Open ground or a planted building? Soil first

Is it better to plant in open ground or to green the building itself? To our mind the answer is short: it is almost always simpler, cheaper and healthier to plant in the soil that is already there than to rebuild an artificial one on a slab or up a façade. A tree in open ground finds its own water, anchors itself, strikes up underground alliances with fungi and earthworms; the same tree in a planter will depend for life on a substrate, a pump and a maintenance contract. One grows free. The other stays on a drip.

Living soil works. It drinks the rain instead of flushing it into overloaded networks; it cools the summer air through the water it holds and the shade it makes possible; it shelters a discreet, considerable fauna, from earthworm to ground beetle; it connects. For free. A garden in real soil can be crossed: a hedgehog passes through, a seed travels, a root meets its neighbour’s. That is what an ecological corridor is: not a green strip on a masterplan, but an actual path for living things. No rebuilt substrate, however well designed, renders all of these services; at best it imitates a few, at the price of depth, structure and irrigation.

The most telling proof we know stands in Paris, along the old rail belt. La Ferme du Rail, by the Grand Huit practice, farms 1,800 m² of open ground for 833 m² of building: an orchard, a permaculture mound, a greenhouse-restaurant. The soil produces vegetables, jobs and coolness, and the site’s own activity pays the hands that tend it. No maintenance line waiting to be cut in ten years: a soil that, quite literally, pays its own gardener.

Alongside it, the great planted buildings have proven real things, and they deserve credit. Bosco Verticale in Milan has shown for over a decade that a structure can carry trees thirty metres above the ground and keep them alive; its social-housing sibling in Eindhoven, Trudo Vertical Forest, shows the principle can fit a social budget; in Sydney, One Central Park’s green wall casts shade that genuinely lightens the air-conditioning load. Yet none of these projects touches the ground. Everything sits in planters, watered for life, tended by rope-access gardeners; in Milan, upkeep runs to around €63/m² a year. These feats leave the question of the soil wide open: they prove that a building can be planted, not that it should come first.

Sometimes, though, there is no open ground to be had: a parking slab under the courtyard, a dense centre without a spare square metre, polluted soil that cannot be opened. Then yes, rebuilding a soil on the structure is legitimate, and at times it is the only purchase a site offers the living world. But it must be done with everything it entails: a real depth of substrate, a structure sized to carry it, irrigation designed from the first sketch, and above all a caretaker named and budgeted before the first line is drawn. The hierarchy itself does not move: open ground first, rebuilt soil on a slab next, planters as a last resort.

Planning law, as it happens, is heading the same way: France’s trajectory of land sobriety rewards de-sealing, and open-ground ratios are appearing in recent local plans. Thresholds vary and must be checked plan by plan, but the direction is clear: the soil is becoming a part of the project in its own right, like the structure or the envelope. So before drawing the green roof, one simple question: what have we done with the ground? If a square metre remains to be unsealed, planted, connected, start there. A wall goes up in a season; a tree takes a generation to give its shade. All the more reason to plant it first.