A Sound Idea, In Theory
A green wall is often a feature in contemporary landscape proposals. In urban areas and along roadsides, the idea of a vertical panel of living plant material offers an ecologically sensitive solution to increase the natural world in tight spaces with limited real estate. A green wall is skinny, but tall, hiding blight and introducing fresh air and softness into the concrete hardscape. As a green feature, it delights stakeholders and evokes optimism. In theory, a green wall makes perfect sense.
A green wall starts as a structural planter with a permanent trellis system. The trellis system is usually made of fixed, modular units—small, boxed openings stacked to provide planting holes for vertical plant placement along the face of the wall. The entire plant system is dependent on water, so green walls often have expensive irrigation delivery built within the structure. Plants are inserted individually into openings, or entire trays of pre-grown plant material are removed and replaced as needed, usually on a monthly basis. A green wall system is expensive and requires constant maintenance, but a green wall is often sold as a carefree, innovative idea.
Why are green walls and the concept of vertical green spaces absent along roadsides and in parks, especially in urban areas? The idea is a good one, but plants struggle to survive when planted on their sides in tiny box planters with almost no access to rain water or nutrients. For indoor green walls, artificial methods of providing adequate water tend to promote fungus and mold. The astronomical labor and maintenance expense of outdoor green walls is a luxury almost no one can afford. The limited volume for plant growth in the planters, the unnatural positioning of the plants, and the unlikely possibility that rain water could penetrate into each planting box evenly and reliably make large-scale outdoor green walls impractical.
For long-term greening of narrow bits of urban land and urban roadsides with limited rights of way, there are other choices. Tall ornamental and native grasses offer an inexpensive an easier-to-maintain wall of living plant material, even if there is a need to cut back clumps at least once a year. The original, old-school green walls were called hedges, but hedges need skilled trimming on a regular basis to keep them narrow so they don’t outgrow their limited space, and this has caused their use to decline in recent decades. A successful establishment of vertical green space is possible if vigorous vines are planted in adequate ground soil (not in planters) and allowed to ramble up and over a sturdy trellis structure or wall. Vigorous vines need to be trained and tamed to avoid a tangled mess of wiry stems and overgrowth, and the structures holding vines need periodic repair. Even with their issues, grasses, hedges, and in-ground vines are far more practical than modular green wall systems, and they are far more horticulturally sound concepts. These proven methods of establishing vertical, green growth still require extra work and expense.
My advice for green walls is this. If you propose them using other people’s money with the risk falling on either your client or the taxpayers, do so only after testing and monitoring maintenance costs and plant success on real-life experience over several years. Then, be honest about the new responsibilities they bring to the stakeholders.