There are several factors that can contribute to poor garden drainage. On new housing estates, it is often caused by compacted soil as a result of builders' trucks repeatedly driving and sitting on the ground. This combined with the mixing of sub-soil and topsoil when all the trenches and foundations were excavated often leads to a heavy, wet plot.
Of course, drainage problems are not always caused by human activity, the soil may, by its nature (e.g. clay soils) be water retentive or, if it is low lying, be affected by the natural water table of the area. This is often the case in our area in Northern California especially Meadow Vista with it's red clay and low water table. On clay and other water retentive soils, the flow of water though the soil is very slow, this can be exacerbated if the contours of the land form depressions in which the surface water can collect. On land which has a very shallow water table, not much can be done to avoid water logging after heavy rain as the water table can rise to the surface of the land or actually above it, in this latter case a pond will naturally form.
How to know if a garden needs better drainage?
Wet weather in the winter or spring will show how good or bad the natural drainage is. Patches of water on the surface are the most common signs of problems. To check how well the land drains, dig a hole about 24 inches deep and 12 inches square, then fill it half full with water. Leave it for 24 hours in which time it should empty on well-drained soil. On very wet land, the hole may actually fill.
What sort of drainage system should be used?
If the top-soil is very poor, it may be worth stripping off the top 6 inch and importing new topsoil. Where the topsoil is just very water retentive, the problems may be reduced by double-digging and incorporating large amounts of bulky organic materials, but where the soil is inherently heavy and waterlogged, some form of drainage will be necessary.
The type of drainage used depends largely on the extent of the problem. If the water logging is not severe and there is only excess surface water, it may be possible to overcome the problem by shaping the garden surfaces so that the water flows off into ditches. These ditches should be 3 to 4 feet deep, with sloping sides.
More extensive issues may need items such as perforated plastic pipe which is crush-resistant and the holes allow the entry of water from the surrounding soil. The pipes can be supplied in lengths of up to 50 ft which can just be laid into a trench and surrounded by gravel. With a large area to drain, the most efficient way is to lay a number of pipes in a "herring-bone" pattern.
Underground pipes should always be laid above the local water table otherwise the pipes will just be moving the groundwater instead of draining the topsoil On level ground, the pipes should be sloped underground to encourage water to drain away quickly and on sloping land, the main drain should run down the slope. Drained water obviously needs somewhere to go so all pipes should terminate in a soakaway or ditch.
It is always best to seek professional advice before attempting to extensively re-direct the drainage issues in your garden or yard to best save you time and money. Most landscape experts offer a free consultation to help assess the severity of your issue.
