Zone 12a: Gardening in Hawaii's Uplands and Interior Valleys
Zone 12a exists only in Hawaii, in places such as Kauai, central Oahu around Wahiawa and Mililani, and upcountry Maui. It never freezes, so the hardiness zone mainly tells you the garden is tropical. Elevation, rainfall and wind decide what grows, from bananas and lychee to head lettuce and broccoli in cooler uplands.
- Coldest winter night
- 50 to 55°F
- ZIP codes in the zone
- 22
Frost in zone 12a
In 100% of zone 12a's ZIP codes with weather records, a 32°F night happens in fewer than half of years, so frost dates do not organize the gardening year. Cool-season vegetables grow from fall through spring, and heat, not frost, ends the season for many crops.
What a zone 12a winter means
Zone 12a never freezes, so frost never shapes the garden. USDA added zones 12 and 13 to describe tropical climates, and their main use is comparing how much cold a tropical plant tolerates. In practice, Hawaii’s gardens are shaped by microclimates. CTAHR notes that conditions vary greatly over short distances, with elevation, rainfall and wind changing from one valley or slope to the next, and that wind protection is critical.
Where zone 12a is
Zone 12a covers much of Kauai, including Lihue, Kapaa, Hanalei and Kalaheo; central and some leeward Oahu, including Wahiawa, Mililani, Pearl City, Waipahu, Waialua and Schofield Barracks; upcountry Maui around Kula, Makawao, Pukalani and Haiku; Kualapuu on Molokai; and parts of the Big Island such as Waikoloa and Kurtistown.
Fruit
- Bananas: CTAHR describes ideal conditions as an average of about 81°F, full sun and 100 inches or more of well-distributed rain a year, in a site protected from wind.
- Lychee: needs a cool, dry winter to flower (see the FAQ), which interior and upland 12a sites provide more reliably than hot coasts.
Vegetables
Hawaii’s cooler, higher areas extend the season for crops that struggle in lowland heat. CTAHR’s trials found:
- Head lettuce grows year-round at high elevation; at low elevation only in winter. High night temperatures make lettuce bitter.
- Broccoli cultivars ‘Waltham 29’, ‘Atlantic’, ‘Premium Crop’ and ‘Express Corona’ are best in cool uplands above 2,000 feet; ‘Greenbud’ and ‘DeCicco’ suit warm lowlands.
Special concerns
Wind is the most common limit. A University of Hawaii Extension agent recommends siting gardens away from the windy north and east sides of buildings and notes that sorghum-sudangrass grows into a 6-foot windbreak in about 40 days. Water is the other: CTAHR’s long-standing home garden bulletin advises against growing vegetables without irrigation in most of the islands, because even frequent showers may add up to too little rain. After heavy rain, CTAHR suggests waiting 24-36 hours before planting.
Where zone 12a is
States with zone 12a ZIP codes, the largest first: Hawaii.
Zone 11bZone 12bAll zones and the zone finder
Questions
Can I grow lychee in zone 12a?
Often, yes. University of Hawaii CTAHR says lychee grows from sea level to about 2,000 feet with 50-80 inches of rain, but needs a cool, dry winter (about 59°F days and 50°F nights) to start flowers. Cooler interior and upland sites are more likely to give it that than hot leeward coasts.
What vegetables grow best in Hawaii's cooler areas?
Cool-season crops. CTAHR finds head lettuce grows year-round at high elevations but only in winter in the lowlands, and recommends broccoli such as 'Waltham 29' and 'Premium Crop' for uplands above 2,000 feet.
Where should I put a vegetable garden in Hawaii?
A University of Hawaii Extension agent advises that the south side of a house gets the most intense sun, the north and east sides the most wind, and the southwest side is often best. Protect gardens from wind with a windbreak.