What Plants Are Safe to Grow Over Septic Tanks and Drain Fields?

Your Best Choices and a List of What to Avoid Planting

Pussy willow catkins.
Avoid planting any type of willow (including pussy willows) around a septic system. David Beaulieu

Certain trees and shrubs can cause damage with their aggressive roots when planted around septic tanks and drain fields. Learn which plants are the worst to grow over a septic system and which are safer choices.

Plants Safe to Grow Over Septic Tanks and Drain Fields

Do not become so paranoid over the potential of damage to septic systems caused by roots that you abstain from planting these areas altogether.

Growing the right kind of vegetation here is not only permissible but actually advisable. Plants will prevent erosion and suck up some of the excess moisture from the drain field. At the very least, grow a tall fescue grass, Kentucky bluegrass, or other lawn grass over that patch of ground. Even letting weeds grow there would be preferable to leaving the ground bare. Creeping Charlie and jewelweed plants will multiply and cover a septic space nicely.

Perennials and grasses (including ornamental grasses) work best around your septic tank and drain field. Their shallow root systems are less likely to invade the underground system and cause it damage. For the same reason, small, non-woody ground covers are a good choice. There are, of course, many examples of such plants, so you will want to narrow down your choices. A good way to start is to consider growing conditions:

  • But if the spot does not get much sun, you will probably be happier with these shade-garden plants.
  • The soil around septic tank drain fields is sometimes wetter than average, sometimes saltier than average, and sometimes both. Cover both bases with perennials such as bee balm, hollyhocks, and wild violets, which tolerate both wet ground and salt.

It is not safe to eat food crops grown in the ground around a drain field because eating them might entail ingesting harmful bacteria.

If you must grow trees and shrubs, shallow-rooted kinds are better to grow around septic tank drain fields. Shallow-rooted trees and shrubs include:

The Worst Plants to Grow Over Septic Systems

Generally, avoid planting large, fast-growing trees. But, in addition, some of the worst offenders are trees and shrubs with root systems that aggressively seek out sources of water. They are not fussy about the water source they tap into, meaning the pipes in your septic tank drain field are very much fair game.  Weeping willow trees are a notorious example. There are many trees and shrubs to avoid, but here is a small sampling:

Let's say you have avoided growing any of the most problematic plants directly over your septic tank drain field. Are you out of the woods? No. There is still a danger posed by any large, mature trees that may be growing anywhere near your septic system. The general rule is that such a tree needs to be at least as many feet away from your septic drain field as it is tall. So a specimen 50 feet tall at maturity should stand at least 50 feet away. Failing that, it is possible to install root barriers to try to keep tree roots from invading your septic drain field (similar to the bamboo barriers used in controlling invasive bamboo).

The Basics of How Septic Systems Work

Once you acquire a basic grasp of what a septic tank is, what a septic drain field is, how a septic system works, and what its purpose is, you will have a better understanding of why it's a bad idea to plant certain kinds of plants over drain fields.

This will take some of the mystery out of what is going on down there for average gardeners so that they can grow plants over this portion of the landscape without fearing that they will damage their septic systems and incur the consequent costs. 

  1. Problem: Something has to be done with all household wastewater, be it from the toilet, shower, kitchen sink, or clothes washer.
  2. Solution: In rural areas (which lack sewer systems), septic systems serve as wastewater treatment facilities. 
  3. A pipe brings the wastewater out of your home and stores it in the septic tank, which is an underground, watertight receptacle.
  4. The septic tank is designed so that solids ("sludge") and scum will separate from liquids. The solids sink to the bottom. The scum rises to the top. The liquids form a middle layer, in between the scum and the sludge.
  5. The liquids eventually exit via a T-shaped baffle pipe. The catalyst for their exit is the entry of additional wastewater from the house. The ingenious baffle is designed so that only the liquids can exit through the pipe.
  6. Their discharge takes the liquids into a much larger part of the septic system known as the "drain field" or "leach field."
  7. The drain field typically consists of a number of perforated PVC pipes laid in underground trenches. The trenches are filled with crushed stone or gravel. They may be covered with drain field fabric to keep dirt out.
  8. Because the pipes are perforated, they allow wastewater to exit into first the crushed stone or gravel, then the soil below. The wastewater is said to "percolate" through the ground. This process removes most of the harmful bacteria present before it can reach ground water. Excess moisture in the soil will be taken care of through evaporation unless you (mistakenly) do something to impede it.
  9. At some point (often about three years), you must pay a septic service to pump the sludge and the scum out of the septic tank.

Why You Have to Be So Careful Planting Over Septic Tank Drainfields

It is primarily the drain field pipes that you have to worry about when planting around septic tanks.

You do not want roots penetrating the perforations and gumming up the works. All of the parts of this carefully tuned system must be functioning properly, else the result is a mess (and a costly one, at that).

While annual flowers such as Impatiens are sufficiently shallow-rooted to serve as plants for septic fields, what makes them less than ideal is that they have to be planted every year. The less gardening work you have to do in a septic tank area, the better (both for you and for the septic system). Always wear gloves when digging in a drain field to protect yourself. Never dig deeply (you could damage the system). 

All of the following are bad ideas because they may interfere with the normal evaporation process that removes excess moisture:

  • Adding soil to the area
  • Mulching too heavily
  • Watering the plants more than you absolutely have to