Provided by the Western Massachusetts Master Gardener Association

April 2002 - Planting A Slope: Tips and Plant Suggestions
By Judy Lochbrunner, Master Gardener

Slopes and banks are found in yards of all sizes, shapes and locations. A city front yard may be higher than street level and contain a bank down to the sidewalk or a suburban backyard may slope away from the home to a gully or stream below the lot's boundary. Hills and valleys may both wind through a large yard in a country setting.
Perhaps the slope or bank in your yard is a difficult or even dangerous to mow area. Ground covers are generally recommended for slopes steeper than 2:1 (1 foot vertical change for every 2 horizontal feet). You probably don't need to measure how steep your slope is, as you are well aware of those areas that require routine and time-consuming hand trimming or weeding. Or, perhaps, the slope or bank in your yard is that rocky, eroded area where nothing will grow. You regularly have to clean up the gravel and sediment that washes to other parts of your yard or pavement after a heavy rain or spring thaw.

Plan to rid your yard of ugly-looking slopes and their high maintenance by making them assets to your landscape design with only reasonable maintenance requirements. Many landscapers build mounds or excavate depressions on level ground knowing that changes in elevations add interest to a garden design; you too can use this landscape asset to bring beauty to your yard. Begin the process by examining your slope or bank.

First, be aware of the size of your slope and the extent of the erosion that has occurred. On small slopes that are not too steep, recontouring, which is defined as creating a series of level planting beds or terraces, can be done on a small scale with shovel and wheelbarrow. Large and extremely steep slopes that require major changes also require the professional services and advice of a soils engineer, landscape architect or other landscaping professional. Erosion will occur on any hillside or slope not covered by vegetation or structures. Signs of erosion are tree roots exposed above ground, small stones and rocks on the surface, small gullies, and silt or sediment built up and deposited in low areas. If you suspect a serious erosion problem, consult with one of the professionals previously mentioned. Plants that are properly selected and planted can solve less serious erosion. Plants are one of the best erosion controls as their roots permeate the soil to hold it in place and their foliage breaks the force of falling water.

Second, be aware of the microclimates or areas that are a little warmer or cooler, wetter or dryer, or are more or less windy than other parts in the yard. Slopes are generally dryer than other parts of your yard as water rapidly drains instead of soaking the soil. Plants on a slope need to be more drought-tolerant than those do in other parts of the yard. Cold air also flows downhill and settles at the bottom. More tender or less hardy plants will have a tougher time coping in a cold hollow at the bottom of a hill. The amount of sun or shade is another plant requirement that needs research on a slope. Improper amount of sun or too much shade will prevent a plant from thriving on a slope. Collect soil samples on the slope and do a soil test. This information will allow you to amend the soil as needed when planting and make more informed plant selections.

Third, planting on a slope requires a constant watch on erosion. Plan to work on only a small area at a time. Clear only the areas that you will immediately plant; preserve the rest of existing vegetation as a "net" to hold the slope until your next step of the project. Clean out weeds (tops and as much of the roots as possible) and remove grass with a spade or sod cutter. This turf can then be used to patch other parts of your lawn or added to the compost pile. If you choose to use a systemic, non-residual herbicide such as glyphosate (Round-up (tm)) read the directions completely and follow them carefully, specifically note the time requirement before re-planting. Once the grass has been killed, do not disturb the areas you are not immediately planting. Use the dead grass's root system to hold the soil in place. Dig out only the planting holes and leave the dead grass to form a protective "netting" against erosion until the new plants fill in the area.

When planting the slope amend the soil with organic matter (and any other amendments determined from the soil test). The soil may be rocky, shallow, and difficult to dig or may even roll down the slope as you loosen it. Choose young plants in small containers. Dig individual planting holes that are big enough to accommodate the root system. Avoid planting in straight lines or rows. A staggered placement will prevent water from running straight downhill. Mulch in between plants until they grow to fill in the area. For vines, plant at the top allowing the stems to drape downward and also plant at the bottom placing the stems up the slope securing with U-shaped pins for more uniform coverage. Seeds can be planted using a jute netting (or coarse weave burlap) to hold in place. The jute or burlap can then be left and allowed to slowly decompose in place.

Another option for planting on slopes is to create level planting spaces on the slope. Open terraces can be dug into the slope in the shape of steps. The existing slope can be cut and the excavated soil can be used as fill. A low soil berm can be formed at the front edge of each step or terrace to slow the flow of water. A boulder terrace can be created by partially burying the stone and filling in behind the rock with soil. This can give the slope the look of a rock garden. Raised beds built sturdy enough to hold the plants and soil in place can terrace the slope. Logs, lumber, and landscape timbers are just a few of the more common materials used for raised beds.

