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WET AND DRY
On all the islands except Hawaii, the north and east (windward) sides are wet, the south and west (leeward) sides are dry.
From the ocean off the windward sides, evaporation rises. Carried by trade winds, this warm, moist air passes over land, increasing clouds and showers, then over mountains, where it cools. Heavy rains are the result.
Leeward, the air warms as it descends, leaving these south and west sides mostly sunny and dry.
The Big Island, though, is different. Its high volcanic peaks (Mauna Loa, Mauna Kea, and Hualalai) block and deflect the trade winds, leaving local winds to drive Kona Coast weather. Here, especially in summer, upslope winds give rise to afternoon clouds and rain at elevatio
ZONE H1: Cooler volcanic slopes: 2,000 to 5,000 feet
This zone includes Kula and Ulupalakua on Maui and Kamuela and Volcano on Hawaii (the Big Island). Cooler air temperatures make these areas higher up the volcanic slopes better for growing standard cymbidiums, hydrangeas, proteas, and sweet bulbing onions (Maui onions).
Low-chill varieties of apples, peaches, and plums grow throughout this zone. The slightly higher elevations are also better for growing plants that can’t abide much heat, including Douglas fir and Japanese maples. Lower soil temperatures and cooler nights, too, help some gardeners at the 4,000- to 6,000- foot elevations on the dry sides succeed with plants from Mediterranean climates.
Warm-season highs range from 65 to 80°F (19 to 27°C); cool-season lows can dip to the mid-40s. Frosts can occur above this zone. The growing season is year-round.
ZONE H2: Coconut palm belt: Sea level to 2,000 feet
Zone H2 includes the Ewa plain, Hawaii-Kai, Kailua, Kaneohe, Honolulu, and Waikiki on Oahu; Kahului, Kihei, and Lahaina on Maui; and the Hilo, Kailua-Kona, and Kohala coasts on Hawaii.
Most lowland lees in this zone get their heaviest rains between November and March, and May through September is relatively dry. Sunny days urge bougainvilleas, poincianas, plumerias, and shower trees (Cassia) into spectacular bloom.
On the wet windward sides, where rain comes from passing storms and yearround tradewind showers, bananas, gingers, and heliconias do well. The Kona Coast, however, gets more rain in the warm season than in the cool season.
The same plants grow here as elsewhere in Zone H2 but may have slightly different cultural requirements. Highs in this zone hover in the 80s; lows can dip to the mid-60s. The growing season is year-round.