Planting Slopes and Hillsides: Gardening at an Angle

Headshot of Nan StermanMany San Diego subdivisions are cut into hillsides and out of hillsides, thanks to our wacky topography and policy of building on every inch of usable space. That leaves homeowners to deal with pretty challenging slopes and the baffling issue of how to plant a hillside.

Most people think “groundcover” when it comes to a hillside. But have you ever seen a mountain covered in groundcover? Nature doesn’t do that and neither should we.

Slopes and hillsides are simply gardens at an angle. You have to choose the right plants and avoid over irrigating, but it’s not that difficult.

One of the biggest issues is how to keep hillsides from eroding and from washing away. The best plants for the job are ones that make deep roots to stabilize the soil, yet require very little irrigation. Native trees and shrubs are ideal: California lilac, toyon, oak trees, and sugar bush are good examples. Include Romneya coulteri, the giant Matilija poppy. This six foot tall perennial is known as the “fried egg plant” for its round, white crepe papery flower with a yolk-yellow center. Underground, it forms a vast network of creeping roots. California natives also create habitat for birds and other wildlife.

Shrubs and trees from other Mediterranean climates work well too. On garden tours I lead in these regions, I marvel at how similar the native habitats appear to ours, yet are entirely different plants. In Italy, for example, the macchia (also called maquis) is the equivalent of our chaparral. Plants like rockrose (Cistus), cork oaks (Quercus suber), rosemary, carob (Ceratonia siliqua), and bay (Laurus nobilis) grow wild there. All are excellent choices for hillside gardens too.

Plant succulents and other shallow rooted plants around and under these plants, but as accents rather than for structure and stability. Sprinkle in some California poppy seeds and you’ll have a lovely springtime display.

Avoid Eucalyptus and other trees and shrubs with shallow roots and brittle branches. As we see whenever storms hit, these plants are among the first casualties.

Don’t plant ice plant, especially Hottentot fig (Carpobrotus edulis). This spreading succulent is many people’s choice for covering vast slopes. However, the plants grow new succulent blades over top the old dead, brown, blades and stems. Eventually, the mass grows so heavy that it slides, taking the slope down with it. In addition, Hottentot fig is an invasive plant.

SlopeA single, uniform planting of one plant across an entire slope is never successful. It is simply impossible to maintain an even, uniform look. Over time, a plant dies here and there, gophers burrow up from below, and weeds blow in. Our eyes focus on the flaws. It simply looks crummy.

How to plant a slope:

  • Choose deep-rooted woody shrubs and trees with low-water needs, suited to your garden’s microclimate. Use trees for structure, shrubs for substance, succulents and spreaders to fill the spaces between.
  • Group plants in sweeps of threes, fives, or sevens to create a beautiful, undulating mosaic. Don’t alternate plants in a hopscotch pattern.
  • Irrigate slopes with in-line drip irrigation. Overhead spray can worsen runoff and erosion. The County of San Diego Landscape Design Manual mandates that any slope greater than 25 percent (one foot elevation change for every four level feet) be irrigated at a precipitation rate of no more than .75 inches per hour. Drip is the most direct way to achieve that slow rate.
  • Top the soil and cover the irrigation with a thick layer of woody mulch.

Just like any other garden!

Nan Sterman is a garden expert, author, designer, and educator. Nan leads international garden tours and hosts the show A Growing Passion on KPBS TV. Learn more about Nan at plantsoup.com.