Planting sites: help trees in the city!

A tree survives best in open ground in an area where the ecological cycle is still intact. In an urban area, trees are surrounded by roads, pavements, car parks and other development. It’s a tough combination because civil engineering and trees almost always have conflicting interests. For instance, a pavement has a soil compaction of around 4 MPa, whereas trees cannot grow roots properly in a soil where compaction is more than 2.5 MPa. Knowing this, it is no surprise that the condition of many trees along pavements leaves something to be desired.

Problems below ground

Moreover, underground space, or rather the lack of it, is a very common problem. Roots have to compete with an ever-growing web of pipes, sewers and cables and there is often too little room for them to be able to develop in the long term. Neither are the soil organisms in urban areas always in great condition because fallen foliage gets cleared away removing the source of nutrients from the soil organisms and breaking the food chain. In former agricultural soils, the soil organisms have often completely disappeared thanks to the intense use of chemical fertilizer. Finally, the structure of the soil is also compromised and compacted during installation and construction work which results in problematic water and oxygen management.

Planting site improvement

In urban areas then, the circumstances for trees are generally unfavourable, so when we create planting sites, measures need to be taken to improve them. Tilling the ground, i.e. turning the soil to improve its structure, is necessary in almost all situations. Soil improvement by adding organic matter is desirable in many cases.

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Root protection systems

Depending on the situation it is also possible to decide to use a root protection system. This involves actually ‘reserving’ part of the underground space for root growth and you make sure that the conditions within this space meet the requirements that the tree sets. Root protection systems lighten the load in the soil and prevent compaction. Most systems have concrete or plastic structures and are installed underneath the paving. The choice of system depends on the soil conditions and the demands that are being placed on the use of the space. Much will depend on the expected traffic density and its distance to the trees. When a root protection system is combined with a substrate that is also acceptable to the tree, then a good planting site can be created. In addition, the higher the quality of the soil, the less penetrable space the tree will eventually require. Lack of space can therefore be partially compensated for by increasing the quality of the planting site. By investing in a good planting site at the outset, subsequent problems such as roots forcing their way up through the paving or a deterioration in condition can be prevented so that even trees in urban surroundings have a future.

Conditions for a good planting site:

Pore volume

The soil has a pore volume of 50%. This means that 50% of the soil consists of solids, i.e. minerals and organic matter. The other 50% can then be filled with 30% water and 20% air. Habitat layers beneath paving have less pore volume. Trees need a pore volume of at least 30% under which roots die off.

Oxygen percentage

The oxygen percentage in the soil is 20-21%. 18% is just sufficient for trees to grow. At 10 to 16% the tree will survive but it will not develop in a positive way. If this situation persists the tree will deteriorate further. Below 10% the tree will slowly die, below 5% there is acute root mortality and the tree will die very quickly. Pay particular attention to this if the planting site is beneath the pavement. It is essential that gas exchange can take place between the soil and the open air.

Moisture content

The moisture content of the soil is between 14 and 30%, depending on the composition of the site. Below 14% a tree will start to wither. Above 30%, the water will hinder the presence of oxygen resulting in root mortality. Trees will then develop negatively and slowly die.

Organic matter

The organic matter content in the soil is between 4 and 13%. A tree needs organic matter to provide its nutrients. The soil organisms convert organic matter into nutrients that the tree can then absorb. Organic matter must be of a good quality. When it is not fully mature or too wet, a process of decomposition will occur, which will extract all the oxygen from the soil with root mortality as a consequence. The optimum organic matter content differs from soil to soil but on average, 7% is ideal.

Soil organisms

The presence of adequate organic matter in the ground. Life in the soil (worms, beetles, moulds, bacteria etc.) ensures that organic material from the leaf litter gets into the soil and can be absorbed by the roots. Healthy soil life also contributes to there being a good soil structure as it keeps it open and crumbly.