Turnips in Hawaii don’t follow a frost calendar, because there isn’t one below the summits. What actually decides sowing time here is elevation, rainfall pattern and how close a garden sits to sea level. The cooler months, when night lows sit nearest the state’s winter average, give roots their best shot at bulking up before heat turns them woody and bitter.
The short answer for Hawaii
Start with what the weather station data actually shows. At Honolulu’s reference station, NOAA’s 1991-2020 Climate Normals report zero days per year at or below 32°F, and every freeze percentile at 32°F and 36°F comes back flagged as not-computable. That’s not a rounding quirk. It means the entire mainland method of counting days before or after “last frost” has nothing to count from here. There is no last spring frost and no first fall frost in the places people actually garden. Frost exists only on the summits of Mauna Kea, Mauna Loa and Haleakala, thousands of feet above any turnip bed.
So the real question isn’t when the cold arrives. It’s when the heat backs off enough for a cool-season root crop to behave itself. Turnips size up best in the 60s, and they turn pithy, bitter and quick to bolt once air temperatures push into the mid-70s and stay there. The closest thing this state has to a limiting cold snap is the coldest month’s average low, and at the Honolulu reference station that figure is 66.8°F. That’s a mild number by any mainland standard, but it’s also the coolest window Hawaii’s lowlands offer all year, and it tells you when to sow.
Practically, that means timing sowing to the months surrounding that seasonal dip, and favoring whatever elevation a garden can offer. A yard near sea level in Honolulu is working with a much narrower cool window than a garden a few thousand feet upslope. This page doesn’t have a county-by-county sowing chart with exact dates, because that data lives with the state’s own agricultural extension service, not in a nationwide climate dataset. What the climate record does confirm is the mechanism: no frost date to build a calendar from, and a coldest-month minimum near 67°F that sets the upper edge of the growing window rather than a lower one.
That reversal is the single biggest adjustment a mainland gardener has to make when growing turnips in Hawaii. Everywhere else, the sowing calendar answers the question “how do I avoid the cold?” Here, it answers “how do I find the cool?”
What will not work here
The first failure is importing a frost-based planting chart wholesale. Seed packets and mainland calendars routinely say “sow four to six weeks before last frost.” In Hawaii’s inhabited zones, that instruction is meaningless, since the underlying NOAA freeze percentiles are flagged not-computable and the annual freeze count sits at zero. A gardener who waits for a frost date that will never arrive is simply waiting past the state’s actual cool window and sowing into rising heat instead.
The second failure is growing turnips through summer at low elevation in the driest leeward pockets. Areas on the leeward, sun-baked side of the islands, the kind of dry, low-rainfall terrain found around parts of Hawaii Island’s Kona coast or central Oahu, push well past turnip’s comfort range for long stretches. Roots that mature under that kind of sustained heat come out woody and sharp-tasting rather than sweet, flea beetle pressure climbs, and plants often bolt to seed before the root has filled out at all.
The third failure is expecting rutabaga to behave like turnip anywhere near the state’s warmest pockets. Rutabaga needs a longer, steadier cool stretch to size a proper root, and Hawaii’s lowland warmth simply doesn’t relax long enough to provide it. In the warmest reaches of the state’s hardiness range, essentially zone 13a, rutabaga rarely finishes a usable root at all; turnip’s shorter timeline is the far safer bet in that heat.
The fourth failure is treating rainfall the same way statewide. Windward slopes, the wetter sides facing the trade winds, stay consistently damp, and turnip beds there planted at typical mainland spacing trap humidity around the leaves and invite rot and fungal disease. Leeward gardens face the opposite problem: dry-season rainfall drops sharply, and a watering habit built for a wet windward yard leaves leeward turnips stressed and bolting early. One watering schedule, applied statewide, fails one side of the island or the other every time.
What changes across Hawaii
Hawaii’s USDA hardiness range runs from 9a to 13a, a five-zone spread inside a single state, which is unusual anywhere else in the country but ordinary here. That spread doesn’t come from latitude, since the islands sit close together near the tropics. It comes from elevation and from which side of an island a garden faces, windward and wet, or leeward and dry.
