Design a Rockery for year-round interest
By Anno Torr
Do you have a slope or bank ready to be dressed more creatively than the default retaining wall blocks or mown lawn? An eye-catching cocktail dress rather than a comfortable pair of jeans; a brightly coloured pashmina to replace a worn monotone windbreaker? Aside from adding a beautiful feature, this style detail/ element increases the sustainability of the garden too, for it saves on lawnmower fuel, provides level planting pockets for better water penetration, and provides habitats for small insects and reptiles. All add to the garden's overall biodiversity. Many gardeners see rockeries as low-maintenance components, but, as with any planted area of the garden, this is only true if construction is good and the correct plant species are chosen.
Rockeries work best in full sun and on sloping ground, typically where one would find a natural rock fall.
Place rocks, so they appear to be part of the ground, rather than dispersed randomly over the surface – in the ground rather than on top of it. Rocks are expensive, so we want to display them as much as possible, but burying the odd large specimen almost entirely imitates a natural outcrop, giving your rockery the appearance of permanence and age. This idea works best on a more inclined slope helping to stabilise the steepest section.
Build up a tier of rocks beginning at the lowest part of the slope. Back-fill with soil behind each stone, tamping down as you go, to secure it in place and stabilise each level. Don’t position rocks in too regular a line; group a few together, and spread them out elsewhere.
Providing a wide base reduces the steepness of the slope; extra soil can be found to fill in the planting levels and makes for a more secure feature.
There are some rules-of-thumb when placing rocks:
Use the same type of rock – don’t mix different colours and materials.
Place them flat rather than on their end.
Partially bury the rocks rather than simply laying them on the surface, both for a natural look and to secure them.
The strata (lines or grain) must lie in the same direction.
Place them tilting slightly backward, so they don’t roll down the slope.
Use a variety of sizes and shapes, and create irregular groups of rocks set to ensure there is no pathway down which the soil can be washed down during a storm.
Drainage is essential; if soils have a high clay content, lighten the load that will sit behind the rock face by removing soil to just over a spade depth and adding in sand, gravel or even building material. Put back the soil and tamp it down before planting.
Make planting spaces between rocks large enough, so plants don’t outgrow their allotted space too quickly. Know the final spread of your plants and provide sufficient space to retain individual shape and form; when too many plants intermingle, while an attractive style in the right place, a rockery loses structure and form quickly.
Rocks absorb and radiate heat, so use plants that enjoy these conditions closest to the rocks.
Buying rocks: Firstly and this point needs frequent mention, buy quarry-mined rocks rather than weathered rocks, and only from a reputable seller, someone who can prove the provenance of their supply. Do not buy from a roadside hawker or any landowner for that matter, who is not in possession of the necessary permits. These rocks are more than likely taken directly out of the environment, destroying vulnerable habitats and increasing land degradation.
The first consideration is the micro-climate of the rockery. Slopes are generally dry as a result of run-off; rocks also absorb and radiate heat, raising the ambient temperature of the immediate surroundings. So if the rockery is north-facing and open to the sky, choose full sun plants with low water needs. Be aware too, of the prevailing winds that will exacerbate the dry conditions. However, rocks also shade the roots of plants growing around them keeping the soil cool and moist compared with more exposed areas; extra moisture also runs off the rock surface to pool on the ground around it. Depending on how steep the bank is, the new planting pockets you create can provide reasonably level platforms that aid water retention and percolation. So don’t feel restricted to spiny, desert plants; the article images of a South Coast garden designed and constructed by Elsa Pooley Landscaping, show a range of succulents, perennials, and groundcovers, all thriving in this sun-baked, wind-swept rockery.
The rockeries in the images on the left create features of diverse beauty on these moderate slopes, providing a focal point and small habitats for insects and lizards. Image 1: This feature rockery is viewable from the pool patio where it tucks into a small space off the path. Crassula multicava knits together a variety of aloes, succulents, and groundcovers. Species: Kalanchoe thyrsiflora, Aloe vanbalenii, Agapanthus praecox, Strelitzia reginae and S. juncea, Aloe arborescens, Arctotis sp., Aloe chabaudii, Bulbine natalensis and Encephalartos villosus. Acid yellow flowers of Kalanchoe longifolia add some height at the top of the bank in the second image.