Managing tree roots for minimal damage during pipe/cable laying.
In gardens as well as parks it is often necessary to dig trenches in order to install or repair underground infrastructure (water, electricity, sewage etc.). Digging these trenches can result in tree root damage which can severely impact tree health even to the point of death.
It is our job to minimize root damage while installing the underground infrastructure.
Green Point: http://www.eco-garden.co.il
It is worth remembering that tree roots grow wherever water and air are available. I will describe two extreme states:
- When a tree grows in a dry area and we irrigate with a single dripper the root volume will reach a diameter of approximately 1.5 m and a depth of 0.5 to 15 m. This volume, of course, depends on the soil type and depth, dripper water dosage and the depth to which you wish to irrigate.
- In soil where moisture and air are unlimited then roots will grow unimpeded, limited only by nutrient availability.
In our gardens here at Green Point, we have found roots of trees and woody shrubs at a depth of 7-17 m and a distance of 25 m from the edge of the tree canopy!
This is contrary to popular ‘received wisdom’ that says the root system only extends to the edge of the tree canopy. For example, in a peat mine in Arizona, USA, Prosopis tree roots were found 56 m deep and 200 m from the edge of the tree canopy! Another example of this is from Moshav Nahalal in Emeq Israel where a 15 m pine tree had the herbicide CMU sprayed 100 m away from its trunk and it died soon after. The presence of CMU was confirmed as the causal agent.
In the acclimatization plots of Green Point, tree roots extend at least 25 m from the trunks. This was demonstrated by the emergence of new shoots from tillage-damaged Acacia pendula roots (the tillage was done for weed control purposes). Simply cutting the roots 3 m from the tree, to a depth of 1.5 m solved the competition problem. In a plot adjacent (within 20 m) to a stand of Pinus halepensis we found that small trees and shrubs did not grow well. However, a replica plot of these species further away did grow well. This is because of direct root competition for water which the P. halepensis stand is winning.
In the same plot plants that grew 20 m from Pinus halepensis do not develop. The same species in another replicate plot further away grow well.
In summary, roots grow preferentially where there is adequate water and air. A 200 l root volume, i.e. a large barrel, is sufficient to support the growth of at least a 10 m tree. Extra volume of root system is needed to anchor the tree securely and so that it need not be irrigated several times a day. If the tree does not have these conditions or water is at a distance or depth, the roots will grow there (i.e. leaking sewage system 10 -20 m from the tree and its roots will grow into it).
To demonstrate this, two Cupressus sempervirans trees (12 m high, 50 years old) here died due to a 1.8 m depth trench being dug 4 m away from the trees for a power line. The trees were accessing the water used on the nearby lawns and the trench effectively cut off that supply (see Image 1). Regular watering of the trees in the picture would have prevented their death.
The solutions to prevent tree mortality as a result of digging a trench are:
- Do not dig the trench within 1.5 m of the tree trunk,
- Cut the roots at the edge of the trench precisely, making sure of a clean cut – this will promote better wound healing.
- In the case that a trench cuts off the water supply for a tree, water the tree regularly for 18 – 24 months until the root system has reconsolidated its growth. This is an important point as the new root system must also act as the anchor for the tree. Regular irrigation would have saved the dead trees in Image 1.
- It may be necessary to provide additional support for the tree until its root system has developed in order to prevent windfall.
The best solution is to lay the pipe/cables in a tunnel underneath the tree roots. This is a little time consuming but saves the tree from too much damage (see Image 2).
Around the tree in Image 3 we needed to pave a new bicycle parking area. We had to recognize the anchoring requirements of the tree. In this case Ficus elastic grows buttress roots that develop upwards over time. The tree was preferentially growing roots on the lawn side, due to the extra water used there for irrigation. We also wanted to encourage root growth on the other side of the tree that doesn't receive water from the lawn irrigation system.
- Cover the roots with fine gravel at least 30 cm above the present height.
- Use paving material that allows aeration and water penetration.
- Lay drainage pipes around the tree and install a drip irrigation system in it. The drainage pipe will also help to aerate the soil around the roots.