from that date. NOAA’s 1991-2020 climate normals for the Honolulu station (USW00022521) can’t produce one for Hawaii. The data returns a not-computable flag for every freeze percentile at both 32°F and 36°F, and records zero days per year at or below 32°F. You can see the station data from NOAA yourself.
Frost does reach the high volcanic summits of Mauna Kea, Mauna Loa, and Haleakala. Nobody gardens there. So the frost-based sowing window that governs the mainland simply doesn’t exist in Hawaii.
The coldest month shows why. At the Honolulu reference station, the mean daily minimum of the coldest month is 66.8°F. Even the chilliest nights of the year are mild enough for Asian greens to grow. Cold never forces you to wait, which moves the question to heat and water. Bok choy, mizuna, and tatsoi are cool-loving crops that run to seed when warm weather drags on. In the lowlands, the work is protecting them from that, rather than from winter.
The practical rule: sow small batches often, every couple of weeks, rather than one big planting. Use quick leafy types in hotter months and give them afternoon shade. Save slower, larger heads for the cooler, steadier stretches of the year. The Best Asian Greens for Hawaii sorts varieties by type. The broader guide, Growing Asian Greens in Hawaii, covers the crop as a whole.
This page doesn’t have a verified month-by-month sowing table for the state, and it won’t invent one. What it has is the climate logic above.
What will not work here
The first thing to drop is the frost-based calendar. “Sow six weeks before the last frost” and “wait until the soil thaws” are instructions with nothing to attach to. If a mainland seed catalog or video gives you a date, it was written for a place with a winter your garden doesn’t have.
The second is the idea of a cold season to sit through. With a coldest-month minimum of 66.8°F at the reference station, there is no stretch of chill that keeps a lowland bed cool for months. A planting that stalls in a warm spell can’t be rescued by waiting for winter. Plan for heat on every sowing, not only the summer ones.
Third, weekly deep watering fails on the dry, leeward side. Asian greens are shallow-rooted and mostly water. A bed that gets a long soak on Sunday and nothing else can leave seedlings stressed and leaves bitter by midweek. In a dry, sunny spot, short daily watering with mulch does better than one big weekly dose.
Fourth, close spacing fails on the wet, windward side. Packed rows of bok choy hold moisture against the leaves, and wet leaves invite rot and fungus. Slugs and snails love the same conditions. In Hawaii they are more than a nuisance, since they can carry rat lungworm, so wash every harvest carefully. Give plants room for air to move, and pick a raised bed or sloped ground so heavy rain drains off.
Fifth, direct-seeding right before a downpour. Fine seed like mizuna and komatsuna washes out or ends up buried too deep. Check the forecast, or start seed in trays under cover and set out seedlings when the rain eases.
Last, frost protection. Row covers, cold frames, and low tunnels are standard on the mainland for a reason that doesn’t apply in the lowlands. Here the same cloth is useful for shade and for insect screening, so use it for that.
What changes across Hawaii
Hawaii is not one garden. The USDA Plant Hardiness Zone Map (2023 edition, built on 1991-2020 normals) places the state at 9a-13a. That is a range, not a zone. The system measures one thing, the average annual extreme minimum temperature, and it spans roughly four full zones from end to end. The cold end belongs to high-elevation ground, and the warm end to the sunny coast. There is no single answer for the state, and this page won’t pretend to give one.
Zone figures also measure cold only, which is the one thing that rarely limits Asian greens here. Treat them as a rough guide to elevation, not as a sowing date.
The Honolulu figures used on this page describe one warm, low reference point. A gardener in upcountry Maui, on the slopes of Hawaii Island, or in a high valley is working with cooler air than that. Altitude and cooler air let you grow cool-loving greens through more of the year and slow the bolting that heat triggers. Plants grow more slowly there, so give them more time to reach harvest than a coastal grower would.
Exposure matters as much as altitude. A windward slope gets steady rain and humidity, so drainage and spacing come first. A leeward coast is drier and sunnier, so water and shade come first. Two gardens at the same elevation, on opposite sides of an island, can need opposite habits.
To learn which band your own address falls in, use the USDA’s Plant Hardiness Zone Map and search by ZIP code. This page can’t tell you where you land.
Where to check this for your own county
For local guidance, go to the College of Tropical Agriculture and Human Resources (CTAHR) at the University of Hawaii at Manoa, which runs the state’s Cooperative Extension. Hawaii’s extension offices serve each island, and their staff know the rainfall and elevation patterns where you live.
Look for two things. The first is a local planting calendar for leafy and Asian vegetables, ideally one that names your island or district. The second is a list of varieties trialled in your county, meaning grown and observed in local conditions rather than recommended from a catalog. Trial results tell you which bok choy held up in humidity, which mustard resisted bolting, and which types struggled with local pests.
A county recommendation beats a state one. This page is the state one, and it has to stay general because Hawaii’s conditions change within a few miles. The same page is accurate for a dry leeward lot and misleading for a wet windward one.
If the extension site doesn’t list a calendar for your area, call or email the nearest office and ask what they suggest sowing this month, and why. Their answer will be based on what is working in nearby gardens right now.
Checking this against your own garden
Your own observations outrank every date here. Three are worth watching, and none needs equipment.
Soil warmth by feel. Push your hand into the bed in the morning, then again mid-afternoon. A bed that stays cool and damp holds moisture and drains slowly, which suits seedlings in hot months. One that turns hot to the touch will cook tender roots, so it needs mulch and shade. This shows you which corner of your yard is your cool one.
The first flush of male flowers. Squash and cucumber vines produce male blooms before female ones. When they appear in your yard or a neighbor’s, warm-season conditions have settled in at your elevation. Shift to heat-tolerant greens and add shade cloth from that point.
The first true leaves. Sow a small test row. The first leaves after the seed leaves tell you how fast your site grows greens. Stretched, pale seedlings mean too much heat or too little light. Yellowing or collapsing ones usually mean too much water. Fast, sturdy growth means the timing suits your spot.
Common questions
Can I grow Asian greens year-round in Hawaii?
In the lowlands, yes, since no freeze occurs in inhabited areas. What changes through the year is heat and rain. Choose varieties and shade for those, not for frost.
Do I need to wait for a last frost date?
No. NOAA’s 1991-2020 normals show zero days per year at or below 32°F at the Honolulu station, and no freeze percentile can be computed. There is no date to wait for.
Is there one best month to sow?
This page has no verified single best month for the whole state. The right month depends on your island, your elevation, and which side of it you garden on. Your county extension office is the place to get a local answer.
Do I need to protect greens from cold at high elevation?
Frost occurs only on the high volcanic summits, not where people garden. Higher gardens are cooler, which slows growth and delays bolting, but the Honolulu figures here don’t describe them. Check your local conditions with extension.