Last year, folks were complaining about wet weather in West Hawaii and too little rain in East Hawaii. Now West Hawaii is getting much more rain than usual. Even the koa haole and kiawe are 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, green is a very restful color.
Urbanization of Oahu is impacting the climate as more pavement creates 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 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. 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. 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 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 that is the soil-plant-atmosphere complex determine how much water will be used in a given time.
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.
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.
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 very drought-tolerant but will also grow in our 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 dryer conditions, they will grow as terrestrials.
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.
Proper fertilization will also help with deep rooting. Also, poor soils should be improved with the necessary amendments to help the plants develop good root systems. The use mulches will also help conserve soil moisture.
Proper planning and maintenance of your garden will help in the short run, but we must do something about the overall future of the islands as well.
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!