Making trees and wall plants thrive at steep quay walls

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.

Edwin Dijkhuis, senior project leader at FLORON (Plant Conservation Netherlands) and GreenQuays partner is responsible for choosing the plant species for the small-scale test, defining the preconditions for establishment of these herbaceous plant and monitoring their growth and survival. He is interested in having walls, where native plants settle by themselves, which could also offer space for endangered plants (selected target species) to thrive in the city. He wanted to learn how to enable plant growth under these special conditions of new vertical walls and to speed up the establishment process, which normally takes several decades. This idea has coincided well with the intention of the urban designers that wanted to have the look of old quay walls with a rich wall vegetation.

He contributed with his, other colleagues’ and Natuurplein de Baronie members’ broad ecological knowledge in the design team and collaborated particular closely with the partners from Delft University to find the best type of bricks, mortar and joint patterns and test them at the small-scale test site. Seeds and plants were placed on the various panels of the test site in spring 2020 as there was no time in the project to wait for a spontaneous establishment of plants. 

After one year, monitoring results at the different panels with different combinations of bricks, mortar, joint patterns, and the presence or absence of a substrate layer behind the stones have shown that only the panels with a substrate layer behind the bricks have worked sufficiently well to provide the plants and seedlings with sufficient moisture from capillary water including vertical transport from surface water. Without this substrate layer, the plants and seedlings died fairly quickly. Furthermore, the choice of bricks and mortar and masonry bond (half brick or brick) has been important. As expected, the most porous tested bricks had the biggest water absorption capacity.  The brick wall sections also retained moist longer than the half brick wall segments. During the collaboration, new ideas have been come up such as the “fantasy mortar” made of straw, clay and lime. Unfortunately, that most successful mortar for germination of seeds and plant growth was not strong enough. 

Seeds have germinated mainly in places where there are no joints or where expansion joints are included in the masonry. These open joints offer space where plants can settle. They act as a replacement for the cracks and fissures that normally created by natural weathering of rocks and masonry. The colonization process can be accelerated by artificially applying these open joints. It can also be seen that the plants and moss thrive better at the places where rainwater occasionally runs down the walls. Some of the plants have germinated directly in spring, others only started in autumn under cooler and more humid conditions. Overall, these are promising first results. It came even as a surprise to see that seeds germinated in fresh mortar, and that they survived a hot and dry summer. The team has always thought that fresh cement would be too alkaline to allow plant growth. That does not appear to be the case.

An important gain for Edwin and colleagues has been the monitoring results. For the first time, they are able not only monitor plant growth, but also the conditions that plants need – the combination of surface temperature, brick layout, moisture in the bricks and the influence of weather. That valuable knowledge can be used for other sites in Breda and elsewhere to create even on tiny inner-city plots space that is suited for natural plant growth, boosting wildlife and saving endangered species. 

Creating nature-inclusive quays is an ongoing learning experience. Different disciplines need to go hand in hand to find the right design, materials and plants. There are many unknown variables, a fact which requires to be patient on how plants will develop, observe their growth or spontaneous colonisation, and adjust tree selection, maintenance or construction to find the right combination. The close collaboration with other partners brought up also new ideas like new mortars or joint designs. 

While this process is exciting from the researchers’ perspective, it generates also some practical challenges. Vegetation development happens relatively slow. There have not been final results for the best solutions before the design of the Real-Life Pilot site – the first stretch of 175m of the re-opened river has been finished this year. Therefore, monitoring will continue, and adjustments will eventually be considered during the construction of the Real-Life Pilot site and afterwards, wherever it is needed and its flexible design allows for. 

See also:
Green quay walls along the new river Mark (Dutch)
The brickwork story behind GreenQuays.