Hillside Gardening
Gardening Newsletter
Vol. 10, Issue 11 - November 2015
The design of a hillside garden is influenced by its steepness, stability, soils, microclimates, existing
plant material and costs. Planning such a garden starts with a detailed topographical survey
(imperative for slope calculations and cross-section drawings) along with soil samples. Instability, a
concern on steep slopes, is often indicated by surface soil cracks, debris accumulation at the slope's
base and trees and fences leaning downslope.
There are a variety of landscape options. One is the "do nothing or very little" approach. This works
if the existing slope is gentle and stable with healthy vegetative coverage. Often all that's needed is
judicious pruning to open selected views, removal of aggressive vines such as wild grape and wild
cucumber, and the creation of paths for access and maintenance.
Steep, sunny, stable hillsides could be left to low-growing, deeply rooted, infrequently mown grasses.
Root-spreading ornamental grasses (Volume 7, Issue 7) can be planted in broad bands to provide
further soil anchorage while adding striking form, texture and animation. If you have youngsters,
consider leaving a portion of the grass slope open as a play area. Alternatively, your slope could
be planted with groundcovers such as fragrant creeping thyme (Thymus spp.), moss phlox (Phlox
subulata), ajuga, sedums, barrenwort (Epimedium spp.), purple wintercreeper (Euonymus fortunei
'Coloratus'), heaths and heathers, and low-spreading junipers.
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Hillside Gardening
Gardening Newsletter
Vol. 10, Issue 11 - November 2015
Unstable hillsides that exhibit erosion and drainage problems are the most challenging. Their
conversion to usable garden space may involve moving earth to create terraces supported by retaining
walls. Brush cuttings, contour wattling, planted gabion baskets (stone and cobble filled wire baskets)
and geogrid systems may also be considered. Drainage may take the form of diversion berms and
ditches that direct runoff away from problem areas to interceptor drains. The advice of experienced
professionals along with various permits is often required.
While costly, terraces provide relatively flat areas supported by retaining structures. These are
made accessible by paths, stairs and ramps. A series of terraces can accommodate different garden
functions (e.g. a wildflower garden, play area, vegetable garden, bird habitat, etc.) that take advantage
of various microclimates. Retaining walls, which stand out, should, for harmony and unity, be
constructed from materials used elsewhere onsite (e.g. stone, timber, precast concrete) and softened
with trailing plants. The effect is that each terrace, flanked and screened by maturing plantings,
becomes a hillside garden room.
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Hillside Gardening
Gardening Newsletter
Vol. 10, Issue 11 - November 2015
Where bedrock comes to the surface of a sunny exposed hillside, which is often the case at cottage
and country properties, a different type of hillside garden is necessary. Vines and creepers can be
planted at the top of the slope and allowed to cascade, adding color, texture and pattern. These could
include Virginia creeper (Parthenocissus quinquefolia), bigleaf wintercreeper (Euonymus fortunei var.
vegetus), creeping bearberries (Arctostaphylos spp.), cotoneasters and weeping forsythia (Forsythia
suspensa). Conversely, bittersweet vine (Celastrus scandens), virgin’s bower clematis (Clematis
virginiana) and hardy climbing roses could be trained on wire or plastic netting to ascend the slope.
Cracks, crevices and holes in the rock face can be filled with sempervivums, miniature sedums, ferns,
rock cress (Arabis spp.), pinks (Dianthus spp.) and moss phlox. Furthermore, where bedrock forces
ground water to the surface, a bog garden is a possibility.
Lastly, a sunny, rocky slope could become a rock garden featuring alpine and scree plants. This option
is sometimes implemented on reverse-slope lots (the rear yard slopes toward the house). To avoid
having the rocks and boulders overpower the plants, bury the rocks up to half of their depth and have
roughly the same surface area of rocks compared to planting pockets. These pockets, depending on
their exposure and soils, could favor a diversity of interesting alpines including Gentiana, Saxifraga,
Lewisia, Dianthus and Arenaria species.
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Hillside Gardening
Gardening Newsletter
Vol. 10, Issue 11 - November 2015
Design Considerations
The design of a hillside garden benefits from a top-down and bottom-up approach to maximize views.
Top-of-slope views are generally panoramic, capturing the larger surrounding landscape. Bottom-up
and cross-slope views focus on the slope’s features and plantings.
Paths, stairs and ramps are the organizing backbone of the hillside garden. They can be used
to control views, direct movement and facilitate the maintenance of and provide safe passage to
landings, observation decks and hillside gazebos. Stairs, which often stand out, can be made less
visually imposing by setting them at an angle to the slope and softening the edges using low plantings.
