Trees, shrubs, and other plants over your septic system

I get quite a bit of traffic from people searching for answers about what can be planted over septic tanks and septic systems. Questions such as:

  • Can I plant ornamental grass over my septic tank?
  • Is it safe to plant trees or shrubs near my septic system?
  • Can potatoes harm my septic system?
  • Is it safe to grow plants over a septic tank?
  • What are some safe plants for a septic tank?

Here is my answer to these questions and others that relate to planting over septic tanks and septic systems: It depends.

Planting over a septic tank

A typical septic tank is usually between 1-3 feet underground. This means that the top of the tank is really not that far below the surface of the ground. Small plants that have small root systems are usually safe to plants over the septic tank because the roots will not travel down deep enough to cause problems. However, a plant like a Forsythia can have an extensive root system that can travel down to the tank and even penetrate into it. On top of the roots getting into the tank, they will spread out over the top of the tank, making it difficult to dig through when the tank needs to get pumped.

And how often does that tank need to be pumped? Every two to three years. Ok, ok, I can hear someone saying ” I have lived in my house for over ten years and never had my tank pumped and I don’t have trouble with my system”. Really, are you sure? I highly recommend reading The Septic System Owner’s Manual to understand how the system actually works. I think you may change your mind and get that tank pumped after reading it.

Now, back to the subject at hand:

Small, shallow rooted plants are usually ok to plants over the tank, but remember to leave a space over the cover to allow pumping of the tank.

Large, deep rooted plants like trees and shrubs are typically not ok to plant over the septic tank. These types of plants grow quite large and the roots can cause a lot of trouble with the tank, possibly entering the tank and causing a sewer backup.

Septic systems (aka: leach fields)

Well, the title is a bit misleading, but most folks who ask about planting over the septic system are actually asking if it is ok to plant over what is called the leach or drain field. This field is where the liquid from your septic tank (effluent) goes through a series of pipes to slowly drain into the ground. Although this liquid will probably make your plants grow like weeds, it is usually not a good idea to plant anything over the drain field.

Here’s why:

  • The field is made up of a series of pipes that have holes in them to allow the water to drain out. The pipe is set in a bed of stone to aid in the draining. On top of the pipe is a layer of fill and/or soil, and this layer is usually about a foot deep. Plants with deep roots will penetrate into the stone layer and possibly into the pipe itself. These roots greatly reduce the ability of the effluent to drain or may clog up the pipe entirely. For this reason alone, trees and shrubs should never be planted near a septic system.
  • The drain field needs oxygen to be able to break down all the bacteria, viruses, and other “stuff” that is in the water. This oxygen is absorbed through the layer of soil that is above the pipes. Planting anything other than a lawn over this field can cause the soil to become less porous and reduce the amount of oxygen that reaches the effluent. This is also the reason you should never place raised beds over the drain field.

So what IS safe to plant over septic tanks and septic systems? Well, I guess I will have to cover that in another article, but I will tell you that it is my opinion that any plant that you will be eating should not be planted over the tank or drain field. I will explain why later. Stay tuned…

For more info on septic systems, try:
Wells and Septic Systems
Water Wells and Septic Systems Handbook
Country Plumbing: Living With a Septic System


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