By Bonnie L. Grant, Certified Urban Agriculturist
Once you know what plants grow on slopes, you can use this knowledge to your benefit and plan a garden that both thrives and helps stabilize the hillside. Click here for ideas on choosing plants for sloping areas and how to maximize this difficult planting terrain.
By Jackie Rhoades
Steep hills in the landscape have always been a problem. Anyone who has mowed lawn on a hillside knows it?s no picnic. So what's a gardener to do? Read this article and opt for hill ground cover instead.
What happens if you take a level garden design and tilt it up 10, 15, 25 degrees? In other words lay that design on a hillside? The design has to change, perhaps quite a bit. Many of the gardens on the Master Gardener Bloomin' Backyards tour on June 6 of this year have had to deal with slopes. Two of the more significant hillside challenges were Linda’s and Jan’s gardens. These two gardeners have met those challenges using tried and true solutions, with some innovative tricks.
In addition to normal gardening issues, particular challenges hillside gardening must be considered:
The degree of slope, or steepness affects all of these considerations. A very gentle slope is not much different than a ‘flats’ garden, but a steeper slope certainly is.
See Gardening on a Hillside for more details on these challenges and some solutions.
Linda's garden style is generally Mediterranean--she is quite interested in drought-tolerant plants, natives and in rain gardens and rain collection. She built her now four-year old Prairie style home on a long, narrow sloping lot perched high above the street. The garden was very thoroughly thought out, and installed over a three-year period starting before the house was finished.
She combined both terracing and slope planting in creating her hillside garden. In the rear, the steeper slope was first cut to create level areas, and retaining walls installed to hold the terraces. In the front garden, she left the slope un-terraced, and created a dry creek that allows rainwater to percolate into the soil. Her garden is divided into 'rooms' with those nearest the house containing the thirstier plants: roses, deutzia, philadelphus, rhododendron and some hydrangeas as well as a Japanese maple and some bananas. In fact she's hydrozoned her whole garden, and made sure that plants with similar water requirements are on the same drip irrigation valves. The planting up the slope moving away from the house becomes more drought tolerant, with dodonaea purpurea, ceanothus, teucrium, callistemon, lavandula, lots of grasses, and drought-tolerant perennial flowers and fruit trees.
Linda says "designing for a slope can be a challenge. I found that using retaining walls to create flat, usable spaces was critical. Since it's important to have areas in the garden where one can sit, and, since having patio areas can help cut down on the amount of area needing to be planted, it's important to carve level areas out of a hillside garden. Although this can be expensive, it really makes the garden much more usable."
Like all hillside gardeners, she also had to deal with runoff and irrigation. Linda is a strong advocate of both rainwater capture, and storm water management--in other words, not allowing lots of rain to run into the storm drains unused--so she set up a system of berms and swales to direct and capture that rainwater, noting "I do a Master Gardener Library Series talk on capturing rainwater and keeping it in your garden and I like to practice what I preach!" She also states, "I keep the entire slope heavily mulched at all times--mulch is a critical part of holding water on a slope."
Like Sara's garden, featured last month, Linda's concentrates more on shape and foliar
texture and color than flowers. She says "I planted for color and texture interest with pops of flowering color to give it continual interest." For example in one area she planted purple-leafed sambucus with yellow-leafed deutzia, and on another slope she mixed ceanothus 'dark star' with spiky grey teucrium fruiticans, callistemon and echium fatuosum to form a huge grey-green mass of texture with a hit of dark green.
Another design criteria was the winds in Petaluma. This was accommodated by providing patio areas near the house protected by the wind, and planting hedgerows of trees and shrubs along the southwest property line to help buffer the wind. That hedgerow also includes Vitis californica 'Roger's Red', Sambucus, Chaenomeles, and Rosa californica to provide food in the winter for birds and animals.
