Septic Tank Soakaway - Gardening over the Drainfield

Septic Tank - Planting over or near the Soakaway

Table of Contents

Why Plant over the septic tank soakaway at All?
Which Plants Are Best for My Septic Tank Soakaway?
Herbaceous Perennials and Annuals
Trees, Shrubs, and your Septic Soakaway
Reducing Tree Root Intrusion Into the Septic Soakaway
Does the Septic Tank Effluent Affect the Soil?
Soakaway Vegatable Gardens
Septic Tank effluent is loved by Hollyhocks

Septic Tank soakaways, when correctly installed, do not smell, so there is no need to plant anything other than grass over them. However, many people, especially those with smaller gardens, want to make a 'proper' garden over them.  There are a few rules as to what can be planted, but every soakaway is different. You can decide what will work best on your site, however, by following a few simple guidelines.

Why Plant over the septic tank soakaway at all?

Septic Tank soakaways drain the liquid from the tank into the ground.  Plants can help your septic drain system to work better by removing moisture and nutrients from the soil. Plant cover is also important to reduce soil erosion. In any case, the soakaway should be planted with a dense cover of grass to provide these important benefits. In addition, many homeowners have limited space and want to make the best landscape use possible of their septic soakaway area. Often the only place for a flower garden on a wooded site is in the sunny spot over the soakaway. Or the drainfield may be located in the front garden which needs flowers and shrubs to set off the house. 

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Which Plants Are Best for My Septic Soakaway?

septic tank soakaway is a series of relatively shallow (a minimum of 200mm. below the ground) underground perforated pipes laid in gravel trenches that allow septic tank effluent to soak away over a large area. As the effluent seeps into the ground, it is purified by the soil, which uses bacteria to digest the pollutants and remove excess nitrogen and phosphorous.  Plant roots can help remove excess moisture and nutrients reducing the pollution and helping the soil deal more efficiently with the remaining pollutants. However, roots that clog or disturb the pipes will seriously damage the soakaway. The answer to soakaway gardening is to find plants that will meet your landscape needs but not clog the drain pipes.  In general, shallow-rooted herbaceous plants that are not too water-loving are best.

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Herbaceous Perennials and Annuals

Septic tank soakaway areas make ideal lawns.  Herbaceous perennials and annuals, lawn grass, and many ground cover plants are shallow-rooted plants and unlikely to damage the drains. Don't dig too deep when planting however.  When planting close to a drain, you may also want to choose species that do not require frequent dividing. Also, always wear rubber gloves when planting, weeding or doing other gardening activities that involve contact with the soil over your soakaway. This will protect you from direct contact with any harmful bacteria and viruses that may be present in the soil.

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Trees, Shrubs, and Your Septic Tank Soakaway

Septic Tank soakaways are not suitable for wooded areas.  Be very careful when choosing species as trees and shrubs are much riskier choices for the soakaway area than herbaceous plants. The woody roots of these plants are more likely to block and damage the drains. Especially dangerous for blocking the drains are water-loving trees such as willows and poplars and shrubs such as Hydrangeas (see table). Do not plant these anywhere near a soakaway unless you are prepared to install a new drain field in the near future.

Some smaller and less-aggressive species may be more suitable for planting over the soakaway. Some possibilities include fibrous rooted shrubs such as boxwood or holly, or small trees such as dogwoods. When planting shrubs in a soakaway field, place them between the drain runs. Normally, drainage trenches are 600mm. wide with 2 metres between trenches.

Finally, you must decide what is more important to you - the soakaway or the plants. One septic soakaway we know of was completely blocked by Weeping Willow roots within 3 years of its construction.  The roots had also travelled up the drain and into the biodisc sewage treatment plant, destroying it and the entire system had to be totally replaced.

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Reducing Tree Root Intrusion Into the Septic Soakaway

Choose less aggressive species. By selecting trees with less aggressive roots, (see the table), you can greatly reduce the likelihood of your trees damaging or blocking the soakaway.

Plant trees as far away as possible from the soakaway.  In order to be completely certain that tree roots will not intrude into your septic tank soakaway, trees should be planted at least as far away as their estimated root spread at maturity. One way to estimate this is from the height of the fully grown tree.  For example,  an oak tree can be expected to reach around 20 metres in height when mature. so it would need to be planted at least 20 metres away from the edge of the septic tank soakaway area.

