Belgian architect Luc Schuiten's vision of 'the Vegetal City'. Luc SchuitenBuildings adorned with plants are an increasingly familiar sight in cities worldwide. These “green walls” are generally created using metal frames that support plastic plates, onto which pre-grown plants are inserted. These plants are able to survive without soil because they’re sustained by nutrient-packed rolls of felt and artificial sprinklers.Some are fabulously rich tapestries of luxuriant vegetation, like French botanist Patrick Blanc’s coating of part of the Athenaeum hotel in London. Here, small shrubs sprout from an almost tropical green wall, with an abundance of mosses and ferns. In summer, butterflies peruse the flowers. All this next to Piccadilly, one of the busiest streets in central London.Others are objects of ridicule: the sadly common outcome of poor design and a lack of maintenance (all green walls need careful planning and a great deal of care). If they’re not carefully tended, green walls will quickly turn into brown ones, with the plastic supports all too visible beneath the dying plants.But there are many others ways of integrating plants into buildings beyond simply trying to grow them on walls. Here are five examples that straddle the mundane and the marvellous. A wilted green wall in Tokyo, Japan. Wikimedia Images, CC BY Growing buildingsGerman architectural practice Baubotanik (a word that means “botanic building”) has taken the radical step of creating buildings that flout the conventional idea of architecture as static and inert. After all, plants grow – they are living organisms. Baubotanik uses pre-grown trees to create multi-storey structures, with trees replacing the conventional steel girders of most tall buildings. Its Plane-Tree-Cube in Nagold, begun in 2012, is made of plane trees supported on a steel scaffold, with a built-in irrigation system to water the trees until they’re large enough for the steel to be removed. Baubotanik’s Plan-Tree-Cube is intended to grow into a usable structure. Baubotanik It’ll probably be another ten years before this structure is ready to be used, but as what? It’s hard to imagine making a home in such an unruly structure, let alone plugging in your internet or other electrical appliances.Building in treesBaubotanik takes grafting, an age-old horticultural technique, and uses it to create structural frames for buildings. Grafting joins the tissue of plants so that they can grow together (it’s most commonly used in the cultivation of fruit trees).As the architects themselves acknowledge, there are many interesting historical precedents, such as the Lindenbaum concentrated in a small region of rural Germany in northwestern Bavaria.These are accessible platforms built into large lime (linden) trees to accommodate dancers in a yearly ritual known as the Tanzlinden (“dance linden”), which originated in the middle of the 17th century and still happen in early September.In the surviving Lindenbaum in the small village of Peesten (one of around 12 that are still around), a stone stairwell spirals up to the wooden platform built inside the tree: dancing happens on this platform, while musicians provide accompaniment beneath. Lindenbaum in Peesten, Germany. Wikimedia Images, CC BY Weaving buildingsIt’s possible to take this practice of integrating buildings and trees one step further and imagine whole cities redesigned in this way. This has been the lifelong preoccupation of Belgian architect Luc Schuiten, particularly in his speculative drawings of “vegetal cities”.These are urban environments in which the branches of trees and the stems of climbing plants have become completely enmeshed with buildings made of steel and glass. One of his designs, called Habitarbres, imagines a house constructed within a living tree. The structure would flex as the tree grows, while hot-air pipes and other infrastructure would be embedded in the trunk. It’s an attempt to envisage how the infrastructure of our buildings – pipes, wire, cables and the like – can be accommodated in a living structure with its own vascular network. With Habitarbes, Schuiten proposes a house built within a living tree. Luc Schuiten It’s a speculative proposal, but perhaps not so different from a common building type normally associated with enterprising children, namely treehouses. Schuiten is merely taking a human desire – to live in a tree – and suggesting how it might be squared with our equally strong desire for comfort. Architecture as compostWhen plants die and decay they create the conditions for the next cycle of vegetal growth; they are sustainable in a way that the vast majority of our buildings are not. While there is a drive to recycle existing building materials (metals and plastics mostly), it’s another thing entirely to make buildings truly regenerative. Martin Miller and Caroline O’Donnell’s “Primitive Hut” project from 2017 created a building that does just this. They made a wooden lattice structure to support the growth of four red maple saplings. Another lattice decomposed over time, providing food for the growing trees. Eventually the whole structure was overwhelmed by the trees. Martin Miller and Caroline O’ Donnell’s ‘Primitive Hut’. OMG! In calling this a primitive hut, the architects questioned how western architectural thinking tends to see indigenous architecture as both an origin point and a model for more sustainable forms of construction. It asks whether the industrial technologies that dominate construction in the global north should be more informed by architects that have continued to build with natural and compostable materials for centuries.Letting beIt’s worth remembering that we don’t have to design green buildings; given enough time, they will happen anyway. Moss on the roof of the Sandringham estate’s visitors’ centre in Norfolk, eastern England. Wikimedia Images, CC BY The sloping roof of my house, directly below the window where I’m writing this article, is gradually acquiring its own green patina of lichen and moss. The roof is old and I’ve been told it needs to be replaced soon. A cloud of spores and seeds peppers this and every single roof every day with the prospect of new life. Without any human intervention whatsoever, this process of vegetal succession can produce a complex ecosystem of not only plant but also animal life (from microbes to insects). That architects so rarely call such a surface “green” betrays something that’s deep-seated in ideas about green design. For it is precisely the absence of human control that allows vegetation to colonise a building; there is, in effect no design involved at all – unless, of course, we accept that plants have designs of their own.Don’t have time to read about climate change as much as you’d like?Get a weekly roundup in your inbox instead. Every Wednesday, The Conversation’s environment editor writes Imagine, a short email that goes a little deeper into just one climate issue. Join the 45,000+ readers who’ve subscribed so far.Paul Dobraszczyk does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.