Landings should occur where paths change direction and at terrace locations. With paths, avoid
straight-up-the-hill alignments. Instead, position the path gently meandering up the slope, which
makes it appear more natural and user friendly. Path material can vary widely from wood chips and
crushed stone to flagstone and precast paving units. Pay particular attention to the material’s slip
resistance. Lighting is a real asset for safety and security.
A hillside garden offers the opportunity to incorporate some special features such as a cascading
stream with waterfalls and naturalistic pools. When built adjacent to paths, these features add focal
points, mystery, animation and sound, and attract birds. Be sure to confirm any permit requirements
with your municipality.
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Hillside Gardening
Gardening Newsletter
Vol. 10, Issue 11 - November 2015
Planting
An inventory of existing slope vegetation is necessary to assess species performance, slope
coverage, plant biodiversity, habitat value and possible new plantings. Hillside plantings frequently
rely on mass plantings for both impact and root-holding soil stability. Plantings parallel to the slope’s
contours help direct views across the breadth of the slope, making it seem larger and more interesting.
Avoid straight-line plantings in favor of more natural gently meandering ones. Use plant species that
are also found in adjacent tablelands to visually unite the areas.
Plants with arching and drooping forms are well suited for hillsides, as they stand out when viewed
from below. Check out shrubs such as cutleaf stephanandra (Stephanandra incisa ‘Crispa’),
beautybush (Linnaea amabilis), bridal wreath spirea (Spiraea x vanhouttei) and bush clover
(Lespedeza spp). Popular slope perennials include Solomon’s seal (Polygonatum biflorum), rose
twisted stalk (Streptopus roseus) and various native ferns. Pay close attention to their predicted height
and spread at maturity and be sure to confirm plant hardiness.
Selected plants need to quickly establish themselves and preferably have dense foliage to intercept
erosive raindrops. Both may be achieved by denser original plantings. Plants that clone rapidly by
spreading roots, such as trembling aspen (Populus tremuloides), sumac (Rhus spp.), chokeberry
(Aronia spp.), Japanese kerria (Kerria japonica), Saskatoon serviceberry (Amelanchier alnifolia) and
various shrub dogwoods, are cornerstones of many naturalized hillside plantings. Coppicing (pruning
trunks almost to ground level) encourages some of these shrubs to send up new spreading basal
stems.
Large drift and block plantings of rhododendrons and azaleas light up many slopes with their abundant
colorful spring blooms and attractive summer foliage. When planted on hillsides, ensure that these
plants take advantage of positive drainage, partial shade and frost-free pockets. Hedges may be
required on the flanks of the hill to lessen wind exposure for these colorful species.
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Hillside Gardening
Gardening Newsletter
Vol. 10, Issue 11 - November 2015
When planting trees and shrubs on slopes, dig a planting hole two to three times the width of the root
ball and use the excavated soil to build a rim on the downhill side of the hole. This helps deflect runoff
water into the planting area. Bare-root cuttings called whips spread quickly by creeping root. These
can be wrapped in newspaper containing soil and fertilizer and inserted into slope holes. On larger
sites, phase the plantings to allow time to monitor results and make changes if required.
Seepage areas along slopes where moisture oozes to the surface lend themselves to moistureloving sponge plants. These might include skunk cabbage (Symplocarpus foetidus), marsh marigolds
(Caltha palustris), Japanese candelabra primula (Primula japonica), rodgersia, umbrella leaf (Darmera
peltata), hostas and various ferns. On dry, sunny slopes, you can plant many fine drought-tolerant
xeriscape plants (Volume 7, Issue 5).
Hillside gardening is not maintenance free. Establishing new hillside plantings can prove challenging
and may require the use of soaker hoses and drip-type irrigation and the addition of mulches and
erosion-control mats. Invasive, weedy plants such as coltsfoot (Tussilago farfara), dog-strangling vine,
horsetail (Equisetum spp.), bracken fern (Pteridium aquilinum), wild cucumber (Echinocystis lobata),
garlic mustard (Alliaria petiolata) and common buckthorn (Rhamnus cathartica) can quickly take over
a hillside garden. Due diligence is required to remove offenders. You’ll also need to continually monitor
the hillside plant communities.
Gardens built on hillsides are spectacular and rewarding but developing them does pose challenges.
Those who have created such gardens cherish their special features and beauty. I hope this article will
prove helpful to those about to tackle this exciting endeavor.
Text by Frank Kershaw
Photos by Marnie Wright
Frank Kershaw is an award-winning horticulturist with more than 35 years of experience. He teaches
garden design and horticultural courses at George Brown College in Toronto and at the Toronto
Botanical Garden. Frank is also a seminar presenter at the Toronto Lee Valley stores.
Marnie Wright ([email protected]) is a lifelong gardener, writer and passionate garden
photographer. Her Rocksborough Garden, developed over 30 years, is located in Bracebridge,
Ontario.
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