Folks are becoming more aware of the importance to be water wise when it comes to gardening.
West Hawaii is experiencing a long dry spell and even East Hawaii is dryer than usual. It is a good time to participate in the 10th annual Nahelehele Dryland Forest Symposium at the King Kamehameha’s Kona Beach Hotel from 9 a.m. to 4:30 p.m. Feb. 26.
Protecting Hawaii’s dryland forests is vital for many of the endangered species of plant and animals found only in Hawaii. Also many native plant species can be incorporated into our water wise landscapes. With global climate change, we don’t know what the future holds, but we do know that protecting our forests and reforestation is essential to our quality of life. This includes urban reforestation that is a must as our low land coastal areas are developed.
Several well known resource conservation experts will be making presentations including Susan Cordell, Tom Gillespie, Art Madeiros, Flint Hughes, Matt Keir, Mike Donoho, JB Friday and Christina Hoffmann. Forest field trips will also be available. For more information and reservations contact Stella Caban at scaban@kohalacenter.org, (808) 887 6411 or Kathy Frost, kjfrost@hawaii.rr.com, (808) 325 6885. Space is limited so register quickly at www://nahelehele16.eventbrite.com.
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 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 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 to call upon. It’s important to vegetate these areas so that our islands don’t look like Kuwait 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 turn the leaf back or edge-on to the sun. Others root deeply to tap and have available for day periods, any accumulated moisture of sub-soil.
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-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 heated-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, 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 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 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, temperature of the air and the vapor pressure or relative humidity of the atmosphere. If water is conducted away by rapid wind movement or low relative humidity, we may 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 like 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 of all, we can irrigate only when the soil-water 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. 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 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 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 on water, or will we make the same mistakes that Southern California and Oahu have already made?
