According to “Best Practices Manual: Landscape Architecture” by Worchester Polytechnic Insititute, Feb 2010, p.35 (WPI), a vertical garden system can weight from 1 kg²/m (0.2 lb/ft²) to 50 kg/m² (100.2 lb/ft²) . WPI also recommends, for deciduous plants, doubling the design weight to take into consideration potential snow and ice loads, and tripling it for evergreens for these harsh weather loads. Also, for those considering large structures, more from WPI: “The physical structures’ weight must be measured with the height above the ground incorporated. If the structure is supported at the top and bottom, then the top should carry the whole load and ½ the wind load while the bottom needs to support only ½ the wind load (Stainless Steel Solutions, “Green Wall Trellis System” 2009).”
Patrick Blanc, the father of fabric-based vertical gardening systems, concurs. In his book, The Vertical Garden, he says his gardens range from 20 kg²/m to 50 kg/m².
Here at our Singer Hill Test Garden, we think in feet and pounds and we buy our expanded PVC in 4′x8′ sheets. Applying these standards, a 32 ft² standard vertical garden panel should be designed to weigh as much as 327 lbs. For a panel grown with 100% evergreen species, the design weight for a 32 ft² panel would be three times that or 983 lbs.
Our walls, before plant maturation, are about 3 lbs/ft². This is less than a third of the weight of covering a wall with a typical commercial installation of stucco (10.5 lbs, according to cement.org.) At this weight density, a 4′x8′ panel is movable by one strong person.
We use light weight construction materials to keep the weight down. In our movable panels we use 3mm (1/8th inch) expanded PVC for the required surface, rather than the 10mm that Blanc reports that he uses for his commercial grade vertical gardens. And, we use a frame made of very light-weight ¾” PVC pipe and fittings rather than using metal rods for a frame. We hang the panels on the walls so that there is a gap between the panel and the wall. However, on a free standing cinder block wall, and a terracotta block commercial building wall, we have had very good success — so far — attaching the sheets of 3 mm expanded PVC directly to the masonry, dispensing with the frame completely. Most other sources on vertical garden construction (including Blanc) recommend a space between the wall and the back of the panel, but so far (2 years) we can not see any difference between the ones that “breath” and the ones that do not.
Keep the biomass down. We trim plants to keep their center of gravity close to the wall. Typically, we keep our shrubs and trees pruned to within a foot from the vertical surface. Wood material can get heavy of course, so planting big plants obviously can add weight over the years. If you decide to plant a fig tree, like Patrick Blanc did at the Cartier Foundation for Contemporary Art in Paris, and then NOT trim it, of course it makes sense to design a wall to hold up loads of 30 lbs per square foot. However, one can keep the maximum weight down through light weight materials, plant selection and pruning.
Clearly, building a vertical garden using robust metal rods for frames will make a very strong wall that will allow the growth of larger plants that grow further from the wall, but at Singer Hill we have shown that gorgeous walls can be built for much less with less beefy materials.