Creating a garden on a sloping site brings a host of challenges, but, says designer Charlotte Harris, with careful planning it is also an opportunity to make a really interesting space.
How to design a garden on a slope: inspiration for your sloping garden
Natural changes in elevation can help break up a space. They enhance movement and offer up new perspectives and vistas. Gradients also offer exciting opportunities for thinking about planting differently. This is why, despite some potentially daunting slopes, a new garden site that we’re working on in northwest Scotland excited us from our first visit. Leaving aside the breathtaking views across a sea loch and the rugged terrain, this sloping garden has dynamic potential.
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In this case, we wanted to design a beautiful garden that could be sensitively woven into the landscape, while also minimising any heavily interventionist construction. A thorough topographic survey,
with contours and levels at regular intervals, was critical. This detailed mapping provided a precise understanding of the levels – an understanding that became even more nuanced through repeated on-site explorations of the site’s characteristics and features. Taking that time to consider, ideally on different days and at different times of the year, is invaluable.
A survey also allows for the calculation of volumes that might be required to fill areas for flattening or levelling out. That volume can then be matched up with areas of the garden that would benefit from a reduction in levels. Balancing the ‘cut and fill’ will ideally remove, but certainly reduce, the carbon-heavy and expensive options of removing waste material or bringing in new. It’s also essential to store the soil correctly on site to ensure it remains healthy.
For this Scottish project, we tried to avoid chunky retaining walls, designing them in as several smaller ones that are gently staggered, rather than a couple of huge whoppers. They’re made using the traditional craft of mortar-free, dry-stone walling, which allows water to pass through the crevices and gaps in the stonework, without the need for formal weep holes, which most retaining structures will need. Land drains have also been added behind the base of the walls to ensure a belt-and-braces approach in preventing a build-up of hydrostatic pressure.
The paths zig-zag and meander, working with the contours of the land, to make ascents and descents smoother and less tiring. When choosing materials for sloping paths, consider those that offer good traction, durability and permeability. On this project, where the slopes were gentle, along contours, we decided to use angular gravel sourced from a nearby quarry. It locks into place better than round gravel, and this angular look is also reflected in the surrounding geology. A compacted sub-base beneath the gravel provides a firmer base, reducing movement, and timber or metal edging also helps stop it moving downhill.
Steps and ramps
In steeper sections, steps and terraced landings are needed to make the garden easier to navigate. In our Scottish example, the steps in the lower loch-side garden are simple strips of reclaimed granite lintels, chosen for durability and landscape relevance, with a uniform riser height to ensure that people’s natural walking rhythms are not disrupted. A similar effect could be achieved with railway sleepers on edge, or timber planks – just ensure they are properly treated to withstand outdoor conditions.
Steps should be designed in scale with a garden. Here, they’re wide to suit the expansiveness of the setting; they open the space up and enable easier movement. Narrow steps might feel pinched here,
but in a more intimate space like a courtyard or small garden, they would feel entirely suitable. A hard landing at the base of a step run – gravel or natural stone – is a small but significant detail to prevent wear.
One alternative to steps is to use a ramp (no steeper than 1:12 for independent use and 1:10 for assisted use), which can be helpful for future gardening. Planning now how you’ll move barrows and bulky items about will leave your future self less sweaty and grateful for your foresight.
Managing drainage is particularly crucial in a sloping garden, and it’s worth taking specialist advice early on to plan it. In the Scottish project, land drains, planted swales and the narrow diagonal channels that are set into the ribbon tracks were designed to handle water together as it moves downhill.
Planting for slopes
lanting helps to stabilise a slope and manage water movement. Choosing species suited to the demands of slopes and to your specific setting (including exposure to prevailing winds) can deliver a beautiful, ecological and a practical ‘bioengineering’ function.
Slopes also require us to rethink our planting design, as they alter perspectives – taller species
at the back and shorter at the front won’t work here. There are different ways of experiencing the planting: from the side as you move up and down steps; from the top looking down; and from the bottom looking up. Also, the soil on a slope will hold more moisture at the bottom and be dryer at the top, so be sure to double check your right plant is in the right place.
