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Norman Bezona Prof emeritus University of Hawaii College of Tropical Agriculture

Norman Bezona
Prof emeritus
University of Hawaii
College of Tropical Agriculture

A few months ago, folks were complaining about dry weather in West Hawaii and too much rain in East Hawaii.

Now, West Hawaii is getting much more rain than usual. In fact, we have had more than 30 inches in the last three weeks at Kona Cloud Forest Sanctuary in Kaloko Mauka. Even the koa haole and kiawe are lush green from makai Kona to Kohala. Climate warming definitely seems to be affecting us locally as temperatures also have hit record highs this year.

How does this affect our gardening practices and our state of mind?

We should remember a healthy green landscape helps minimize the extremes of hot and cold. Vegetation helps reduce noise and pollution and produces oxygen that makes us feel better. Also, the color green is a very restful color.

Urbanization of Oahu is impacting the climate as more concrete and asphalt create desert-like conditions. On Hawaii Island, we have vast areas covered with lava as well as pasture lands that once were forest. Reforestation can help increase precipitation as several studies show.

With water rates on the increase, some folks might consider concrete lawns. But don’t be hasty. You can have a beautiful garden even if you live in a drier area. It’s just a matter of planning and proper planting.

A garden planted with no thought given to dry spells will do well in rainy periods but deteriorates without irrigation in dry periods. Even in East Hawaii, the on going El Niño has proven we need to use plants that will tolerate extremes of wet to dry conditions.

Fortunately, many garden plants in Hawaii are fairly hardy when it comes to short water supply, so we have a long list from which to draw. It’s important to vegetate these areas so our islands don’t look like Southern California in years to come. A good reference to help you select the right plants is “Sunset’s National Garden Book.”

There are two factors that make plants able to survive moisture stress.

First, some plants are notably resistant to drought. This quality is centered largely in the cellular structure and has a bearing on the economy with which the plant functions. Some plants have the ability to carry through extended dry periods because of a happy faculty of closing the pores of the leaf against transpiration, or turn the leaf back or edge-on to the sun. Others root deeply to tap and have available for dry periods any accumulated moisture of subsoil.

The garden environment is the other critical factor.

Water use is a process controlled by energy. The source of that energy is the sun. To move water out of the soil directly or through the plant and away into the atmosphere requires energy. The amount of energy available and the nature of the conducting medium that is the soil-plant-atmosphere complex determine how much water will be used in a given time.

One effect of Tokyo's heat-island effect in summer, which caused this picturesque 'guerrilla storm'

One effect of Tokyo’s heat-island effect in summer, which caused this picturesque ‘guerrilla storm’

Consider the amount of energy available on a piece of the landscape. The total available is the solar radiation that reaches the earth’s surface, plus the heated air that reaches the earth’s surface by wind. The amount of energy reaching the earth’s surface is limited by the cloud cover and the dust or vog in the atmosphere.

Air that is heated in another and drier part of the landscape and moves across the area of land in which we have our plants growing also adds heat. The result is a larger amount of water evaporated than we would predict purely on the basis of solar radiation.

This is why the more shade and wind protection from trees we have in the garden the less water is required to keep moisture levels up. And conversely, the more asphalt and concrete to heat up, the more rapidly our planted area dries up, even in normally high rainfall areas such as Hilo.

Our lava lands are unusually prone to moisture loss, so when we develop these areas and plant trees, shrubs and grass, we actually create a cooler more comfortable environment. We actually might increase the rainfall in places such as Hualalai, Kukio and Mauna Kea Beach when we change lava flows to develop “urban forests,” parks and gardens.

It helps us understand the reason for the common observation that an inch of general rainfall is much more useful and long lasting than an inch of irrigation water.

In effect, when we irrigate a small area, we are creating an “oasis.” If we have low, relative humidity and enough wind to move hot air across our irrigated surface, we can have losses of water nearly double that we would expect from direct solar radiation.

Besides the moisture of the soil, the nature of the plant itself has considerable effect on the amount of water lost into the air. The height of the plant and the roughness of the surface have an effect on the wind movement and mixing of air across the surface of the vegetation. A rough surface will cause more water loss than a smooth surface.

The amount of water conducted away from the soil and the plant surface depends on wind movement, wind speed, air temperature and the vapor pressure or relative humidity of the atmosphere. If water evaporates very rapidly by wind movement or low relative humidity, we might have high rates of water use.

Allagoptera

An Allagoptera arenaria palm. Both drought and salt tolerant.

Plants tolerant of salty beach conditions often use less water than many soft, luxuriant jungle plants because they are streamlined for water conservation. Beach naupaka is a great salt-resistant shrub but also is used in the landscape inland. Plants such as the bird of paradise, dracaena, monstera and many philodendrons give a luxuriant look and still are drought resistant. Many palms also have this quality. Heritage plants such as noni, hala and kukui are very drought tolerant but also will grow in our wet, humid lowlands.

Relatively new plant introductions such as tropical vireya rhododendrons have an amazing capacity for adjusting to environmental extremes. In wet areas, they might grow as epiphytes. Under dryer conditions, they will grow as terrestrials. To learn more about this amazing family, connect with the Hawaii Chapter of the American Rhododendron Society. The next meeting is noon Sept. 20 at the Keaau Community Center. For details, call Sherla Bertelmann at 808-966-9225.

The tropical Vireya, comes in many colors these days.

The tropical Vireya, comes in many colors these days.

What can we do in managing the soil to take advantage of our knowledge of the factors affecting water-use rates?

First, we can irrigate only when the soil-water content becomes low and plants begin to show evidence of wilt during the hottest part of the day. This forces deep rooting. Daily watering tends to promote shallow roots.

We can understand we will have to irrigate sooner following a previous irrigation than following a general rainfall. And we can provide soil with good physical and chemical properties for deep rooting of plants.

Proper fertilization will help accomplish this. Also, poor soils should be improved with the necessary amendments to help plants develop good root systems. Addition of well-rotted organic matter or compost often helps increase moisture- and nutrient-holding capacity. In many Hawaiian soils, available phosphorus is lacking. This is essential to root growth, so addition of this element is particularly important.

The use of mulches also will help conserve soil moisture.

Proper planning and maintenance of your garden will help in the short term, but we must do something about the overall future of the islands, as well.

A series of past dry years and increased pressure on water supplies have made us aware that water is an exhaustible resource. Limits on our water resource mean we can sustain only a certain level of population. Too many people can seriously threaten our water supply. This includes keeping our parks, gardens and perhaps even house plants alive if the shortage became critical. Limited water could mean a definite reduction in the quality of life in Hawaii.

Will the time come when we are islands teaming with too many people? Will we be so limited for water we can no longer have gardens or parks or landscaped highways?

urbanheatislandReforestation and regreening of our urban areas and lava lands will help, but the trend toward global warming and continued extremes of drought and flood require creative planning, planting and maintenance. For our mental and physical health, we need to focus on our own gardens and, at the same time, work with our local politicians and planners to keep Hawaii the green paradise it is meant to be.

This information is supplied by the University of Hawaii College of Tropical Agriculture and Human Resources.