Soil compaction and altering ground levels

Soil compaction and altering ground levels

There are many things that can damage trees and vegetation when completing construction works.  It is your responsibility to manage these risks when undertaking a new project.

So to help out, we’ve compiled some of the main factors you may come across and some preventive measures you can put in place when working around trees.

What is the root zone?

The root zone of any tree is the area you most need to be aware and take care of and the easiest way to identify the root zone will depend on the tree.

For large spreading trees such as pohutukawa, the root zone is the same as its dripline. The dripline is the furtherest outside point of the tree canopy. You can draw an imaginery circle on the ground around the tree.

On more vertical trees such as conifers, the easy way is to take half the height of the tree and apply it as the diameter of the root zone, as a guide.

Soil compaction

Most roots are found in the top 200mm of soil.  These are the fine roots that collect the water and nutrients that the tree needs to stay healthy.  These roots, like humans, need oxygen as it is necessary for root respiration.

Compacted soil makes it harder for roots to breath.

Compacted soils also can change the moisture levels of the soil that can lead to root rot or dried roots.

What causes soil compaction?

Soil compaction can be caused be vehicles or machinery driving inside the drip line (the outer canopy line), over the root zone, or from stacking building materials or placing temporary buildings there. Soil compaction can also be caused by foot traffic, and is something to be particularly aware of on larger sites.

How to prevent soil compaction

The best way is to cordon off the root zone with fencing.  The fencing should be positioned outside the dripline or half the height of the tree whichever is the greatest.  Fencing should be sturdy and not easily climbable. Temporary mesh fencing is ideal for this and is a cheap option.

Signs can be used in conjunction with fencing to warn people that this is a protected area and to keep out.

Aged mulch rings can be spread under the canopy.  This will help keep people off and minimise compaction if people are to stray into root zone.  Mulch is also great for helping to keep moisture in the soil and add nutrients to the top layer of roots.

Raised walkways/ platforms can be created from scaffold or the like, to keep foot traffic out of root zones and limiting compaction of the soil.

Raising/ reducing the levels

Often on site you will be required to cut into or raise the level of the land, and often there are trees nearby.

Raising levels on a site inside the root zone/ dip line can cause similar problems to soil compaction.  The soil that is bought in is generally required to be compacted. This can suffocate those fine surface roots.  The story is the same for cutting of the levels. Cutting the top off the soil inside of the root zone can damage those vital roots that are found in the top 2-300mm of soil.

So what can you do?

Firstly try and think about how the lay of the land is going to change during the planning stage and look at how you can design your project around any mature trees on site. 

When raising:

  • In the early stages of planning consider using retaining walls or terraces to minimise the soil level being raised around the trees.
  • Never stock-pile soil or other materials close to the tree. 
  • Avoid raising the level more than 50mm-100mm.

When lowering

  • Use mulch in excavated root area to help maintain moisture in soil.
  • Water more during dry periods
  • Prune roots to prepare for root loss caused by the lowering of the level.

If all else fails…

Think about how much of the area will require cutting and whether this falls within the root zone?  If so it may be more sensible to consider transplanting or removal and replanting somewhere else. It’s very hard to rectify the effects from raising/ cutting the levels in a root zone.

Better still, speak to one of our experts. Together we can create the right environment where everyone’s a winner.

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