Plant selection and design ideas can come from a drive around your area (most gardeners will gladly discuss their plantings), an arboretum or botanical garden, local nurseries, a garden symposium or lecture, a visit to the library or a search on the internet. The list that follows is just a place for you to begin your search and gather ideas. Don't limit yourself to one plant or one type of plant on a slope. Take the time to do your research it will save you much time and increase your enjoyment in the future.

A selection of groundcovers and low growing shrubs for a slope
(Zones 4 & 5):
Many of these plants are very aggressive spreaders. Their tenacity is their key to success on covering a slope. They should be contained with an edging or planted in an area where spread in not a problem. Heed this caution to maintain your enjoyment and prevent maintenance problems in the future. A plus is that the stone, brick or metal edging used can also be a benefit to your garden design by allowing you to visually separate areas for a more pleasing result.
Achillea tometosa (Wooly Yarrow) Good choice for dry slopes. Contain by edging or plant where spread in not a problem.
Ajuga reptans (Ajuga) Requires little maintenance. Full sun to partial shade. Use edging strip to keep away from lawns.
Akebia quinata (Five-leaf Akebia) Climbs by twining; vigorous. Fine textured foliage with dark green leaves in summer.
Arctostaphylos uva-ursi (Bearberry) Native; evergreen, forms mats of woody stems with shiny green leaves (bronze in winter). Difficult to establish so obtain as pots or sods. Prepare soil with acid peat moss and sand.
Berberis thunbergii (Japanese barberry) This plant, which is included on many planting lists, is considered an invasive woody plant in parts of New England. Avoid its use in your home landscape
Coronilla varia (Crown Vetch) Ideal for difficult sites and steep slopes. Can be invasive.
Euonymus fortunei (Wintercreeper) Low growing evergreen vine; best choice for dry and shady sites.
Calluna vulgaris (Scotch heather) Acid, well-drained, poor soil. Many varieties to choose from to provide your slope with year-round color. Generally grow to a height of 4-18 inches with spread of 1-3 ft.
Chaenomeles lagenaria (Flowering quince) Height 6ft with 4-5 ft spread. Many varieties available in different spring blooming flower colors.
Cotoneaster horizontalis (Rockspray cotoneaster) Dense branching habit, horizontal growth spreads as branch tips touch the ground and take root. Mulch underneath plant for best weed control.
Convallaria majalis (Lily of the Valley) Spreads quickly by creeping roots; good for shady spots. Leaves turn brown in mid-late summer so take care to place where they are not a focal point during this time. Orange fruits are poisonous.
Erica carnea (Spring heath) Moist but well drained acid soil. Low creeping evergreen plants. Investigate the many varieties available.
Forsythia 'Arnold Dwarf' (Arnold dwarf forsythia) Good foliage cover; but blooms sparsely and only after it is well established. Height 2ft with 3-5 ft spread.
Hedera helix (English ivy) Use on slopes for erosion control. Can become invasive. Surround with edging strip and ruthlessly cut any shoots that cross.
Hemerocallis fulva (Tawny daylily) Spreads by thick tuberous roots. Excellent choice for erosion control on slopes and hillsides. Pest-resistant.
Juniperus horizontalis (Creeping juniper) Use for covering even steep slopes where other plants won't thrive. Mulch underneath for weed control. Evergreen plants with many colors, sizes and growing habits. Will not tolerate foot traffic.
Liriope spicata (Creeping lilyturf) Prefers acid soil. Spreads by underground runners and can become invasive. Contain with an edging strip.
Microbiota decussata (Siberian carpet cypress) Soil of pH of 5-7. Grows well in shade. Green feathery foliage turns bronze in the fall.
Oenothera speciosa (Showy sundrops) Spreads quickly by underground runners. Contain with an edging strip that extends a few inches below ground level. Good choice for dry, sunny slopes.
Pachysandra terminalis (Japanese pachysandra) Popular evergreen groundcover that provides good coverage for shady slopes. Easy care.
Parthenocissus quinquefolin (Virginia Creeper) This native plant, which is included on many planting lists, is considered an invasive woody plant in parts of New England. Avoid its use in your home landscape.
Phlox subulata (Moss pink, thrift, ground pink) Slightly acid soil. This low growing evergreen plant provides spring color.
Potentilla fruticosa (Shrubby cinquefoil) Low growing shrub that flowers all summer long.
Rhus aromatica (Fragrant sumac) This native plant is a poison ivy look-alike but has no skin reaction. Good fall color. Suckering growth habit enables it to spread rapidly as groundcover. Contain, as it can become invasive
Rosa rugosa (Rugosa rose) Mulch heavily to prevent weed and grass problems. Upright grower (6ft) with many arching canes with unlimited spread.
Rosa spinosissima (Scotch rose) Height 3 ft with 3-6 ft spread. Dense habit.
Rosa wichuraina (Memorial rose) Canes root to ground to form mat. 12 inches tall with 3-6 ft spread.
Stephanandra incisa (Lace shrub) Groundcover to fill large areas on slopes. Height 4-7ft with 3 ft spread or more if lower branches take root.
Symphoricarpos alba (Snowberry) Upright loose shrub 3-5 ft tall. White fruit on arching branches. This native plant tolerates poor soil.
Symphoricarpos orbiculatus (Coralberry, Indian current) Hardy in cold and drought conditions. Spreads rapidly by underground stems. Contain.
Vinca minor (Common periwinkle) Plant this spreading evergreen vine where spread is not a problem or surround with edging. Excellent choice for steep and shady slopes.

