Plantable Green Walls

By Dean Sandri, PE of Soil Retention Systems, Carlsbad, CA

Landscape architect and owners desire to build attractive, colorful and plantable walls, and segmental walls can do just that: add a green, ?EUR??,,????'??visually-soft?EUR??,,????'?? grade transition imparting charm, serenity and beauty to any landscape.

A few plantable wall systems, ranging in height from a few inches to over 50 feet, have been created to serve this niche market. These plantable walls provide excellent structural integrity and design flexibility at competitive prices.

The long-term success of ?EUR??,,????'??green?EUR??,,????'?? walls, which blend into the landscape, while providing safe, engineered, retaining structures, require consideration of drainage and irrigation?EUR??,,????'??+two seemingly contradictory concepts. If you?EUR??,,????'???re interested in implementing ?EUR??,,????'??green?EUR??,,????'?? wall systems, this article will shed some light on important elements you need to consider.

?EUR??,,????'??Trough?EUR??,,????'?? type blocks are open at the back and top to allow roots horizontal and vertical access to the soil mass, where they gather moisture and nutrients as they mature. Inset: Vegetation established in ?EUR??,,????'??pocket?EUR??,,????'?? blocks may initially prosper, but eventually become "root bound" as they mature.

Drainage

The need for drainage in retaining walls cannot be overemphasized. Nor can the need to provide an adequate long-term source providing moisture to ?EUR??,,????'??feed?EUR??,,????'?? the vegetation. For long-term, structural stability, retaining walls need good drainage to ensure that hydrostatic pressures are dissipated?EUR??,,????'??+but plants need adequate water to grow. These two requirements seem to conflict. However, providing drainage for the ?EUR??,,????'??retaining wall?EUR??,,????'?? and ?EUR??,,????'??water?EUR??,,????'?? for the plants can be done if the wall structure and plantable facing system is configured properly.

Drainage is most effectively provided at the interface between the soil back-cut and infill. This is specified in the design provided by the geotechnical engineer. This zone is typically responsible for interception, collection and discharge of moisture. While not needed for all wall systems, the existence of a granular-transition soil layer behind the retaining wall fascia (often incorrectly referred to as a ?EUR??,,????'??drainage?EUR??,,????'?? layer) is often provided to facilitate compaction near the wall face and protection against soil loss via piping.

The drainage layer at the back-cut/infill interface has no significant impact on the wall vegetation. However, the granular-transition soil zone behind the fascia does. The proximity of the wall plantings to the transition soil zone (if used) will impact the moisture available for plant growth and root system development for the long-term health of wall plants.

With future plant growth, this retaining wall walkway will blend naturally into the hillside. The drip irrigation system attached to this wall face incorporates long horizontal distribution lines with smaller vertical lines curving in to hydrate each individual planting pocket.

Troughs and pockets

Fascias for plantable retaining systems typically consist of either a ?EUR??,,????'??pocket?EUR??,,????'?? or a ?EUR??,,????'??trough?EUR??,,????'?? type product. The ?EUR??,,????'??pocket?EUR??,,????'?? type products are characterized as having discrete planting ?EUR??,,????'??cups?EUR??,,????'?? formed into the concrete retaining wall fascia unit; some of which enable root growth to promulgate vertically. ?EUR??,,????'??Trough?EUR??,,????'?? type products provide a trough shaped system in which vegetation is established and allowed to root horizontally and vertically.

Continuity with backfill

Providing continuity between the retaining wall backfill and the planting medium encourages healthy root growth extending well into the reinforced soil mass. That same continuity assures the ability of the wall system to ?EUR??,,????'??retain?EUR??,,????'?? moisture within the planting zone or to ?EUR??,,????'??draw?EUR??,,????'?? moisture from the mass of the retaining structure.

Conversely, during wet periods these systems are able to drain moisture away from the planting medium. Decoupling the planting medium from the backfill results in root-bound plants, which increases plant mortality, especially during periods of irrigation malfunction (root rot or dry pots), severe exposure to heat and/or direct sunlight (burned plants) and after plant maturity (restricted, bound roots). In any event, the wrong mix of backfill, block type, and planting results in disaster.

This completed plantable retaining wall blends in perfectly in this tropical residential setting. Observe how color and texture can be used to sculpt designs into the wall plantings.

Surface mounted irrigation lines

Irrigation lines should access the retaining structure from the base and be surface mounted. Irrigation via supply lines buried within the wall mass or routed above the wall significantly increases the risk of undiscovered irrigation line leaks that compromise the walls structural integrity. As an alternative, above grade supply lines can be routed along a V-ditch near the wall crest, which facilitates the collection of excess moisture should a supply line rupture. Sprinkler, drip, mist or bubbler type distribution heads can be used to irrigate the wall face. However, the use of drip heads can become complicated, especially when the area covered is large and if redundant heads are desired for each planting area.

Attaching irrigation lines to the wall

Where the wall face is too large to irrigate from one line, either vertical or horizontal irrigation circuits can be ?EUR??,,????'??hung?EUR??,,????'?? from the wall face. This can be accomplished by a combination of anchoring methods including supporting on block ?EUR??,,????'??ledges,?EUR??,,????'?? anchoring to hooks driven into the wall mass, or other mechanical means of connection to the wall fascia. Irrigation attachment suggestions are typically available from the wall provider.

Spray, bubble, mist or drip

Spray type irrigation systems perform well on walls that exhibit large face areas. Vertical and horizontal spacing is dependent on plant type, wall fascia, and irrigation head selection. Bubbler type irrigation heads and drip irrigation systems work well where discrete watering pockets can be identified and maintenance access is convenient. Mist systems work well where small areas need to be covered. While minimal, some consideration for maintenance of the irrigation system should be budgeted. A landscape architect along with the wall system provider will be critical to the success of a complex irrigation program.

Drip irrigation and connection shown with redundant irrigation drip lines to each planting pocket. Two individual drip heads serve each planting pocket.

Irrigation visibility and UV

Face-mounted irrigation lines may initially be visible, however, they will disappear as vegetation becomes established. If UV is deemed to be a long-term concern, UV resistant pipe should be considered. Once established, the vegetation will help to protect the irrigation lines from the destructive effects of UV, prolonging the ?EUR??,,????'??rated?EUR??,,????'?? life of the ?EUR??,,????'??naked?EUR??,,????'?? irrigation line.

Long-term maintenance

Long-term maintenance requirements for the planting materials and irrigation system include maintenance of plant materials, fertilizing and irrigation head maintenance, which is typically conducted on an as-needed basis. However, periodic visual observation (at least yearly) of the operating irrigation system is advisable. While extremely infrequent, broken lines must be dealt with as soon as they are discovered.