I always feel the best designs are ones that don't use too many different species of plants

Guy Bowden

These are systems designed to assist with the treatment of grey water and effluent from a septic tank or a treatment plant. All homes that are not linked to a district sewage system need to treat and dispose of their own effluent.

Evapo–transpiration seepage systems use both soil soakage and plants for the disposal of treated effluent. These usually consist of perforated pipes laid on the ground covered with bark, or laid in shallow trenches filled with gravel. Effluent flows out of the holes in the pipe and soaks into the surrounding soil. This area is then planted with specially selected plants that can tolerate wet feet. The plants absorb some of the effluent using the nutrients to grow and loose the liquid portion as water from the surface of their leaves through the natural process of evapo-transpiration. There are no odors or running water, they are child safe and mosquito free. Generally there is an even distribution of treated effluent (no boggy areas), and although you are limited in the number of species you can use due to the wet soil types, there is no reason why these areas cannot be made into a real feature.

I always feel the best designs are ones that don't use too many different species of plants. Mass planting of half a dozen species can look the best. Grasses, rushes, flax and cabbage trees will perform the task of soaking up and filtering effluent with the greatest efficiency. There are also a number of large trees that will tolerate wet conditions well.
There are numerous plant combinations which can be used.

Carex secta planted on a wet bank.

These below are some of our favourite suggestions.

Carex grasses provide some particularly suitable species, they look attractive throughout the year and are easy to maintain. When grasses are planted en masse they bring a wild look to any garden, gracefully moving in the wind.

Phormium tenax, large green flax growing to 3 metres high and almost as wide. While very useful in this type of planting, flax is best kept to the middle of the bed where it is more easily contained.

Apodesmia similis, Oioi or jointed rush is another plant that looks best planted en masse. It is a creeping plant growing to 1.5 metres high forming dense patches. Naturally it is most commonly found growing in tidal areas and salt marshes.

Cordyline australis, cabbage tree, grows 6 – 8 metres high and will grow quite happily in moist/wet soil. It is also useful to giving the planting some character and structure by adding height.

Grasses, sedges & rushes:-
Carex secta, Carex virgata, Apodesmia similis, Baumea species, Juncus maritimus.
Phormium tenax only.
Cordyline australis, (cabbage tree),
Plagiantus regius, Coprosma propinqua, Coprosma robusta, Kunzea ercoides (Kanuka),

Dacrycarpus dacrydioides (Kahikatea), Laurelia novae-zealandiae (Pukatea), Rhapolystlis (Nikau).