If You're So Inclined – Slope Planting
Hillside properties have the potential to create the most beautiful of landscapes–combining a backdrop of trees, green shrubbery, trailing vines and spikes of color for a lush, layered effect. The rich, rolling tapestry of vegetation evokes a sense of privacy and serenity, setting the tone for the entire landscape.
When designing a level property, landscape designers often intentionally create elevation changes using raised planters and varying heights of plantings to achieve a layered look–so embrace your slope!
Caution must be taken, however, as slopes do require special considerations before planting to prevent erosion and runoff.
Before you start selecting plants for your slope, consider what you want out of the space. Is the slope to be viewed or used? Is it a focal point or on the side of your property rarely seen? Do you want to create a viewing deck or terrace the slope to cultivate a productive garden? If you simply want to protect and plant it, read on.
If the pitch of your slope is more than 30 degrees, consider terracing or building retaining walls, especially if your soil is sandy. A quick way to roughly determine the angle of a slope is use the “Measure” app on your smart phone by using the level feature and placing your phone on its side on the slope to read the angle directly.
Since slopes are prone to erosion and mudslides, strategically planning a mix of trees, shrubs and ground covers will ensure you have varying root depths that will weave the hillside in place.
Long-term studies have shown that a well-designed hillside garden planted in native plants has no measurable erosion1, so it is critical to include our California natives as part of your planting mix.
Choosing plants that have the same watering requirements is also critical. Overwatering California natives is a common cause of death. In fact, native plants in cultivation die within 3 years when overwatered and fertilized compared to their 100-150 year lifespan in the wild – since regular watering and fertilizing is not part of their native habitat in arid California.
Clarity on how to water your slope is more complicated. The more water you put on a slope, the more unstable the slope will become. Common convention calls for drip irrigation, however it depends on the type of soil and slope you’re planting – see these two informative links:
laspilitas.com/garden/howto/slope.html and laspilitas.com/drip.htm.
The video on Las Pilitas’ site advocates against drip irrigation for highly eroded slopes or bluffs, since water is likely to run off and not soak into the hill. If you choose low to no water plants, occasional deep watering is all you may need once the plants are established, which may be more efficient with an overhead system and most closely simulates their natural environment.
Native species can even help protect your property from fire damage, always a threat in our dry, dynamic region. See this table for fire-resistant plantings: https://www.laspilitas.com/classes/fire_burn_times.html
There are a number of commonly-used plants that cause more problems than they solve. The common ice-plant Carpobrotus edulis that dominates SoCal’s freeway medians and beach bluffs is an introduced, invasive species that grows heavy enough to pull down hillsides. This plant also competes directly with several threatened and endangered plant species for nutrients, water, light and space.
Ditto for the ubiquitous English Ivy (Hedera helix) . This woody vine and ground-cover will rapidly overpower everything in its path, “throwing shade” on your understory, and even thwarting and killing trees by dominating the canopy.
If you have either of these plants anywhere in your garden, especially on slope sides, remove them and find indigenous alternatives: