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Gardening/planting over your soakaway (drainage field)

The ground over your soakaway (drainage field) will seem a bit of a mess when it’s newly installed, as trenches will need to be excavated for the pipework and reinstated. However, newly-planted grass will soon grow and you shouldn’t notice any difference to the rest of your garden.

Some customers like to plant flowers and plants over their soakaway, which is generally fine (with a few exceptions). Soakaway systems are designed to drain the water from the septic tank into the ground, and are constructed from a series of perforated pipes in trenches. Plant roots can improve this drainage by removing moisture and nutrients from the soil, and the soakaway should always be planted with a dense cover of grass to reduce soil erosion.

However, plants with aggressive root systems can disturb the pipework and damage the soakaway – it’s best to stick to shallow-rooted plants that will not encroach on the soakaway, such as herbaceous perrenials, annuals, bedding plants and lawn grass. Just make sure not to dig too deep into the soil, and always wear gardening gloves so as not to expose your skin to any potentially harmful bacteria existing in the soakaway soil.

A soakaway is not suitable for installation in a wooded area. Tree root systems can often be extensive and aggressive, and are likely to cause damage to the soakaway, particularly willows and poplars. Simply ensure any new trees are planted as far as possible from the soakaway – estimate this by the expected height of the fully-grown tree (for example a tree that will grow to 15 metres high should be planted at least 15 metres from the edge of the soakaway area.

Effluent from septic tank soakaways understandably affects the soil conditions. You should take this into account when selecting plants to grow, as some plants prefer dry, sandy soil whereas others are happier in wet soil. Discharged effluent contains high levels of salt so take this into account when you’re planting. Violets, dogwood and hollyhocks are both salt and moisture tolerant, and are good options.

Vegetable Planting

Effluent from septic tanks contains high levels of bacteria and viruses. The soil will kill these eventually but in very dry, sandy soil this bacteria can travel several feet before they are filtered out. Your decision to plant a vegetable garden over the soakaway depends on how effectively your soakaway drains and their depth below-ground.

If you do decide to plant vegetables over the soakaway, the following guidelines will help:

  • do not plant root vegetables over the trenches
  • any plants/bushes with low-level fruit or vegetables could risk contaminated soil splashing onto the lowest fruit. Cover the ground with straw or mulch, or discard the lower fruit and vegetables
  • any plants/bushes where the fruit or vegetables are hanging should be securely tied so they are not trailing on the ground
  • wash anything you harvest thoroughly!
  • don’t install raised beds over the soakaway as the drains will then be too deep for the effluent to be adequately treated aerobically
  • never install paths or greenhouses over the soakaway as these contravene Building Regulations