Improve Soil with Simple Garden Drainage Techniques

herringbone garden drainage system

Garden Drainage Systems to Improve Soil

Waterlogged gardens are a misery. They are impossible to cultivate and plants won’t thrive in such infertile soils.

What is needed is a good garden drainage system.

Every garden needs moisture to enable plants to flourish, and in most gardens the balance is about right; rain falls, seeps into the soil and drains away naturally. But if something disturbs this natural balance, the result is a garden that is a soggy swamp most of the time.

Few plants can thrive with waterlogged roots, and wet soil is very hard to work when it comes to digging and planting. Worst of all is a lawn that is always soft and wet underfoot; every footfall leaves a mark and mud is constantly carried round the house from the garden.

Waterlogging occurs for a variety of reasons. One of the commonest problems — and the easiest to put right — is soil that has become compacted because it has not been recently worked. Heavy clay soils are particularly bad in this respect.

Compacted soil keeps water standing on the surface so that it cannot reach plant roots. The solution is a vigorous programme of double digging, incorporating plenty of organic matter such as manure, straw or compost to help break up the soil and improve its water-holding capacity.

A similar problem is common in the gardens of newly built houses. When clearing a site, builders often remove the topsoil and spread excavated material over the garden area. They then inadvertently compact it with the movement of heavy machinery, and later cover everything up with a thin layer of topsoil or poor quality subsoil. The result is an impermeable layer just below the surface, preventing adequate garden drainage. Again, the solution to improve soil is to double-dig and to incorporate organic matter.

However natural problems are the worst, such as a layer of non-permeable rock close to the surface, or an area where the water table — the natural level of ground water — is particularly high.

In the latter case, prolonged rainfall causes the water table to rise until it reaches the surface, resulting in standing water. With both these problems, the only practical solution is to install a land-drainage system.

If you move house in the summer, you may not discover that you have a garden drainage problem until winter. The best way of spotting a potential disaster area is to look at what is growing in the garden; stunted sickly looking shrubs, rushy grass and bare patches of lawn are all tell-tale signs of garden drainage trouble.

Gauging Soil Porosity

To gauge how well the garden drains, dig a hole about 60cm (2ft) square and 60cm (2ft) deep. Leave it for 24 hours to see if any water collects in it; if it does, the garden is suffering from impeded drainage caused by a high water table or non-porous subsoil, and definitely needs drainage. If the hole remains dry, fill it with water and see how quickly it drains away. If it empties in about 12 hours drainage is very good — in fact too good — probably because the soil is very sandy. Rapid drainage can be slowed down by enriching the soil with lots of organic matter.

If the hole takes 24 hours to empty, drainage is adequate; if the water is still standing in the hole after 48 hours, the soil is probably too dense and needs breaking up thoroughly by double-digging. Some simple drainage may also be necessary.

Simple Garden Drainage Techniques

Soggy areas can sometimes be improved by installing garden drainage trenches. These help to divert water running across the garden from higher ground, conveying it to a soakaway or a natural outlet such as a ditch or stream.

On no account must any land drainage system be connected to the main sewage system.

The simplest type of garden drainage system is a trench about 75cm (2-1/2ft) deep with a lining of coarse rubble (brushwood is less satisfactory because it eventually rots and allows the trench to collapse under the weight of the covering soil). Unless you have access to a reasonable quantity of rubble, contact a local demolition firm and arrange to have a load delivered.

Give the trench a fall along its length of about 1 in 100, running to the ditch or soakaway. This is just a hole about 90cm (3ft) deep and filled to within about 30cm (1ft) of the surface with loose-packed brick rubble or clinkers.

Spread the hardcore to a depth of about 30cm (lft) along the base of the trench. To prevent soil washing in and clogging up the hardcore, cover it with polythene sheeting or glass fibre matting before back-filling the trench with the excavated soil.

Herringbone Garden Drainage System

In extreme cases of waterlogging, install a series of land drains running across the whole property. They should discharge into a natural outlet, a ditch for example, or a soakaway. Drainage trenches are laid out in a herringbone pattern, running from the highest point of the garden to the lowest and then into a soakaway. On level sites the trenches should have a fall of about 1 in 100 to ensure that the water flows comfortably through the system.

The depth of the trenches depends on the soil porosity. Dig down to about 75cm (2-1/2ft) in clay, 90cm (3ft) in loam and 90-120cm (3-4ft) in sandy soil to compensate for the differing rates of drainage of each soil type. The side drains should be spaced about 3m (10ft) apart in clay, 6m (20ft) apart in loam and 15m (50ft) apart in sand, and should join the main drain at an angle of about 45°.

Start by marking out the positions of the side and main drains with pegs and string lines. The main trench needs to be about 30cm (l ft) wide, the side branches about 15cm (6in) across. Then, lift the turf along each drain line, setting it aside so that you can replace it later, and excavate the trenches. Give them sloping sides so that they don’t collapse, and store lawn turf, topsoil and subsoil separately on tarpaulin sheets as you dig.

After digging each trench, use a spirit level on a long batten — or a water level made from a length of garden hose and two pieces of clear plastic pipe — to check the fall. Then lay a 5cm (2in) thick bed of sand and gravel along the bottom of the trenches ready for the drainpipes to be laid.

You can use plastic, concrete or earthenware pipes. Plastic pipes are perforated, and are laid with the perforations facing downwards so that water can enter the pipe without washing in fine soil and causing a blockage. Successive lengths are linked with straight couplers, and tees are available to make connections into the main drain run. Plastic pipes are best — they are easy to lay and the joints help keep the system clear of blockages.

Concrete and earthenware pipes are laid with 12mm (½ in) gaps between successive lengths. The joins are covered with pieces of slate or roof tile to stop soil falling in. However, they are more difficult to lay and silt up easily.

Whichever sort of pipe you choose, lay 7.5cm (3 in) diameter runs for the side branches of the system, and 10cm (4in) diameter pipe for the main drain leading to the soakaway. This should be dug so its base is about 1m (3ft) below the bottom of the main drain run (or 2m / 6-1/2ft) deep on sandy soils. Slope the sides and shore with sheets of plywood or similar materials to help prevent the trenches from caving in.

Then fill up the soakaway with rubble to the level of the main drain and start laying the pipe-work. The last length of the main drain run should project into the centre of the soakaway.

As you reach each branch drain, fit a tee piece (with plastic piping) and carry on laying the pipes until you reach the top end of the system. Check that all the pipes are laid correctly and that the joints are secure. Then cover them with 15cm (6in) of gravel, then a layer of glass fibre matting to help prevent soil seeping into the pipes. Finish off by backfilling the trenches with subsoil and topsoil tamped firmly into place, and replace the turf. Lastly, complete filling the soakaway with gravel to within about 30cm (1ft) of the surface and lay soil and turf over the top. A sheet of polythene placed between the soil and the rubble will prevent soil from being washed in by rainwater or forced into the hardcore base by people walking across the lawn or the movement of garden implements over the surface.

28. October 2010 by Dave Pinkney
Categories: Garden Management, Soil Cultivation | Tags: , , , , | Comments Off on Improve Soil with Simple Garden Drainage Techniques


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