Must Cut the Roots of Italian Cypress to Put in a Block Wall

Q. We are thinking of taking out our wooden fence and replacing it with cinder-block wall.  The Italian cypress trees grow along 2 sections of the fence and are about 20 years old.  The new block fence will go on the other side of the wooden fence but there’s the footing to consider which will cut into the root system.  My question to you is, what are the chances of these trees surviving since we won’t know how much of the root will be cut into?  I can send pictures of the base of the tree with the irrigation and the existing fence post to give you a better picture of the area or I can send measurements of what I am planning.  Not sure if the pictures will give you enough detail to make an informed prediction. 

This is a picture of the soil around some oleanders after a
block wall had been removed and was being replaced. This
is not the readers but a friends. The oleanders were being
watered in a shrub bed with bubblers. Notice that there
is not alot of root development next to the wall.

As you know the trimming of these trees are time consuming and/or expensive to hire out.  We wonder if the block wall will bring more heat to the yard and could the heat from the wall burn the tree?   The reason we are considering this project is that with our dogs we are simply worried that if a plank snaps that our dogs will get out and possibly hurt.  We like to rustic look of the wooden fence but we wonder if the block wall might be a better choice for security.  Any thoughts on your part would be appreciated.
 

A. Let me talk about things I know something about, the damage to the trees if you decide to move ahead with a block wall, replacing a wooden fence.

    In the desert, plants grow where there is adequate water. If a good supply of water is on your side of the fence, the roots will tend to grow more in that direction. If there is lots of water on the other side of the fence, they may tend to grow in that direction.



Notice the difference between the shallow fleshy roots
of the palm tree (left) and the oleanders on the right. Palm
roots grow where there is more air, near the wall.

    So try to picture that your tree's roots will grow more in the presence of water. Would this be on your side of the wall or your neighbor's side? So if a cinder block wall is constucted, a trench will be dug and a footer laid to support this very heavy wall. You are right, this will definitely eliminate a portion of their root system.

   If these tree's roots are growing toward your neighbor mostly, then these trees will suffer significant damage. If your landscape is dry and the neighbor's is wet, this might cause severe damage to your trees.

    If their water is coming mostly from your yard and your neighbor's is dry, hen cutting the roots on the side toward your neighbor will probably have a more minimal effect.

This picture from Washington State University shows
how some plant roots will grow toward a good balance
of air and water such as close to the container wall.

    Likewise if their water is coming from a water source in small amounts from drip irrigation close to the trunk adn the resest of the yard is dry then the impact will probably  be minimal. In other words,if you are trenching in soil that stays dry for most of the year you will probably be okay.

    Another word of caution. The week before they put in the footer water the trees very well. Then let the soil dry out until they dig. Encourage them to dig, put the footer in and backfill the soil as soon as possible to minimize damage to the trees. As soon as possible, irrigate the trees again.