How to improve sub-surface drainage in the garden

The issue of drainage in your garden needs to be considered. This is vitally important for plants that require good drainage, as waterlogging will usually lead to death by root or collar rot.

By far the best strategy is to install sub-surface drainage (if it is necessary) before new gardens are created, as it invariably requires extensive soil disturbance that will be harmful to established plantings. It is also important to recognise that sub-surface drainage is a rather technical subject that may be best left to professionals such as engineers and landscape contractors where large projects or sensitive (eg steep slopes, unstable soils) areas are concerned. It is also important to check with relevant authorities (eg local council, gas, water and electricity suppliers) if you will be digging holes that may disturb existing underground services or changing existing drainage patterns. You also need to consider where the water will end up once it is drained, as you don’t want to merely push the problem to your neighbour.

Do I need to worry about sub-surface drainage?

If your topsoil becomes waterlogged for more than a few hours during wet weather then you should investigate as follows. Dig holes in several parts of your garden approximately 50 cm deep, as this is usually the depth where problems can occur as the topsoil changes to subsoil. The subsoil usually contains a lot more clay than topsoil, often resulting in poor drainage at the interface. Fill the holes you have made with a bucket of water and observe how long the water takes to disappear into the subsoil. If it takes longer than an hour then subsurface drainage is highly desirable.

Installing sub-surface drains in the garden

The most effective and easiest to install option is corrugated plastic pipe, sometimes known as ‘€œagpipe’€ or ‘€œdraincoil’€ as it is flexible. It is also has slots made in the corrugations to allow easy entry of drainage water. It is readily available from hardware stores and garden centres and unless there are very large volumes of water likely to be carried the 100mm diameter pipe will be sufficient.

The drain can be installed up to 80-90cm below the surface depending on your soil depth. The drains should also have a slope of at least 1:70 (ie a fall of 1metre for every 70metres in length). On very heavy clay soils the drains may need to be as little as a metre apart, while if the soil is lighter they can be up to 5 metres apart. Where large areas are involved the most common type of drainage system is a herringbone pattern.

It is very important that the pipe is surrounded by a very porous gravel such as blue metal, road base or pea gravel to prevent it clogging with silt. Put a 50 mm layer in the bottom of your trench before laying the pipe and cover it with a further 50 mm on top. A useful tip is to backfill over the gravel with sand right up to the soil surface, which will help get the excess water into the pipe more efficiently. It is also advisable to use the ag pipe that has a knitted cover, as this also helps to prevent dirt getting into the pipe, which will eventually block it up.

Where does the water from sub-surface drains go?

There are a couple of choices as to where you divert any excess water collected in sub-surface drains. Given that water is a valuable commodity it can be a good idea to retain as much as possible on-site, particularly if you have a large garden with lots of greenery such as deep-rooted trees and shrubs that can use the extra water. The option here is to install an absorption (transpiration) pit similar to those that have been used extensively in the past with septic systems. Firstly a trench several metres long, a metre wide and up to a metre deep is dug (it must be lower than the level of your sub-surface drain). This trench is filled with coarse gravel such as blue metal and then covered with a plastic semi-cylindrical trench-liner that is perforated to allow the entry of water but not sediment. The water is retained in the trench during periods of heavy rain and gradually seeps into the sub-soil when things dry out.

The second option for the excess water is to divert it into a stormwater outlet if one exists on your property – check with your local council. Not every property has a stormwater outlet (for example old inner-city properties or those that are lower than street level). In such cases an absorption pit is the only viable option. In either case it is important to seek advice from a plumber or landscape contractor to ensure that your system is adequately designed.