6 Things You Need to Know About Green Walls

Green walls are regularly used throughout Europe and Asia, and in Tokyo they are considered more valuable than green roofs for cooling the city. But the vegetated green wall is still new to the U.S.—with the exception of ivy-covered buildings, whose aggressive, self-clinging plants grow without specially engineered support and are known to damage walls and hinder building maintenance.

The green walls being advocated today are designed and engineered with a support structure. Based on current applications and data from the experience of green roofs, green walls can offer considerable cost savings to both the public and private sectors. For example, the reintroduction of vegetation into cities has been correlated with the reduction of the urban heat island effect, and therefore will reduce energy consumption. Cities are cooler and quieter through shading, evaporative transpiration, and the absorption of sound by green walls.

To guide Building Teams who are unfamiliar with the basics, BD+C asked green wall expert Randy Sharp to answer six fundamental questions to help grow your green wall knowledge.

What are Green Walls?

Green walls are vertical structures, that have plants or greenery attached to them. The are usually planted in growth, if it is a living wall. If it is a living green wall, they consist of built-in irrigation systems.

1. What are the different types of green walls?

There are two major categories: green façades and living walls .

Green façades are wall systems where climbing plants or cascading groundcovers are trained to cover specially designed supporting structures. Plant materials can be rooted at the base of the structures, in intermediate planters, or on rooftops. Green façades can be attached to existing walls or built as freestanding structures.

Living walls (also called biowalls, “mur” vegetal, or vertical gardens) are composed of pre-vegetated panels or integrated fabric systems that are affixed to a structural wall or frame. Modular panels can be comprised of polypropylene plastic containers, geotextiles, irrigation, and growing medium and vegetation. This system supports a great diversity of plant species, including a mixture of groundcovers, ferns, low shrubs, perennial flowers, and edible plants. Living walls perform well in full sun, shade, and interior applications, and can be used in both tropical and temperate locations.

Due to the diversity and density of plant life, living walls require more intensive maintenance (regular water, nutrients, fertilizer) than green façades.

2. What are the different types of green façade support structures?

The two primary types are modular trellis systems and cable and rope wire systems.

Modular trellis systems —rigid lightweight panels—are installed vertically as either wall-mounted or freestanding systems. They can be used on tall buildings in conjunction with intermediate planters or on rooftops. These planters may be required where the growth of climbing plants is physically restricted.

Freestanding structures, such as green columns or canopy tree forms made of rigid panels, can be placed on either urban streets or rooftops where space is limited or weight is restricted. The panels can also be used in horizontal applications such as arbors or as shading devices over the upper level of a parking deck.

Cable and rope wire systems consist of a kit of parts that includes high-tensile steel cables, wire trellises, anchors, spacers, and supplementary equipment. Vertical and horizontal wires can be connected through cross clamps to form a flexible trellis system in various sizes and patterns. To cover large areas, stainless steel wire-rope nets can be supported on flexible or rigid frames.

3. What are the best plant types to use for green walls?

Green façades use climbing plants, which are divided into self-supporting plants (root climbers and adhesive-suckers) and plants that need supporting structure (twining vines, leaf-stem climbers, leaf climbers, and scrambling plants). Climbers vary by hardiness, orientation, and climate.

Plants used in vertical exterior applications are exposed to harsher climactic conditions than those at grade or indoors, and as a result, hardy species should be selected for projects that intend to reach great heights. Similarly, climbers with a tolerance for wind, frost, and heat should be selected for projects in less hospitable climates. Rooted at the base of a green facade, climbing plants may take 3-5 years to achieve full coverage.

Plant selection will also impact the design of the supporting system. For example, a denser, faster growing plant will require a greater space between supports than less aggressive species, which allow for smaller intervals between supports. The density of plant life will have further implications for the underlying structure, given that the greater the leaf surface area, the more impact snow and rain will have on the weight of the system.

Living walls are comprised of a variety of plants in pre-vegetated panels grown in greenhouses and assembled four to six months later on a frame attached to a structural wall. Species are usually selected based upon their tolerance of growing system, site-specific environmental conditions, color, texture, rates of propagation, and root systems. The panels support groundcovers, ferns, low shrubs, perennial flowers, and edible plants. Pre-vegetated living walls offer an instant green wall for immediate impact.

4. How are green walls installed and maintained?

Wall-mounted green façades can be flush with walls or set 3-18 inches from the wall surface using mounting clips or “standoff” brackets. A waterproof membrane is not required. The depth of the trellis modules protects building surfaces by keeping plants from attaching directly to walls to prevent problems that could otherwise compromise a building's integrity. The structures also help distribute the weight of climbing plants across the screen structure and wall. In cable and rope wire systems, anchors and turnbuckles are installed at one end of each cable for tightening and adjustments as required by plant development.

Climbing plants require a good supply of moisture and occasional pruning, depending on species, appearance, and wildlife control. Supporting structures require minimal maintenance, with only occasional monitoring of the tension and structural connections.

Installation of pre-grown panels for living walls requires a lead time of 6-12 months prior to delivery for plants to propagate and grow first as plugs and to fill in the panels. The panels can be grown horizontally until the date of shipment to the site, when they are then mounted vertically. High-density concrete walls may be watertight; however, a waterproof membrane may be required for installation on metal or wood frame structures.

Approximately 500 sf of living wall panels with frame can be installed in one day. At least one month prior to the installation of the wall, water supply and power supply for an automatic drip irrigation system should be connected (at the top of the wall or for individual panels, depending on the system). A standard fertilizer loop is recommended for the injection of liquid nutrients for the plants.

The growing medium within the panels should be fully saturated once a day during the first week after installation; thereafter, water is provided to maintain moisture without oversaturation. The modular system allows panels to be taken out and replaced if required.

5. Do green walls attract unwanted pests, insects, and birds?

For both green façades and living walls, climbing plants can be selected that do not bear fruit or provide a food source. Also, property managers prefer closely cropped vegetation to discourage shelter or nesting sites for birds. Any excessive growth or dead wood should be removed, and standing water should be avoided. A continuous gravel strip at the base of the building is recommended.

6. Are LEED points available for green walls?

Green walls can contribute to several LEED credits when used in combination with other sustainable building elements:

Sustainable Sites Credit 7.1: Landscape Design That Reduces Urban Heat Islands, Non-Roof (1 point)

Exterior green walls reduce the solar reflectance of a structure, thus reducing the urban heat island effect.

Water Efficiency Credits 1.1, 1.2: Water Efficient Landscaping (1 to 2 points)

Buildings can incorporate a stormwater collection system for irrigation of the green walls and other landscape features. Using only captured, recycled, or nonpotable water may enable the project to achieve this credit.

Water Efficiency Credit 2: Innovative Wastewater Technologies (1 point)

Green walls can be utilized as wastewater treatment media. Other features, such as the incorporation of compost tea from a composting toilet, is another way for green walls to aid in the reduction of wastewater.

Energy and Atmosphere Credit 1: Optimize Energy Performance (1 to 10 points)

Green walls can provide additional insulation and natural cooling, which reduces a building's reliance on mechanical systems.

Innovation in Design Credits 1-4: Innovation in Design (1 to 4 points)

Green walls may contribute to innovative wastewater or ventilation systems.