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Tropical gardening: Urban forests help keep Hawaii green

Urbanization of Hawaii is impacting Kona and Hilo, but Puna, Kohala and other population areas also are showing signs of construction and a trend toward removing trees.

With water rates on the increase, some folks may even consider concrete lawns! But don’t be hasty. You can have a beautiful yard even if you live in a dryer 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 deteriorate without irrigation in dry periods, even in East Hawaii. Fortunately, many garden plants in Hawaii are fairly hardy when it comes to short water supply, so we have a long list upon which to call.

It’s important to vegetate these areas so that our islands don’t look like Death Valley in years to come. “The New Sunset Western Garden Book” is a great guide to plant selection. Also, you may contact the UH College of Tropical Agriculture and Human Resources master gardener helpline (Hilo, 981-5199; Kona, 322-4893).

There are two factors that make these 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 on edge to the sun. Others root deeply to tap and have moisture available for dry periods.

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, which is the soil, plant and atmosphere complex, determine how much water will be used in a given time.

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 heat in air radiation that reaches the earth’s surface by wind. The amount of energy reaching the earth’s surface is limited by the cloud cover and dust or haze in the atmosphere.

Air that is heated in another 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 better. 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 high rainfall areas like Hilo.

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 than we would expect from direct solar radiation.

Besides the moisture content of the soil and the plant, 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.

Plants that are tolerant of salty beach conditions often use less water than many soft, luxuriant jungle plants because they are streamlined for water conservation. However, plants like the Bird of Paradise give the luxuriant look and are still drought resistant. Many palms also have this quality.

The overall conditions for a high rate of water loss are rapid movement, high temperature, low humidity and bright sun with no cloud cover or shade.

What can we do in managing the soil to take advantage of our knowledge of the factors affecting water use? First, we can irrigate only when the soil moisture 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 that we will have to irrigate sooner following a previous irrigation than following a general rainfall. 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 the plants develop good root systems. The 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 the addition of this element is important. The use of mulches will also help conserve soil moisture.

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

A series of dry years and increased pressure on water supplies have made us aware that water is an exhaustible resource. Limits on our water resource means that 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 houseplants 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 are teaming with too many people? Will we be so limited in food and water that we can no longer have gardens or parks or landscaped highways?

As our population increases and the climate continues to warm, water rates are sure to go up. Will we be wise and plan for a future that is not quite so demanding on water, or will we make the same mistakes that California and Oahu have already made?
Source: Hawaii Tribune Herald

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