In the small garden, the stabilization of steep slopes is usually achieved through the building of retaining walls. These can be built using a variety of materials such as concrete blocks, quarry rocks, old railway sleepers or treated pine logs – the choice of building material is usually dependent on what is available locally at a reasonable cost.
However, in the larger garden and on acreage properties, the building of retaining walls may be prohibitively expensive. In these circumstances, the best solution is to use plants to stabilize slopes.
In the natural environment of a tropical rainforest, it is easy to see the role that roots play in holding slopes in place. In some rainforest localities, the roots of trees have retained slopes that are nearly vertical for hundreds of years. When such trees are removed from the forest through logging, it should be no surprise that landslides occur. In some countries, whole mountainsides have collapsed causing considerable loss of life.
In addition to the important role of roots, the leaves of the trees and forest undergrowth break the force of the rain, and the leaf litter and decaying organic matter on the floor of the forest help to soak up the water to prevent surface soil from being washed away. These are the scenarios that need to be replicated in the garden to successfully use plants to stabilize a steep slope.
In 2010, the Malaysian Public Works Department’s Slope Engineering Branch funded a study to determine the mechanical properties – specifically the pull-out and tensile strength – of the roots of some tropical trees and shrubs.
The tests were carried out by the Department of Engineering of the University of Malaya. In a research paper authored by Faisal Ali, it was stated that: “The role of root strength is important in stabilizing steep hill slopes . . . It appears that root systems mechanically reinforce soil by transferring shear stress in the soil to tensile resistance in the roots”.
The study found that amongst the species tested, Leucaena leucocephala had the highest root tensile strength and recommended it for slope stabilisation projects as it had outstanding root mechanical properties.
However, Leucaena leucocephala, which is a large scrawny shrub or small tree, is regarded as an invasive plant in many countries because it forms impenetrable thickets and crowds out all other plants. It is not an attractive plant, especially when covered with its unsightly brown seed pods. It is commonly known as Mimosa Bush, Wild Tamarind, Jumbay, White Leadtree or White Popinac in western countries, and locally known as petai belalang or petai tiga bulan in Malaysia and ipil-ipil in the Philippines. In Central America, from where it originated, its Spanish name is ‘guaje’.
Acacia mangium (commonly called Black Wattle, Brown Salwood or Hickory Wattle) was another species that was tested. This is a fast-growing hardwood tree that is widely planted throughout the tropics and harvested for paper and timber production. The results were not as good as Leucaena leucocephala but still proved to be quite effective at stabilizing slopes. However, again there is the problem that in some parts of the world it has become an invasive species.
Whilst Leucaena leucocephala and Acacia mangium may be suitable for large scale slope stabilization in ‘wasteland’ areas where control measures can be taken to prevent their spread, they are not species to be recommended for the home gardener. So what alternatives are there?
Most trees with extensive root systems can be used for slope stabilisation, but avoid those with very aggressive root systems because they can do more harm than good. For example, the popular Ficus benjamina (commonly known as the Weeping Fig) has an extensive root system, but the roots of a mature tree can lift paths and block drainage pipes, so is suitable for only the largest acreage properties.
The same applies to F. benghalensis, F. elastica and F. macrocarpa which are also popular in largescale landscaping projects, but not recommended for the home garden.
There are however a few Ficus shrubs that can be used for slope stabilization and are not difficult to keep under control. One of the most popular is a shrub form of Ficus microcarpa which is sold as Ficus ‘Green Jade’ in some countries and Ficus ‘Green Island’ in other countries. The downside of using these Ficus shrubs for slope stabilization is they are quite slow growing, so are better used as ‘fill-ins’ between other trees or shrubs.
Another tree that is popular in tropical gardens – Schefflera actinophylla (commonly known as the Umbrella Tree) – is a good choice for slope stabilization because it has an extensive root system but does not get as aggressive as many of the Ficus tree species.
Schefflera actinophylla (also known as the Octopus Tree in some countries because of the appearance of the red flower stems which arch out from the top of the tree like the arms of an octopus) is an attractive tree with glossy green leaves, and can be quite spectacular when in full flower. It can grow in either full sun or partial shade but won’t flower as prolifically when shaded.
