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 have also hit record highs this year. How does this affect our gardening practices and our state of mind?
We should remember that a healthy green landscape helps minimize the extremes of hot and cold. Vegetation helps reduce noise, pollution and produces oxygen that makes us feel better. Also the color green is a 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 that are covered with lava, as well as pasture lands that were once 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 deteriorate without irrigation in dry periods. Even in East Hawaii, the ongoing El Nino has proven that 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.
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 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 that 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 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 dust or vog in the atmosphere.
Air that is heated in a 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 like 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 may actually increase the rainfall in places like 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 the irrigated surface, we can have losses of water nearly double of 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, temperature of the air and the vapor pressure or relative humidity of the atmosphere. If water evaporates very rapidly by 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. Beach naupaka is a great salt-resistant shrub but is also used in the landscape inland. Plants like the bird of paradise, dracaena, monstera and many philodendrons give a luxuriant look and are still drought resistant. Many palms also have this quality. Heritage plants like noni, hala and kukui are drought-tolerant but will also grow in wet, humid lowlands. Relatively new plant introductions like tropical vireya rhododendrons have an amazing capacity for adjusting to environmental extremes. In wet areas, they may grow as epiphytes. Under drier 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 at noon Sept. 20 at the Keaau Community Center. For details, call Sherla Bertelmann at 966-9225.
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.
Mulches will also help conserve soil moisture.