Erwin van Herwijnen from Tree Ground Solutions signs responsible for the concept and its implementation to grow trees out of the wall. He gets inspired by forests. How are these organised and functioning, and how could the basics be transferred into the very artificial environment of the vertical quay walls? For sure, trees need something to grow in that is obviously bigger than the scarce space in joints between bricks. Hence, the wall segments include containers with substrate, and trees, which have been grown already bended in the nursery Van den Berk, have been set in. The space is still small; willow and elm trees have been planted and seem to be species that can cope with these conditions.
Another challenge is about the right amount of water in the substrate. The design foresees that water comes only passively from the river. There are neither pumps installed nor can rainwater infiltrate from the top of the quays, which is sealed. As the trees’ base is higher than the water level, different substrates have been tested that can soak water in like a sponge and bring it up to all roots. The challenge is to find the right substrate in which the water and oxygen balance is in the right proportion, and which always works reliably. Two strategies are being tested, one based on Lava, and one based on sand substrates, both have already been applied in (several) wall projects, i.e., in the Hague and Amsterdam as well as a substrate used to form mounds on a roof in Heerlen to grow trees in it and which soak sufficient water from the roof’s surface to the roots, have been found appropriate. The advantage of the solution is constant water availability even in longer periods of dryness because the growth site always makes contact with the river. In addition, the substrate and the way of watering ensure that there is always oxygen in the containers. It is important for trees and other plants that the growth site is balanced and that no anaerobic conditions are created, this way the trees continue to develop positively.
Erwin has constantly monitored how the trees thrive and what the conditions in the substrate have been. This helped to get to know the properties of these substrates well during the different periods of the year. For example, the planting on the north side suffered very badly from burning just after planting. the reflection of the sun through the wall was extreme while not enough roots had yet emerged from the root ball. So it is important to give the planting extra water after planting. After 2-3 months, watering was no longer necessary, and the trees grew independently in the growing area.
The learning continues now. What will the trees need in the coming decades? It is clear that they will need different maintenance techniques compared to ordinary street trees. Because they are limited in growing space, they are more likely to be "bonsai trees" or perhaps become pollarded trees. In any case, they will need to be pruned to keep them vital. Furthermore, because of the design of the wall, the substrate cannot be replaced and falling leaves will not bring nutrition to the soil. However, dying roots will have to provide circulation of organic material in the container. Another open question is, how a tree should be replaced if one dies. Erwin has learned a lot from this test setup, and he can use that knowledge for this project as well as other future projects. An example for such spin-off is a facade wall that can be greened the same way with herbaceous plants.