'Trees live longer in a tree bunker'

In the past, planting a tree was pretty simple. You dug a hole, planted a tree and then filled the hole with local soil. If we want trees to live longer, something has to change, believes Bert van Gils, product consultant at GreenMax and TreeBuilders. "We designed a solution in the form of a compromise between the city and the forest."

Planting trees in the city has many advantages. They are good for biodiversity, provide shade and absorb CO2 and particulate matter. But before you can enjoy those benefits, the tree must be planted properly. And that can be a challenge in the city. Van Gils: "Ultimately, a tree belongs in the forest. There you have rich soil, with leaves that rot, creating new resources. You don't have that in the city."

Then you also have the risk of mature trees damaging infrastructure with their roots. "Mature trees are strong enough to break sidewalks and pavements, or damage pipes and sewer pipes. You want to prevent that, of course." 

TreeBuilders has specialized in tree bunker systems for 20 years. "It's a construction of piles and a platform on top, creating space underground for the tree roots. The material we use ensures that the bunker can withstand an imprint of 70 tons per square meter. We fill the bunker with optimum soil. Not tree sand or tree granulate, but tree soil. This soil gives much more space for rooting as well as for the biological ecosystem that you normally find in the forest."

The result? It's quite good, according to Van Gils. "Part of that tree soil consists of humus, and that has a sponge effect. This allows you to catch much more water. Because of this natural cycle in the soil, trees also grow a lot older. We expect 60-80 years old, but it could be much longer. Time will tell."

The beauty of the tree bunker is that it can be combined with water buffering. "Think of those heavy rain showers in the summer. Those 10 to 12 milliliters of rainwater normally flow away into the sewer system, but that can also just go into the tree bunker. The tree is very happy with that, when a few days later it suddenly becomes 30 degrees again. A nice win-win situation, it seems to me." The company also supplies potato polymer, which allows soil to retain 40x as much water. "In the early years of a tree, that can make all the difference, because then there is always enough water available."

The need to plant trees is becoming more and more apparent, Van Gils notes. "Municipalities now recognize that planting trees really makes cities more livable. It's also great that the trees we plant immediately advertise us. They grow much faster than trees in other soil. You don't have to be a tree expert to see that a tree looks much better in livable soil."