Folks are becoming more aware of the importance to be water-wise when it comes to gardening. West Hawaii is experiencing a long spell of water restrictions because of pump problems, and even East Hawaii is drier than usual. ADVERTISING Folks
Folks are becoming more aware of the importance to be water-wise when it comes to gardening. West Hawaii is experiencing a long spell of water restrictions because of pump problems, and even East Hawaii is drier than usual.
With global climate change, we really don’t know what the future might bring, but scientists are saying temperature extremes and more drought conditions are likely.
Urbanization of Hawaii is impacting Kona and Hilo, but Puna, Kohala and other population areas are showing signs of construction and a trend toward “degreening.” With water rates on the increase, some folks might even consider concrete lawns.
But don’t be hasty. You can have a beautiful yard 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. Fortunately, many garden plants in Hawaii are fairly hardy when it comes to short water supply, so we have a long list of plants to assist us. It’s important to vegetate these areas so our islands don’t look like the deforested Greek Isles in years to come.
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 turning the leaf back or edge-on to the sun. Others root deeply to tap into deeper soil during dry periods, taking advantage of accumulated moisture in the 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 at 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 heated air radiation that reaches Earth’s surface by wind. The amount of energy reaching Earth’s surface is limited by the cloud cover, dust and 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 high rainfall areas such as Hilo.
Besides the soil moisture 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.
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 is conducted away by rapid wind movement or low relative humidity, we can have high rates of water use.
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 such as the bird of paradise and monstera give the luxuriant look and are still drought-resistant. Many palms also have this quality.
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 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 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 the 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.
Will the time come when we are islands 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, water rates are sure to go up. Will we be wise and plan for a future that is not quite so demanding of water, or will we make the same mistakes Southern California and Oahu already made?
