Discouraging Root Growth Around Irrigation Pipes

Q. Having had several somewhat expensive repairs because of root damage to my sprinkler system, I am wonder if you have any suggestions for preventing/lessening such damage. I was told by one sprinkler technician that putting copper (he suggested pennies) in the ground would discourage root growth in the area. Is there any truth to this?

A. I am guessing the root damage was lifting the pipe out of the ground because roots were growing under the pipes. Roots will not grow into pipes unless there is existing damage to them, allowing for their entry but then there would be water everywhere. The other type of intrusive growth would be in drip irrigation emitters.

The best ways to avoid damage are to bury pipe deep, plant woody plants far enough from irrigation pipe so that it does not become a problem (preferably outside the irrigated zone of the plant) and use the appropriate type of irrigation pipe.

The best way to keep roots from lifting pipe and damaging them is to bury the pipe deep, as it should be. Irrigation laterals (pipe coming from irrigation valves directly to a sprinkler or other type of emitter) should be a minimum of twelve inches deep (0.3m).

Irrigation lines that are under constant pressure (pipes before the valves) should be a minimum of 18 inches deep (0.5m). All plastic pipe used for irrigation should be a minimum of Class 200 PVC when installed after or downstream of the valve. All pipe under constant pressure (before the valves) should be Schedule 40 PVC.

Class and Schedule refer to the internal pressures that these pipes can withstand which is related to the thickness of the walls of the pipe. All pipe installed should be "fresh" pipe, undamaged by the sun.

Class 200 PVC pipe on the left and Schedule 40 on the right

The PVC in most irrigation pipe is rapidly damaged by the sun if left exposed to it. You will see this damage by discoloration of the pipe. In extreme cases it will turn the PVC black from discoloration due to damage. Once this damage from the sun occurs, the pipe becomes very brittle and has no capacity for bending. It will shatter easily if bent.

Sun damage to white PVC irrigation pipe

Do not use, or do not let a professional use, PVC pipe which is discolored. Pipe exposed to the sun should be painted or wrapped. Paint will protect PVC from the sun's damaging ultraviolet radiation.

As far as pennies are concerned. This idea comes from the fact that copper is very toxic to plants.

Copper sprays are used to control some fungal diseases (Bordeaux sprays), copper is used to control mildew which are plants, copper is used to control algae and mosses which are plants, copper will kill plant roots when used as an appropriate pesticide (yes, it is considered a pesticide if it kills plant roots!) and copper nails will kill trees if pounded into the trunk a few inches apart.

The copper that kills is called elemental copper and must be available to react with plant roots. Perhaps plant roots in direct contact with pennies will be killed but plant roots just a few inches away probably will not. This would mean that if you were to protect the pipe you would have to line the pipe with pennies.

Just a side note. Whenever we pound anything into a tree we need to make sure the damage is minimal. Of course pounding anything into a tree damages it but some things are more damaging than others. Zinc galvanized nails will damage a tree more so than stainless steel. Whenever anything is put into a tree it is always best to make sure it is stainless steel if it has to be done at all.