A sea-level garden in a warm leeward pocket can land in zone 13a, among the warmest classifications on the entire USDA map, where nights barely cool off even in the state’s coldest month. Move upslope, to the higher farming districts on Hawaii Island, upcountry Maui, or the higher elevations of Kauai, and the classification drops toward zone 9a, a range that’s actually cooler at night than much of the Deep South. For a cool-season crop like turnip, that difference is the whole game. Elevation is standing in for the winter this state never gets.
That means there is no single statewide sowing date, and this page won’t invent one. A garden near the 9a end of the range gets a real, sustained cool stretch and more flexibility in timing. A garden near the 13a end is working against heat almost year-round, and timing has to hug the coolest weeks tightly, or shift toward the elevation solution instead of the calendar one. Since this page can’t tell an individual reader which end of that 9a-13a range their own yard falls into, the right next step is the USDA Plant Hardiness Zone Map itself, checked by ZIP code, rather than guessing from an island name or a rough sense of “up here it feels cooler.”
The practical takeaway is that two turnip beds ten miles apart in Hawaii can be running on effectively different growing calendars, one shaped by a wet windward slope, the other by a dry leeward flat a few hundred feet higher. Neither is wrong. They’re answering to different microclimates the state map already documents.
Where to check this for your own county
Hawaii’s land-grant extension service is the College of Tropical Agriculture and Human Resources, and its statewide directory sits at ctahr.hawaii.edu/site/extension.aspx. That’s the source for county-level detail this page simply doesn’t carry, since a national climate dataset can confirm the absence of frost and the coldest-month average, but it can’t hand over a working sowing chart tuned to Hilo’s rainfall versus Kona’s dryness or Kula’s elevation.
On that CTAHR page, look specifically for two things. First, any planting or sowing calendar published for the island or district closest to the garden in question, since Hawaii’s counties are effectively its major islands, Hawaii County, Maui County, Honolulu County and Kauai County, and each has its own extension office with its own local guidance shaped by that island’s rainfall and elevation pattern. Second, look for the variety list that office has actually trialled locally. A turnip variety that thrives in a cooler upcountry Maui plot may perform poorly in a hot, dry Oahu backyard, and county extension offices are the ones running side-by-side trials that catch that difference before a home gardener has to learn it the hard way.
A county recommendation beats a state one every time, and a state-level page like this one beats a generic mainland calendar every time. This article can confirm the mechanism, no frost season, a 66.8°F coldest-month floor, and a 9a-13a zone spread driven by elevation and exposure, but it can’t replace the office that walks that specific stretch of coastline or upland valley every growing season. Treat what’s here as the reasoning, and treat CTAHR’s county-level material as the specific dates and named varieties to act on.
If you follow the almanac, here is when to plant turnips by the moon. It is a tradition, not a proven method.
Checking this against your own garden
Every figure on this page is a state average pulled from a reference station, and a reference station is not the same as the actual soil in a specific yard. Three simple checks, no thermometer or instrument required, tell a gardener more about their own microclimate than any statewide number can.
First, feel the soil itself before sowing. Turnip seed germinates and grows best in soil that feels cool and slightly damp to the touch, not warm the way a sun-baked leeward bed often does by midday. If the soil feels warm through the whole morning, that’s a sign the plot is running hotter than the state’s cool-season window and sowing should wait, or shift to a shadier, higher spot on the property.
Second, watch how fast true leaves appear after the first seedling leaves. A quick, vigorous push to true leaves within the first couple of weeks signals conditions close to that ideal cool range; a slow, stalled seedling that sits for weeks without adding leaves is usually sitting in soil that’s too warm or too waterlogged for the roots underneath to size up properly.
Third, watch for an early flowering stalk. Turnips that shoot up a central flower stalk well before the root has filled out are bolting under heat stress, the plant’s own signal that it’s sensing conditions past its comfort zone regardless of what the calendar says. That’s the garden overriding the almanac, and it’s the single most reliable signal a Hawaii grower has that it’s time to shift sowing later, higher, or into shade rather than trust the date on a seed packet written for a mainland winter that never happens here.