This ornamental tree can also be used for screening purposes if the main stem is topped before the plant becomes too large. This will encourage the growth of multiple trunks. Alternatively, the planting of Schefflera actinophylla can be combined with a related species – Schefflera arboricola (sometimes called the Dwarf Umbrella Tree) – which is a large shrub that can be used to form a hedge.
If screening is not required (such as when it is desired to preserve views) then simply prune any multiple stems that develop from the base to force the tree to grow with a single trunk. In these circumstances, on well drained soils, Schefflera actinophylla can grow as tall as 12-15 metres, with most of the foliage at the top of the tree (thus not blocking views lower down). With multiple stems, usually the maximum height is around 7-10 metres.
When using Schefflera actinophylla for slope stabilization, don’t plant the trees too close together (3-4 metres apart is ideal) otherwise the root systems will crowd themselves out and this will defeat the purpose of planting them to stabilize the soil.
Bamboos are also a good choice for slope stabilization. The underground rhizomes of bamboo radiate out from the base of the culms to stabilize the plant, and in doing so will help stabilize the soil.
There is a wide range of ornamental bamboos from groundcovers to species with giant culms that can be used for different landscaping purposes. Most tropical species are of the clumping type and not very invasive. However, larger species can take up a lot of space eventually, so the choice of species will depend on how much space is available for planting and the extent to which screening is required.
Many species have attractive variegated culms that make handsome feature plants in the garden. The availability of species varies a lot in different countries, so check with local nurseries for information on species that are recommended for the region in which you live.
Mango trees (Mangifera indica) can also be used for slope stabilization because they have extensive root systems with a long tap root. However, a problem with mango trees is that they will eventually cast a lot of shade, so as the trees mature, the choice of understory plants becomes more limited.
Once the main trees and shrubs have been planted, attention can be turned to the understory planting. This can be almost any type of groundcover or small shrub, but bear in mind that as the larger plants grow and cast more shade, the type of understory plants being used may have to be changed into more shade-loving species.
Sphagneticola trilobata (syn. Wedelia trilobat), commonly known as the Creeping Daisy, Singapore Daisy or Bay Biscayne Creeping Oxeye in different parts of the world, is a particularly good groundcover because it will grow in both full sun and partial shade. It is fast growing, does a good job of crowding out weeds and looks attractive with its bright yellow daisy-like flowers. However, it will become straggly if the site becomes too heavily shaded.
In some countries Sphagneticola trilobata is listed as an invasive species, so it is best used only in areas where its spread can be controlled.
For large scale slope stabilization, nitrogen-fixing groundcovers should be considered as they help to improve the fertility of the soil as well as preventing soil erosion. Arachis pintoi (commonly known as Perennial Peanut or Pinto Peanut) is widely used in the tropics as a cover crop in fruit orchards, and is effective as a groundcover for slope stabilization. It will also grow in both full sun and partial shade.
However, Arachis pintoi does take longer to establish than Sphagneticola trilobata (more weeding is required in the first season) and once established, it can be hard to get rid of, so may not be the best choice for smaller properties.
Turf grass is not recommended for slope stabilization unless water run-off is not going to be a problem. When slopes are grassed, they do not absorb water as quickly as one on which groundcovers and shrubs have been planted – especially if the grass is a broad-leafed type like many of the Paspalum species. And turf grasses are heavy feeders, so they take more nutrients out of the soil than groundcovers and shrubs.
While the understory shrubs and groundcovers are getting established, it is best to keep the slope mulched as heavily as possible to prevent soil erosion. On steep slopes it can be a challenge to keep the mulch in place in heavy storms. Placing small bamboo or wooden stakes in the ground can sometimes help to prevent the mulch from washing away. Usually this is a problem only through the first rainy season. By the time the second rainy season comes around, the understory plants should be sufficiently established to hold the mulch in place.
For slopes in full sun where there are reasons for not planting trees or bamboo (for example, to avoid blocking views), a good choice is psittacorums. These are a small species of Heliconia that generally don’t grow more than a meter high. They are excellent for erosion control because their root systems are creeping rhizomes, and they can cope with periods of drought, unlike some other species of Heliconia. If they are kept well-watered and fertilized, they will become a source of attractive cut flowers as well.