Gardening in Hawaii: zones, frost dates and when to plant

Elevation sets the season

Hawaii is the only US state in the tropics, and almost half of its land lies within 5 miles of the ocean, which keeps temperatures steady. Elevation drives most of the variation. Below about 1,000 feet, winter nights rarely drop under 50°F. Only the high summits of Mauna Kea and Mauna Loa see hard cold. An old University of Hawaii home gardening bulletin put it simply: apart from the highest areas where frost occurs, every vegetable will at least try to grow. The real question is which crops grow well, and when.

That bulletin, Home Gardening in Hawaii (Bulletin 91), sorted planting seasons into three elevation bands: low (sea level to 1,000 feet), medium (1,000 to 2,000 feet) and high (2,000 to 3,500 feet). It is still a useful frame:

• Low and medium elevations. Carrots, beets, chard, mustards, eggplant, green onions and beans perform fairly well all year. Cool-season crops struggle here in summer.
• High elevations. Cabbage, broccoli and lettuce join carrots, beets, chard and green onions as year-round crops. Leaf diseases can be serious on some crops at these elevations during cool, rainy winters.

Rain, wind and the two seasons

Rainfall changes over very short distances. NOAA’s state climate summary describes windward mountain slopes that can get more than 300 inches a year, while leeward coasts and the highest slopes average under 20 inches. The year divides into a drier season from May through October, when steady trade winds bring light showers, and a wetter season from November through April, when trade winds weaken and winter storms bring more rain. Kona storms in the wet season can cause flash flooding.

Wind is the other constant. The Bulletin 91 authors said there were few places in the Islands where a garden should go in without a windbreak. They suggested quick living screens such as thickly planted corn or sorghum, or pole lima beans grown on a fence.

Heat, not frost, limits the lowlands

At low elevations, the warm months cause most of the trouble for cool-loving crops:

• Lettuce. CTAHR’s Lettuce for the Home Garden says head types rarely do well above 75°F. Heat brings tip-burn, early bolting and bitterness. ‘UH Manoa’, a CTAHR selection of ‘Green Mignonette’, is heat-tolerant and slow to bolt, so it can be grown all year near sea level. True head lettuce is dependable only at higher elevations such as Kula on Maui and Volcano and Kamuela on Hawaii Island.
• Bell pepper. The pepper fact sheet notes that blossoms seldom set fruit at 90°F. In dry lowlands, plant in late fall and winter for a late-winter and spring harvest. In cooler uplands, plant in spring and early summer.
• Tomato. The Home Garden Tomato guide says tomatoes can grow year-round in most places if you pick varieties suited to the site. CTAHR’s own tomatoes were screened for root-knot nematode resistance: the indeterminate hybrids ‘N-5’, ‘N-52’, ‘N-63’ and ‘N-65’, and the bushy, open-pollinated ‘Anahu’, ‘Healani’ and ‘Kewalo’. The guide warns that heirloom tomatoes bred elsewhere may not perform well in Hawaii.

Soils: lime, calcium and phosphorus

Hawaii’s soils differ from island to island, but CTAHR’s tomato guide notes some common patterns. Soil below pH 5.5 needs agricultural lime. Soils in high-rainfall areas often need lime to supply calcium, and a shortage of calcium shows up in tomatoes as blossom-end rot. Many Hawaii soils are low in available phosphorus. Mix both lime and phosphate into the top 6 to 8 inches before planting. An annual 1 to 3 inches of compost or well-rotted manure, worked in, usually raises yields.

Get a soil test first

CTAHR’s Agricultural Diagnostic Service Center offers soil and plant tissue testing to all Hawaii residents at a reasonable price. A routine test covers pH plus phosphorus, potassium, calcium and magnesium. Other tests, such as organic matter and micronutrients, are available on request. For a home garden, combine 5 to 10 small samples per 100 square feet into one sample. Collect it two to three months before planting so you have results in time to lime and fertilize. Test again at least every two years.

Drop samples at your county Extension office or send them to the center in Honolulu. Include the information sheet: CTAHR’s recommendations depend on whether your soil is heavy (as most Hawaii soils are), light volcanic ash like parts of Hawaii Island, or broken ’a’a lava.