Garden Q&A – Planting on Steep Slopes

Garden Q&A with Lee Gordon, member of CNPS San Diego Chapter

Lee Gordon
CNPS San Diego Chapter

How do I plant on steep slopes?

California native plants beautify and strengthen steep slopes

Gardening on steep slopes can be challenging, but locally native evergreen foundation plants, properly irrigated, can create a garden that provides erosion control and year round beauty.

Choose locally native, evergreen plants

Evergreen plants provide a foundation for your design that looks good all year.  Locally native plants are easier to establish and maintain, and you can see how they look by visiting nearby open spaces. Other resources include your local CNPS chapter, Calscape.org, and Calflora.org.  The good news is that California has many to choose from! Next, fill in the spaces with other locally native plants to add interest and seasonal color.

Before I put plants in the ground, I irrigate to create a deep reservoir of water so the roots have room to grow. It takes 5” of rain to get the clay and cobble soil in my neighborhood wet to a depth of 2’. To make sure the water gets into the ground, I water slowly, sometimes taking several days. I plant my plants in holes just large enough for the pot. Then I water the hole a little more to get the clay particles to cohere to one another, which helps moisture get to the root ball from the surrounding soil.

After this preparation, my plants have done well with only monthly watering, even in the first month. Each watering gives the plants the equivalent of an inch of rain. Native plants can’t be watered like traditional gardens. Instead, they do best with infrequent deep irrigation that gives the near-surface soil time to dry. Native plants seem to need this. Most of a native plant’s roots grow in the top foot of the soil, spreading far from the plant. Therefore, I irrigate the area with overhead water to nourish those roots. Near surface roots prevent erosion. As the surface dries, water migrates downward to the deep roots that keep plants green and that stabilize steep slopes. I do not mulch my plants, but the plants’ leaf litter creates a natural mulch.

Most of the plants on the steep north slope where I live grow without irrigation, but some plants that turn crispy brown in the summer stay green with monthly watering. If you want your plants to survive without irrigation, plant in November the way I describe above. One initial deep irrigation should get them to the rainy season, and after that, they take care of themselves.

Some evergreen foundation plants native to my dry San Diego neighborhood

• Cherries (Prunus ilicifolia)
• Toyon (Heteromeles arbutifolia)
• Lemonade Berry (Rhus integrifolia)
• Mission Manzanita (Xylococcus bicolor)
• Scrub Oak (Quercus berberidifolia)
• Red Berry (Rhamnus crocea)
• Sumac (Malosma laurinius)
• Woolly blue curls (Trichostemma lanatum)*
• Summer holly (Comarostaphylis diversifolia)
• Coyote bush (Baccharis pilularis)**

* needs summer water
** ‘Pigeon Point’ is a non-local selection that is a great ground cover

Visit Calscape.org to find plants native to your region!