Tap-rooted perennials, such as Centranthus ruber, Mertensia maritima or Eryngium maritimum, help to anchor the soil, as do the spreading root networks of ivies and ferns. Ornamental grasses, such as Deschampsia cespitosa, Anemanthele lessoniana and cultivars of Panicum virgatum – showstoppers for slopes that have texture and movement – also have fibrous roots that help knit the soil.
Pioneer plants including willow, broom, rowan, alder and gorse are also well adapted to trickier situations such as this. Using mulches on a slope helps retain moisture and reduce erosion. Heavier materials applied in a thin layer – site-won shredded prunings, larger bark or gravel – are less likely to be washed away.
Gardens on slopes are also wonderful opportunities to look to similar landscape typologies for inspiration – alpines, scree, rockery and crevice gardens are all up for grabs. Slopes move water and nutrients away. In my own sloping garden, I’ve added small linear pockets just above some of the tree whips we’ve planted on a steep slope, to encourage moisture to slow on its downward journey; a home-made version of techniques used in traditional agriculture. The idea of ‘micro-reservoirs’ might be coming on a bit strong here, but hopefully you get my drift.
Watch the drop
Lastly, when we are talking about slopes, I have to put my proverbial health-and-safety hi-vis vest on. Currently in the UK, level drops of more than 600mm require edge protection – usually a balustrade of 1.1m. You should always check current building regulations to make sure you’re on top of them. Even when drops are shallower than 600mm, consider how you can deter people going too close to the edge by designing in planting beds, or softening any small drop to the side. On the Scottish project, we will be using shrubs such as bog myrtle, broom and coppiced willow.
Sloping garden inspiration
- Pausing places avoid it feeling relentless when traversing steps. General guidance is for platforms at every flight or ten to 12 steps; we limit that to five to seven, with landings at least three times the tread width of a step if it’s a place to take a breath. This is increased if it’s a terrace, such as this ‘sitooterie’ with firepit.
- Crib walls are interlocking timber frames that cost- effectively retain steeper slopes. The timber gives a natural finish, especially once seeded with wildflowers and grasses.
- An anchored timber platform on buoyant pontoons to deal with the sea loch tides.
- Planting acts as powerful bioengineering of slopes, with carefully selected vegetation here including robust, structural shrubs and tap-rooted, mat- rooted and fibrous-rooted perennials. Slopes of more than 1:3 (33 per cent) are the top end of what can be planted without additional matting or terracing.
- Experimentation with brush mattressing, a technique used on riverbanks. Bundles of hazel branches are staked into the slope to reduce and slow run-off. Over time, they trap sediment, providing a base for plants and habitats to grow.
- Expansive steps, in scale with the setting, are designed as a feature as well as a practical way to navigate levels. They open up the space, enabling easier movement, and provide places to perch and enjoy the view. Designed to work with the contours, they gently turn and zig zag down.
- Step risers in gardens differ to houses, generally made a consistent height of 150mm. Too shallow, and the steps are uncomfortable to use; too high, and they’re too much effort. Treads are notably wider than the 300mm minimum we use in more compact spaces.
- Site-won boulders from the excavation for the house help with localised stabilisation. They make any grading more naturalistic, and connect to the setting. Varying the size from large to small is the trick to ensure it feels natural.
- Curving and switching back the driveway helps to manage difficult gradients. Brushed- finish concrete ribbon tracks with gravel in-between provide a non-slip surface and avoid heavy hard landscaping.
- Narrow diagonal channels are set into the tracks to help divert water run-off.
Case study for a sloped garden design
How Californian design studio Terremoto uses simple, natural design materials to tackle slopes
Well known for its regenerative, creative- design approach that prioritises stewardship and social responsibility, Terremoto produces work of radical re-imagination.
After talking to David Godshall from Terremoto about its design for two sloping gardens, I came away feeling so invigorated. “A wall with concrete should be the last solution, not the first,” says David, “and only used when all other options are exhausted. Let the land be. Slopes are exciting, and kids love to play on hills.” Responding to, rather than imposing on, the site comes from knowing it, walking it and revering it. In one garden, a simple series of trails avoids all tree roots.
Elegant, timber- fronted steps curve down through the landscape and carefully past the trees. This makes it enjoyable to explore, while at the same time is still mindful of rich, living underground networks.
In the other garden, dead trees from the site have been used to create retaining walls as well as a wonderful new habitat.
Discover more on sustainable garden design from Charlotte Harris