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 one of the benefits of living on the side of a mountain may be the great vantage points from which to view our endless paradise, gardening on a slope can be difficult.
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 one of the benefits of living on the side of a mountain may be the great 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, as there are many ways to plan and plant around it, others seek solutions to have level ground. Terracing a slope by constructing low retaining walls can create a level gardening surface.
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, as well as retaining soil as it tries to move down slope. It can also help an area that has no, or very shallow, soil feasible for the growth of crops. Terracing also allows for cropping on steeper slopes. 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, 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 my garden usability? Will it have a low environmental impact or be environmentally neutral? Can I do this on my own or do I need to hire a contractor?
Also, check with your local building department to see if your project will need a construction permit, as well as with your homeowner community association and homeowner deed for any restrictions.
The slope of your property will dictate what can or cannot be done as a retaining wall can only rise up only so high before drastically affecting the labor and price involved. The rule of thumb is 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. Will it be a straight wall, or will it flow following the contour of the garden? 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 always 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 different ways from 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 local 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 poles or metal pipes, then back filled with soil.
The selection of building materials 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 could lead to muscle strain. 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 by.
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.
For more information on this and other gardening topics, please visit the CTAHR electronic publication website at http://www.ctahr.hawaii.edu/Site/Info.aspx or visit any of the local Cooperative Extension Service offices around the Island.
Dr. Russell T. Nagata can be reached at russelln@hawaii.edu.