In Hawaii, most of us live on the sloping side of a mountain, which means that our homes and gardens are usually built on a slope. While the benefits of living on the side of a mountain may be the great many vantage points from which to view our endless paradise, gardening on a slope can be difficult. While some of us do not view this as a major issue because there are many ways to plan and plant around it, other seeks solutions to have level ground. Terracing a slope by constructing low retaining walls can create a level gardening surface that is desired.
In Hawaii, most of us live on the sloping side of a mountain, which means that our homes and gardens are usually built on a slope. While the benefits of living on the side of a mountain may be the great many vantage points from which to view our endless paradise, gardening on a slope can be difficult. While some of us do not view this as a major issue because there are many ways to plan and plant around it, other seeks solutions to have level ground. Terracing a slope by constructing low retaining walls can create a level gardening surface that is desired.
Terracing has other benefits besides creating a level surface for planting. Terracing can be used to help control erosion on a slope by directing water flow to where it would do the least amount of damage and also to retain soil as it tries to move down slope. It can also make useful an area that has no or very shallow soil that could not otherwise support the growth of crops. Terracing also allows for cropping on steeper slopes where cropping would not normally occur. Because terracing allows for the accumulation of deeper soil within the terrace, precipitation can be better retained, thereby making it available for crop growth over a longer period of time.
When planning to build a terrace, there are several steps to consider before starting construction. First is to determine the objective and scope of the project. Is it to plant a single fruit tree or to have a series of terraces to create a multilevel garden? Will this project improve garden usability? Will it have low environmental impact or be environmental neutral? Can I do this on my own or do I have to hire a contractor? Also, check with your local building department to see if your project will need a construction permit. Check with your homeowner community association and homeowner deed for any restrictions.
The slope of your property will dictate what can or cannot be done since a retaining wall can only rise up only so high before drastically affecting the labor involved and price. The rule of thumb is that the steeper, the slope the narrower your terraces for a given wall height. As the wall gets taller, more design considerations will be necessary. Wall height will also affect the depth of the soil you have for planting and hence what you can plant.
Retaining walls for terracing need not be straight monoliths of uniform appearance, as they can be constructed with nooks and other planting spots that may bring life to the wall. The use of large rocks or boulders can look great in the garden, but will require the proper equipment to move them. Consideration also needs to be given for the design of the retaining wall in terms of will it be a straight wall or will it flow following the contour of the garden. As mentioned on steeper properties, multiple terraces may need to be constructed, giving the project a step-like appearance to avoid very high walls. Consulting with a landscape architect is a good idea if in doubt.
Deciding on what materials to use for your terraces will be determined by what is available and the budget for your project. Retaining walls can be made out of many types of new and recycled materials. Construction materials commonly used for building retaining walls are concrete, concrete blocks, bricks, natural stone, interlocking blocks, sheets of metal and lumber. These can be held in place by many ways including cementing, dry stack, nails, bolts and gravity. On Hawaii Island, many of us have rocks, which can be gathered and used to construct a wall. Another favorite of many islanders is the use of old iron roof panels to create a wall from which to fashion a terrace. Sheets of roofing material are held vertically in place with wooden pole or metal pipes and back filled with soil. Selection of building material will also be influenced by the height of the wall and the amount of soil you need to hold back. Soil, especially wet soil will have considerable weight pushing on the wall and this push is exponentially increased with wall height.
When constructing a retaining wall to build a terrace, always be cognizant of safety. Wall building materials can be heavy and awkward to move and straining your muscles can occur. Uneven work area surfaces create fall and trip hazards. Wall building is hard and strenuous work, so don’t overdo it. And let’s not forget that once the wall is built, there is the possibility of falling off the edge when working close to the edge.
Terracing was also practiced in Hawaii prior to western contact. On Hawaii Island, the terracing can be seen as the Kona Field System. These were long, low walls that created narrow terraces on which taro and other crops were grown. In this manner, soil could be aggregated to a depth that would support crop growth. Prior to planting, the terraces were enriched through the accumulation of organic material that was allowed to compost prior to planting.
For more information on this and other gardening topics, visit the University of Hawaii College of Tropical Agriculture and Human Resources’ website at https://www.ctahr.hawaii.edu/Site/Info.aspx or visit a local Cooperative Extension Service office.
Nagata can be reached at russelln@hawaii.edu.