For Erwin, GreenQuays meant in particular collecting valuable experience on appropriate substrates for roots to grow in. The right substrate plays a particular role for trees where limited space for growth poses a challenge. Due to that, it is difficult for the roots to develop in order to get sufficient water, nutrition and oxygen for the tree to thrive. In the case of GreenQuays, the space has been restricted by the demand to grow trees out of the walls. This requires small containers with an effective substrate providing the tree all it needs. The substrate needs to make the water from the river rise through its capillary system to the roots and at the same time, ensure enough oxygen in the substrate by its structure as the roots need both water and oxygen. In other urban settings, underground space is often limited too, e.g., by building foundations, sealed surfaces of roads, underground infrastructure such as pipes and wires, or underground parking levels. Equally, space is limited on green roof tops.
Tree sand is now being tested at the Hanze University of Applied Sciences in Groningen (BuildinG0) in combination with the Permavoid Capillary Irrigation System, and here too, sensors have been installed to learn more. Erwin was even surprised to see how well the solution worked here, especially how quickly the growing area recovers from excessive rainwater. It is indeed the use of an effective substrate in combination with the Permavoid Capillary Irrigation System and an underground construction that can handle both too much and too little water while maintaining the balance with oxygen and water. The research impressively demonstrates the dynamics of water in the soil.
Results of the small-scale test site of GreenQuays and the trees at the Hanze University on the appropriateness of substrates and its use in a Permavoid Capillary Irrigation System at Hanze University were subsequently used in designing and upscaling these solutions in a much larger system - the Grote Markt square in Groningen, which is currently under construction. The complex underground structure with storage capacity, overflows, a capillary system, and different layers of substrate, tree sand and a forest layer allow the trees to cope with the different amounts of water throughout the year. The site becomes a stable growth area for the trees.
The lessons from the GreenQuays, together with the many tests already carried out, provide the current developments of an integral growing place that will make Europe's cities climate proof and heat resistant. Extreme situations with too much or too little water will increase due to climate change. Therefore, a high demand for solutions like this is expected, where cities remain dense, but trees can still thrive and provide their shade for cooling and other ecosystem services.