Prepare the soil for your trees before planting. Roots tend to grow along  "paths of least resistance", so it is wise to dig up as wide an area as possible for your new tree before planting to provide a good rooting environment. Prepare the soil really well on the side of the plants that faces away from the drain field, digging in plenty of compost, manure, etc. as this is the direction in which you want the roots to grow. If you plant a tree in shallow topsoil over a compacted subsoil - a common problem in new housing developments which have paddled over the existing soil during the build and then landscaped using sandy topsoil - the first easy route in the soil that a tree root finds could be your septic tank soakaway trench.   Good soil preparation can help stop roots from blocking your drain pipes because tree roots will be better able to take up enough water and nutrients from the soil without sekking them in the soakaway.

Use root barrier membranes. Another effective, although expensive, technique is to install a vertical root barrier membrane lengthways between your soakaway and tree plantings. This should be the entire length of the drain field.  To effectively stop tree roots, the barrier should extend from the soil surface to a depth of at least 1.5 metres.  Install the barrier fabric at least 1 metre from the drain field so as not to disrupt the system. Allow at least 2 metres between the tree and the root barrier - more if it will grow to be a very large tree. 

Choosing Trees For Planting Near Your Septic Tank Soakaway  

Not Recommended for Planting
Near Soakaways
Better Choices for Planting
Near Soakaways
Red Maple
Silver Maple
Fagus spp.
Betula spp.
Ulmus spp.
Populus spp.
Acer rubrum
Acer saccharinum
Salix spp.
Sorrel tree
Prunus spp.
Malus spp.
Cornus spp
Tsuga spp.
Pinus spp.
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Does the Septic Tank Effluent Affect the Soil?

Septic Tank effluent alters the soil conditions.  It is important to understand your soil type before deciding on any landscape planting. Some plants prefer sandy, dry soil and some prefer wetter clay soils.  Some plants like acidic soils and some prefer alkaline soils.  Many household chemicals and washing powders are highly alkaline and can raise the pH of the effluent, making it more alkaline.  The pH of the effluent will alter the pH of the soil, so have soil samples from the drain field tested periodically so that you can adjust the soil pH as necessary with products like Sequestrene, if you have plants that like acidic conditions, eg Azaleas. Septic Tank drainfields are ideal for growing Himalayan Balsam

Septic Tank effluent contains a lot of salt (see also sodium binding) and it is essential to plant species that are tolerant to salt in the soakaway.  Plants that are both salt and moisture tolerant include hollyhocks, bee balm (Himalayan Balsam), violets, arborvitae, dogwood, inkberry holly. Other plants that are tolerant of salt include yarrow, columbine, chrysanthemum, delphinium, daylily, peony, clematis, and rose, although these will not do so well on very wet septic tank soakaways. You will also find that you need much less fertiliser, if any at all, as the septic effluent contains many plant nutrients..

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Soakaway Vegatable Gardens

Septic Tank effluent contains a lot of bacteria and viruses.  These are killed, eventually, by the soil, although in very sandy soils, they can travel several feet before they are filtered out. Whether you put a vegatble garden over a soakaway depends a lot on how well your soakaway drains and how deep the drains are below ground.  A properly operating drain field will not contaminate the soil with disease-causing organisms, but it is very difficult to determine if a soakaway is working well.  If in doubt, don't use the soakaway for a vegatable garden - plant flowers there instead, but if you do decide to use it for vegatables:-
  • Do not plant root vegatables over the drain trenches.
  • Low level green vegatables like cabbages could be contaminated by rain splashing soil onto the plant, so either cover the ground with straw or mulch or discard the lower leaves.
  • Fruit bushes should be OK, but you might want to discard any low level fruit.
  • Tie cucumbers or tomatoes onto a support so that the fruit is off the ground.
  • Thoroughly wash any vegatables from the garden before eating it.
  • Do not put raised beds over the field as the drains will then be too deep for aerobic soil conditions to treat the effluent.
  • Do not put paths or greenhouses over the soakaway as this is against Section H2 of the drainage and septic tank Building Regulations.