Slopes in gardens can be challenging for both gardener and plants. The rooting environment of vegetation at the top of a slope is different from that of plants at the bottom. The steeper the slope, the less time water has to infiltrate the soil and the faster water runs off so that plants at the top have to be able to cope with much drier conditions. Unless the soil is held together effectively by root systems, water is not the only thing that runs down the bank.
It can take quite a while before a bank is stabilised by careful and considered planting. A quicker, and, depending on the gradient of the slope, easier way to deal with a bank is to retain the soil within walls of rocks, concrete, bricks, or timber. Rather than one massive reinforced retaining wall, terracing offers a gentler alternative. Terraces created by a series of low retaining walls allow rainwater to soak into the soil instead of running down to the bottom of the slope, and they are also more user-friendly for the gardener.
For a relatively gradual slope such as the one in the photo, a wall of one railway sleeper may be sufficient to retain each terrace.
The property owner has planted an Olearia lineata hedge at the top of the bank, which over time will develop into a fine-textured light grey backdrop for the plants in the terraces, and provide a privacy screen for the house.
A couple of mature trees like the banksia on the left and the golden elm on the right, are worth keeping. Their heights are in proportion to the roof of the house and as such create a link between building and garden. Initially, newly planted gardens for a newly built house often look quite sparse and lacking in depth. The house is usually the only vertical element on the site, and as such is not well-connected yet with its surroundings. Gradually this will correct itself as the plantings grow and gain in height. But for a more instant effect, investing in some large grade trees, or if possible, keeping some existing trees, is a perfect solution.
When planting terraces, try to regard the space as a whole instead of a series of separate garden beds. Choose a selection of plants with different shapes, textures and growth habits; rounded shrubs, plants with spiky foliage, grasses or grass-like plants, and ground covers. Repeat these in groups throughout the terraces. For plants that are visually more dominant than others in terms of shape, texture and/or colour, include more in the lower terraces than in the top ones. In this drawing you can see for example that to avoid the planting to become top-heavy, three rounded shrubs are grouped together with one in a terrace higher than the other two. Likewise the flax-like plants are arranged in such a way that there are more on the lower-level terraces.
The plants selected for this scene include two ground covers, Coprosma acerosa and Bergenia cordifolia. Coprosma acerosa (sand coprosma) is a shrub with fine intertwining brown-orange branches and tiny olive green leaves that forms springy mounds with a spread up to 1 m. Its fine texture forms a great contrast with the large glossy leaves of Bergenia cordifolia. The rounded shrubs are Hebe odora (box-leaf hebe). Spiky foliage is represented by Phormium 'Chocomint', a flax with chocolate brown leaves and green margins. The grass-like leaves in the scene belong to Apodasmia similis (oioi), formerly known as Leptocarpus similis. The latter is a very tough plant found throughout New Zealand in particular in coastal marshland and estuaries. It will grow fast in moist conditions, but also tolerates dry sites. Apodasmia spreads from creeping rhizomes and may need to be kept in check with root barriers.