Jan and her husband didn’t have a lot of space due to their lot being located on an old rock quarry on a slope. Lots of rocks, and huge roots from a neighboring 100-year-old pine were special problems. In addition, a city-owned hillside above their lot would send rainwater sheeting downhill to wash across their front yard.
Their primary weapon was terracing—they built an interconnecting series of raised beds and decks. Don built brick planters and stone terraces using some of that quarry rock, and those became the backbone of their garden. Drip irrigation is zoned to take care of watering the different beds and levels. A berm was built across the slope on the uphill side of their property to divert the rainwater away from their front yard and down the easement into the street gutter—simultaneously solving their runoff problem and allowing the storm water a longer path to soak into the ground before reaching the storm drains.
Jan says of her gardening style that the garden is a sort of a specimen garden, with a leaning toward a Japanese style. She says “I am an admitted plant addict. . . .when I see a plant I love, I usually buy it. Lately I’m trying very hard to purchase only things I have a good place for.” She likes color and cut flowers, and has zinnias, roses, dahlias and sunflowers in a side garden. The front garden was recently remodeled and high-maintenance perennials have been largely been replaced with an all-season garden of shrubs and trees emphasizing shape, form and foliage. She accents the form and foliage with splashes of seasonal color using impatiens and begonias in summer, and a few bulbs and Icelandic poppies in winter. Jan continues “I have many succulents which bloom, ferns, hosta, orchids and a fuchsia house—but I’m trying to cut down on the constant maintenance.”
Hardscape can be a very important part of designing hillside gardens. In level areas or on terraces, flagstone or gravel allows rainwater to percolate into the ground. Stonewalls can be part of the hillside terracing. In that new front garden, Jan’s son built a dry stacked wall to create a garden room, and add further visual appeal to the garden.
These two gardeners faced similar challenges in different circumstances, and successfully used the tools of the hillside gardener to create spectacular AND water efficient gardens.
If you aren't able to find the exact species or cultivars listed above, substitute with others that have similar colors, shapes, and sizes. And because some plants can become overly aggressive and spread out of control in certain climates, always check which species are considered invasive in your area before planting.
In addition to normal gardening issues, particular challenges in hillside gardening have to be considered:
To Terrace, or Not--hillside retention and erosion
Hillside retention and erosion control are two key issues in slope gardening. You really have two choices, which are partly driven by aesthetics, partly by cost considerations and influenced by steepness: to terrace, or not to terrace. The steeper the slope the better the garden will behave if terraced. Terracing is attractive, and also helps prevent runoff and allows rainfall to seep into the soil. And the steeper the slope, the closer together the terraces will be.
Methods of creating and holding terraces include stonewalls, wood frames, like a larger version of half a raised bed, concrete retaining walls, and more. Leaving the slope un-terraced is the less expensive route, possibly by far, depending on the method of terracing you would otherwise employ. In the end, if you can afford it, you will probably have a better, and more useable hillside garden if it’s terraced.
If you leave your slope natural, plant selection is doubly important. You don’t want shallow-rooted plants on a hillside, but rather you want deeply rooted plants to help retain the hill and avoid erosion. Perennials that have deep root systems and well-rooted grasses and small and medium shrubs perform this function well.
What to plant on a slope, and where on the slope to plant various species are key considerations. The larger the root system the better, so trees are great, with their extensive roots. Shrubs require less maintenance than perennials, and in addition, generally have a more extensive root system.
Shrubs which are great for slopes include Arctostaphylos, Ceanothus, Erica, Helianthemum and Euonymous. Drought tolerant selections include Baccharis pilularis 'Twin Peaks', Cistus, Correa, Grevillea, Mahonia, Rhamnus and Rosmarinus.
Larger plants, shrubs and trees should be planted vertically, not pointing out of the hillside. This actually makes each plant a mini-terrace. Build a small half well on the bottom side of the plant to help retain water and allow it to soak into the soil around the plant. Smaller plants and groundcovers to fill in while larger plants take hold can simply be planted flat with the hillside.