Ornamental Grasses
Ornamental grasses offer a variety of design ideas and options to planting a slope. Investigate the use of these plants for your yard.
Fescues, which all form dense clumps, can be used as alternatives to conventional ground covers. Festuca cinerea (Blue Fescue) is one example.
Bouteloua curtipendula (Side oats grammagrass) attractive groundcover. Adapts well to dry soil; good erosion control.
Sorghastrum nutans (Indian Grass) Good groundcover for erosion control. Will reseed in moist areas. Handsome native once covered the prairies.
Wildflowers and other Perennials

Drifts of wildflowers are another excellent choice for a slope. Jute netting or coarse-weave burlap will hold the seeds in place on grass-free and properly prepared soil. Purchase good quality seed mixtures that contain no weed seeds (frequently found in bargain mixes). Wildflower drifts or patches will look best contained in an edging, framed by a hedge of shrubs or bordered by ornamental grasses or a distinctive plant border. Keep your drifts or patches of wildflowers small as they will require careful weeding and extra watering during their first season. After the wildflowers are established, routine maintenance consists of a yearly mowing after a hard freeze when seeds have dispersed, and occasional weeding. You need to be mindful of city ordinances and subdivision covenants that seek to prevent weedy lots. A slope planted completely in wildflowers could fall under the heading of "weeds". From a design perspective, informal drifts of wildflowers look best when contrasted with more formal borders.
Perennials, planted as accents or in odd-numbered (3, 5, 7 plants etc.) groupings are another excellent choice to add color to your slope design. Follow careful plant selection by choosing plants that will tolerate the conditions on your slope. Note the occasional maintenance required by some perennials such as cutting-back and dividing. Do not to place them in a difficult to reach area of the slope.

The Importance of Mulch
Applying: Apply a thick layer of mulch to the soil surface between the plants on your slope. (If you are planting seeds the jute or burlap is your mulch.) Do not cover the plant's crown, which is the area where the roots and stems meet. Avoid lightweight mulches that will blow away in a wind and rounded mulches that will roll downhill. Organic mulches such as shredded bark; pine needles and compost will slowly break down and improve the soil structure. Leaves collected during the fall and chopped-up by a lawnmower make a great mulch for the following spring. Hay contains weed seeds while straw is generally seed-free making it a better choice. Fresh-cut grass forms a dense mat, which prevents moisture from reaching the soil so add this material to your compost bin instead of to your landscaping.
The Value of Mulch: Reduces run-off, conserves soil moisture, suppresses weeds, protects plants from winter-injury, prevents wide-fluctuations in soil temperature, and reduces soil splashing which can prevent the spread of some plant diseases.
Happy Gardening!

Native Shrubs and Vines for Northern New England Landscapes by Norman Pellett published by The Friends of the Horticulture Farm, 2001
Rodale's Successful Organic Gardening Lawns, Grasses and Groundcovers, Lewis and Nancy Hill, Rodale Press 1995
Landscape Plants for Vermont, University of Vermont Extension Service, 1999 Update
Sunset Hillside Landscaping, Susan Lang and the editors of Sunset Books, Sunset Books, 2002
Livable Landscape Design, John F. Collins and Marvin I. Adleman, Cornell Cooperative Extension Publication, 1988