Susan Jasan is a horticulturist and landscape architect with a passion for creating landscapes that provide the human spirit an environment that gives one’s soul the respite from life’s daily challenges.

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An Uphill Battle
by Susan Jasan    

If you’re gardening on a hillside, you’re probably facing some unique challenges. Maybe there area some challenges that you did not totally anticipate when you purchased your beautiful sloping property. 

Or possibly you thought your land was flat or had “just a little bit of slope across it.” Upon development, you came the realization that the property has a pretty significant drop across its entire distance. Now you’re facing the reality of retaining walls in order to fully utilize the land you cherish.

But don’t be discouraged, for as with any new challenge, there are also some great opportunities too, that may ultimately present you with some wonderful benefits.

Sloping, hilly property usually presents the following challenges relative to gardening:
• Little existing topsoil
• High erosion potential
• Hiking the hillside to garden
• Stormwater runoff: Where does it go?

However, it also presents these wonderful opportunities:
• Creating a great soil mix for optimal plant growth.
• Terracing some areas can create added visual interest and facilitates a functioning garden that you desire.
• Being pro-active about managing the stormwater around your property from heavy rainfalls will ultimately save you great expense, and sometimes can be an added feature to your landscape.
• Creating planting areas with great drainage.

Places to Plant
One of the keys to creating a good garden environment on a hillside is building retaining walls to hold your soil. The most common materials used for retaining walls include landscape timbers, stacked wood timbers, dry-stacked stone, pre-fabricated segmented concrete stacking blocks, railroad ties (not recommended) or even a mortared block and brick planter. 

No matter your style or budget, building a good, sound structure is critical on any hillside. Make sure you follow manufacturers’ guidelines whenever using a manufactured product and provide adequate drainage for water to weep out of the walls. Remember too that local codes, as well as the materials you use, may impact the height and width of your planters.

The depth of your planting site will also vary – 9 inches is probably a good minimum to consider, with 18 inches typically the maximum depth of soil mix required for good plant growth. This holds true whether you’re creating a vegetable garden, an herb or flower garden, or even a raised bed for woody ornamental plantings. Consider the condition of the subsoil as well. Compacted soil will prevent proper drainage.

Your planting area should be no wider than 4 feet if you plan to work from both sides of the structure. A 4-foot-wide bed gives you enough room for good root development, but also allows you to reach both sides easily.

If your site is for ornamental plantings, such as shrubbery or trees, the 4-foot width does not necessarily always apply. In fact, on a hillside, you will often vary the depth to complement a structure, or to accommodate the topography of the land. The height too will vary more with ornamental plantings. Be sure to check local codes for any restrictions on maximum wall heights.


The Soil Mix
The soil mix will vary by your area, but a general rule of thumb is to create a mix of one-third topsoil, one-third coarse sand and one-third well-composted organic material. It’s easiest if you mix this well before installing the material into the planter. You’ll find that if you use your planter for flower or vegetable gardening that each year you’ll benefit from adding a little more composted material to the planter, and mixing the new compost really well into the existing soil mix.

The Drainage
NEVER underestimate the power of water! Also NEVER underestimate how much rain can fall in a short period. Very often a “heavy” rainfall that gets our attention with flooding, is just a minor event compared to what a truly severe storm can become. If you’re unsure, consult your local authorities or a design professional such as a civil engineer to review your specific drainage circumstances.

With the above caution, there are some general considerations you’ll want to keep in mind.

When you locate your plantings be sure to consider whether they change any of the drainage on or around your property in an adverse manner. You also cannot change the flow of stormwater as it leaves your property on to adjacent neighbors, thus potentially causing them “drainage issues”.

Here are some drainage basics:
• Drainage is always measured as a percentage, usually referred to as the “percent of slope.” Knowing these guidelines should help you determine what strategy to use for your sloping property and any terracing.
• Percent of Slope is calculated as: Rise/Run = Percent of Slope The Rise is the change in vertical height over a specified distance.
• The Run is the horizontal distance being measured.
• 1/8” Vertical Change in Height over 1 Horizontal Foot of Distance = 1% Slope.
• Ground Surfaces: Minimum 1% slope, Maximum 10% slope is ideal.
• Planting Beds: Minimum 2% slope, Maximum 10% slope (for your raised garden bed, you’ll want some slope for good drainage).
• Above 8% slope on a hillside: Mulch will wash downhill. If you have more than 8% slope on your property, you’ll definitely want to consider creating terracing or raised beds to facilitate gardening and prevent washouts.
• Above 10% slope: Erosion will start, particularly without established vegetation. Thus at 10% slope you really must address erosion on your slope if you install anything other than turf. Above 15%, even mowing a turf area will become dangerous.
• No long walking sloped area should exceed 5% slope. Consider this guideline whenever you plan any walkway across your property, including any walkways that give you access to your garden areas. Hauling materials over any distance that has above 5% slope will become cumbersome at best.

With the above guidelines, begin planning that new raised garden or terraced hillside. Recognize too, that budgeting for raised beds and the soil mix to fill those planters will add up quickly. Consult your local professional landscape contractors to get estimates, or if you like to do it yourself, be sure to be very thorough in your planning and budgeting to prevent any surprises and added costs midstream. 



This article appeared in a previous publication of State-by-State Gardening magazines.


Posted: 06/13/19   RSS | Print


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