Composting to amend soil is usually a good idea—certainly if your hillside soil is sandy or clay. Hillside gardens, like all gardens should be well mulched. But the choice of mulch is more critical. Things like straw, small bark or cocoa hulls will just wash away. Finely shredded redwood, known as angel hair, or the coarser shredded rather than chipped mulches tend to knit together into a mass, and are less (but not completely) susceptible to being pushed downhill by rain. Two that hold well are called mixed fir bark, and vineyard mulch.
Since hand watering of a hillside garden is not very convenient, you’ll want to install drip irrigation. Alternatively, if you’re interested in a low-water slope garden, you can select very drought tolerant plants, and elect to hand water them for their first two summers, rather than installing irrigation.
Plants put at the top of a slope will get the least water, those in the middle somewhat more, and those near the bottom the most, as water running downhill section is soaking in more and more. Accordingly, within whatever water-use segment you’re planning the garden, the most drought tolerant should go toward the top, and those that can tolerate more water closer to the bottom. Exposure plays a factor too. And the direction the slope faces will impact plant selection: southern and western exposures are hotter, and northern and eastern can take plants that can stand less direct sun.
Access and maintenance
Access to a hillside garden is necessary for maintenance, weeding, pruning and watering, and can take different forms. If a very small garden, access can be from the edges. Larger gardens require access to the innards, via paths, walkways or steps. Steps down the slope portions, and paths more appropriately access terraced gardens across the terraces. Naturally sloping gardens can be accessed by paths down and across the grade.
Rainwater and Runoff Management
This is interrelated with irrigation, plant selection and erosion control. Rainwater can be an enemy of a hillside garden. Unchecked, it can cause serious erosion. Rain running down the hill and off the garden into city storm drains is a problem that municipalities and water agencies are striving to correct.
Rain can become an asset, and runoff turned into irrigation with proper design. Terraces accomplish this inherently by providing level garden areas for the rain to soak in and not runoff. Where a slope is not terraced, berms across the hillside with narrow swales behind, and winding stone-lined creek beds can slow the movement of water, allowing it to soak in. A rain garden at the bottom of the slope allows the last of the runoff to gather and irrigate thirstier plants assembled there.
If there is a slope uphill from the house, berms can also be used there to send surface water around the house for use in lower gardens. French drains above the house can be used to keep sub-surface water away from the foundation and crawl space, and also be channeled to gardens below.
So although a hillside garden can present a greater challenge than flat ground—there are solutions to those challenges that can lead to a beautiful, ecologically sound and success garden.
©Sonoma County Master Gardeners
Rise above it
Constructing a deck out over a steep downward slope bags you additional level space for entertaining, with views over the garden. Depending on the angle of your slope, the deck could also be raised on pillars to create storage or even a play space beneath.
Plant a living mural
If your garden slopes upwards, the rear boundary will be clearly visible, rising up at the end of your garden. So make a feature of it by creating a fabulous planted wall, as here. Alternatively, grow trees in pots along the wall or train climbing plants up it, for a green backdrop to life in the garden. Here, strips of lawn and shallow steps with inset lights create a green carpet leading to this lush feature.
Walk on the wild side
Planting a slope with a relaxed mix of hardy, low-maintenance plants, rather than neatly lined-up specimens that require a lot of care, is a great way to make a garden slope attractive without the need to clamber around on it with the secateurs every weekend. This slope has wooden steps leading up it, with a small terrace halfway as a pause point from which to enjoy the planting.
While an outdoor dining space is best positioned close to the house, an area devoted to reading or lounging can be located anywhere in the garden. Make the most of the more private, lower reaches of a downward sloping garden by building a small terrace here and adding garden furniture.
Steps are a necessary ingredient of a sloping garden, but you can create a very naturalistic effect by growing grass on them. These stairs leading up are covered with Korean grass, which is tough and bright.
Liked this? Share photos of clever design solutions in your garden in the comments below.