The marriage of living plants and cold steel ranks among the most enjoyable elements of practicing landscape design in the San Francisco Bay Area.
SCENARIO: Single-family residence on steep 2-acre property in Los Altos Hills, California. The lovely and level back patio was marred by the slope immediately above it: ugly bare dirt, too steep for traditional planting, and eroding at the base of the house's diagonal support beams. The situation called for a bold stroke of design creativity.
QUESTION: How to turn this liability -- a barren and degraded slope -- into a lush and attractive asset?
We terraced up two levels of plantable retaining wall: the lower section is a 4' x 25' rectangle, and the upper section (set back 2') a trapezoid rising from 2' - 4' tall along a run of 16', thus to follow the ascending grade and to allow for a footpath behind and above the wall.
The front of the wall is made of steel panels perforated with 2" squares set on 2-1/2" centers, for a sleek modern facade that holds the growing medium inside but allows the plants to grow through. The final effect strives to achieve a sense of effortless lift, but the construction of this project required clever planning and forward-thinking clients.
We began by setting a run of 4x6 pressure-treated posts along the base of the slope, then attached a expanded aluminum screen behind the posts to hold in the wall's growing medium and to support the grade on the other side. We chose a 1/2" weave to allow roots from new plantings to penetrate the rear side of the wall and grow back into the hill. Then we drilled each post laterally, both at midpoint and near the top, for the installation of an internal loop of irrigation; this will get the young plants established and keep them looking lush. (In truth, most of the plants in this wall are local native species which, once established, will require little to no supplemental water, as these species have been growing here since before the white man, before the red man, and long before houses and gardens and water hoses -- they drink when it rains.)
Here Kevin installs 1/2" drip tubing between the watertight flashings that penetrate the posts.
The hollow space between the posts is filled with coir, a waste product from the coconut industry made of shredded husks, a lightweight and water-absorbant material that serves as a growing medium. The coir contains no nutrients, but we added granules of a 14-14-14 time-release fertilizer to the mix, thus to get the plants off to a good start. Nonetheless, all the plants we chose to compose this wall are natives that can thrive in the natural unfertilized soils their roots will encounter when they grow back through the screen into the existing hillside behind the wall. Pictured left, Kevin and Francisco have used 2x12s as temporary supports for the face of the wall in order to fill the coir up to the first run of irrigation line. After this level of drip line is installed, they will slide a 4'x10' perforated steel plate down between the front of the posts and the back of the 2x12s, then bolt the plate to the posts, remove the 2x12s, fill with coir (behind the plate) up to the top of the wall, and then finally attach the upper run of the irrigation loop to complete the circuit. Plate by plate, the plantable retaining wall comes together.
Pictured left, the first perforated steel plate has been installed and planted.
Here's a view from upslope of the first planted section of wall, showing off its clean hollow core filled with coconut coir. Note the stub-out irrigation supply at the end closest to the house.
The second level of wall rises with the slope, fitted with steel plates cut into a trapezoid to accomodate the footpath running behind it at grade. A slim door of redwood panels hides the irrigation supply.
Before and after the application of an oil-based "transparent stain" for weather protection and lustre.
Trimmed out with redwood caps and filled with California native plants, the green wall cuts a sleek profile on the day it was completed.
Six months later, filling in nicely.
A beautiful spot to sit and relax for a spell.
Don't mind if I do!