A Wooden batten B PVC panel C Mosquito repellent D Matting E Supporting wall

F: So how do you build a vertical garden?
S: The principle’s quite simple. Firstly, you need a wall, either the wall of a building or a boundary wall. This is called a supporting wall. Vertical wooden battens and a PVC panel are secured to it. Currently, Japan is the only country where PVC is recyclable, meaning, elsewhere, vertical gardens are not environmentally friendly inside.
F: What happens after the PVC panel is in place?
S: Irrigation piping and two layers of matting are attached. The matting is made from felt or woven plastic that lasts a long time outdoors, but it can’t be recycled either.
F: Speaking about recycling, unless it comes off a nearby roof, the great amount of water needed to keep the wall growing seems the biggest drawback to me — especially when watering five times a day, as you mentioned.
S: In a tropical rainforest, there’s plenty of water, but not so in downtown Paris. Also, Parisian water has few nutrients, so the amount of fertiliser for vertical gardens is high — 0.4 grams of macronutrients and 0.2 millilitres of trace elements per litre of water. If a designer can ensure the fertiliser is absorbed by the plants or drains into a fishpond, that’s OK, but if it goes into the main water supply — which I bet it does most of the time, it just adds to pollution.
F: Good point. How exactly do the plants grow?
S: They sprout from slots cut into the matting.  
F: What are your designs?
S: One is a mosquito-repellent wall, containing scented plants that drive mosquitoes away. The other is a culinary wall, with herbs, salad plants, and strawberries.
F: Sounds delicious.
S: Personally, I’d be happier planting my veggies in pots on the balcony.
F: Perhaps. But I do find the giant vertical gardens in Madrid and Paris inspiring. Once obstacles like the plastic components and frequent irrigation are overcome, vertical gardening will be an aesthetic and environmental achievement. I’m not sure the whole world will take it up, but richer countries could well do so.