SUPPORTING URBAN TREESby Citygreen Systems
It is common knowledge that in order for a tree to thrive, the root network must be able to access enough water. If there is insufficient water then the tree will be unable to absorb nutrients from the soil and will deteriorate as a result of the water loss that occurs during transpiration.
On the other hand, if the rooting area is continuously waterlogged then the surrounding soil may become anaerobic. This will also likely to result in the destruction of the tree.
Gaseous exchanges that are vital for tree’s survival occur not only above ground but also in the root zone. These cannot take place if all air has been excluded from the soil pores because of too much water.
Because of this, consideration must be given to the substance and structure of the ground surrounding a tree pit to ensure that there is sufficient drainage to prevent waterlogging.
However, since the majority of urban tree pits are covered by a hard, impermeable surface, tree pit designs should also incorporate the means to irrigate efficiently, particularly for the first three years.
An efficient irrigation system will account for the fact that tree roots may have been directed downwards as a result of root management measures, and will deliver water directly to the rooting volume, rather than the area above it.
This irrigation system will help with soil aeration at depth, but it is also prudent to leave sufficient open space around the trunk to allow for gaseous exchange between air and soil (and of course, tree growth).
In urban areas, it is often impractical to allow for bare soil. A variety of grating systems or permeable resins should be considered for these types of environments.
While trees in their natural environment are likely to be at least partially sheltered by surrounding vegetation, many urban trees do not have this protection. They are mostly situated in exposed locations where they are vulnerable both to the high winds that can develop as a result of urban wind tunnel effects and to disturbance from pets, pedestrians and vehicles.
An appropriate form of support is needed to help trees through the establishment phase. Underground guying is widely favoured for urban tree pits as it is unobtrusive. Staking and tying is a cost-effective alternative although this will require maintenance, and can be unsightly.
Aside from needing support, urban trees also need protection especially from the constant barrage of gratuitous vandalism, or construction and traffic damage. Because of this, some form of above-ground protection can be critical to tree survival.
When you are designing a tree pit you will need to decide whether there is a need for tree grates, vertical guards or other protective measures.
By creating a tree pit design that successfully accounts for the issues we mentioned above, you will have addressed over 90 percent of the reasons for urban tree failure. A range of products are available to address any or all of these factors as need dictates, giving you the confidence that your tree pit design will be efficient and successful, producing vigorous and healthy trees.
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