October 2016 | The Garden 65

Improving drainage

Improving drainage

With the intensity and frequency of extreme rainfall events rising, avoiding waterlogged or flooded gardens may mean gardeners need to improve drainage Author: Guy Barter, Chief Horticulturist, RHS Garden Wisley. Photography: Tim Sandall

Garden solutions

Rainfall in Britain ranges from more than 1,200mm (47in) in some western regions to a mere 600mm (24in) or less in the driest parts of East Anglia, much of it falling in winter. Every 25mm (1in) of rainfall equates to 25 litres per sq m (5 gall per sq yd).

Average daily household water use in the UK is 150 litres (33 gall) per person, per day. Rain does not fall in a steady stream, of course – showers may not even soak in – but much falls in heavy rainfall, which quickly saturates soils or runs off into ditches. Most winter rain soaks in, but much summer rain evaporates in the warmer temperatures.

It is easy for soil to become too wet (actually too airless) for roots to thrive – like most living things roots need oxygen, from the soil’s air spaces – or to be workable by gardeners.

As a measure of the importance of drainage, to avoid waterlogging farmers have used costly buried porous pipes to drain 6.9 million ha (26,640sq miles), out of the total 17.2 million ha of agricultural land in the UK. Gardeners are not farmers, but many gardens can benefit from improvements to their drainage.

Flooding (above) can drown plants and leave behind muddy sediment. Low-lying areas known as swales (left) that hold back runoff water can be planted with wet-tolerant species to help flood management.

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66 The Garden | October 2016

Factors influencing drainage

Walking on wet grass compacts the surface, reduces infiltration and can lead to bare patches.

Grass roots open up soils: spring feeding and overseeding thin areas by gently raking in more grass seed should enhance root growth and improve soil porosity. To remedy compaction, you can spike your lawn with a fork or a specialist tool (including powered hollow-tine

aerators), 10–20cm (4–8in) deep. Raking top-dressing (a mixture of sand and organic matter

such as leafmould or peat substitute) into the holes helps keep them open for longer.

Water collecting in the lowest part of the garden can cause waterlogging.

In winter, wet ‘sumps’ limit access and enjoyment, and summer waterlogging can be fatal to all but a limited selection of tolerant plants. In drier regions, areas that stay wet in winter can become parched in late summer, further limiting plants to those that can tolerate both extremes. Often the best approach is to work with the conditions, creating a swale, or a seasonal pond or bog garden.

Water runs off sloping ground, leading to dry soil on the slope but an excess of moisture at its foot.

Steep slopes without plenty of vegetation to retain precipitation are prone to runoff, especially in high rainfall areas, which can lead to soil erosion. While suitable planting is establishing, you can spread coconut-fibre

mats or nets to stabilise slopes, or terrace them into ‘steps’. Terraces can offer well-drained

planting areas where even plants that dislike water logging such as lavender can thrive.

Soils with high clay contents drain slowly and remain wet long after rainfall.

Clay soils are fertile, ‘restructuring’ annually as they shrink and crack in summer when plants extract moisture; although clay soils swell again in winter, they retain many cracks, enhancing drainage. If you want to grow vegetables and herbaceous species on clay soils, the best way is to use raised beds, 15cm (6in) or more high. These warm more quickly, allowing earlier sowing and planting.

Compacted turf

low-lying ground

Sloping ground

Dealing with clay soils

Collecting waterWater butts and tanks, and green roofs, can all intercept and hold runoff from roofs, releasing it more slowly. If more households had such green roofs and tanks, the risk of flash flooding, especially in urban areas, would be reduced.

Permeable pavingMaterials that allow water to soak through into the subsoil, such as gravel or pavers bedded in sand (rather than cement), can reduce standing water and slow runoff in wet periods.

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Soil type and a garden’s physical structure are key to how well it drains.

Installing drainage systemsIf a garden has serious drainage problems due to soil type or topography, a bespoke drainage system, installed by professionals, may be the best option, despite the cost and disruption caused at installation. Drains conduct water away into ditches, ‘soakaways’ (pits or trenches full of free-draining stones), or to nearby streams. Perforated drainage pipes, covered with about 15cm (6in) of gravel, are buried at parallel intervals, deep enough so they are not disturbed by cultivation. They are set on a slight slope (right), so water runs down them by gravity to a collecting pipe, outfall or soakaway.

Creating a swale or temporary, rain-fed pondIn gardens, swales are low-lying areas where excess water can accumulate and be safely stored until it can flow or seep away. In some cases they might be seasonal ponds, in others a grassy depression or, as here, an area of gravel and cobbles that takes the excess runoff from the house roof by simply diverting a downpipe.

improving your garden’s drainage

October 2016 | The Garden 67

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Improving soil structureAdding organic matter such as garden compost (left), leaf-mould or manure helps improve soil structure. It aggregates soil particles into clumps around which water can flow, and holds water inside and between clumps that plants can access.

Directing watercoursesPermanent or seasonal brooklets, rills and streams (right), if well designed, carry surface water away rapidly and look natural. For gardeners lucky enough to have watercourses running through their plot, they offer the opportunity to plant wet–soil specialists such as Iris sibirica and I. ensata, Caltha palustris (kingcups), astilbes, and ferns Matteuccia struthiopteris (shuttlecock fern) and Osmunda regalis (royal fern).

Disruptions from drainageInstalling drainage can be expensive and cause considerable disruption, and there is also the problem of where to direct the water. Byelaws usually prohibit home drainage systems feeding into sewers or storm-water drains, and discharging water into neighbouring properties might constitute a nuisance. Having the drains lead to a soakaway pit (above), full of rubble such as old bricks, avoids this.

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68 The Garden | October 2016

Improving drainage

living the water cycleRain is formed from water vapour that has evaporated primarily from the surface of the sea (the beginning of the ‘water cycle’). Over land, especially if the air is forced to rise over high ground, it cools and can hold less water vapour. The excess condenses, and forms clouds. More cooling causes droplets to form, which fall as rain (precipitation).

In summer, much moisture evaporates before it soaks into the soil, and water that does soak in is quickly taken up and transpired by plants – the combined losses are called ‘evapotranspiration’. Relatively little water is lost by evaporation from bare soil.

The water cycle in winterBetween October and March, evapo-transpiration losses are much lower, and the soil absorbs what is usually higher precipitation (rain and melted sleet, hail and snow) until all its pores are filled with water (called ‘field capacity’). After this, surplus water drains into porous subsoils, or runs off clay and other non-porous surfaces.

Groundwater (water held by porous rocks such as chalk and limestone) is replenished each winter by water soaking through porous soils. Groundwater feeds springs

and brooks, but much is also pumped to the surface for human use, especially in the

southeast. Rain, runoff, ‘grey’ water created by households and

irrigation water goes back to the sea via sewers and water-treatment works, or by streams and rivers, where the cycle can begin again.

Climatic variabilityDryness and waterlogging

vary like the weather from season to season and year to

year. In dry winters in the southeast, there may not be enough rain to restore soils to field capacity, and with reduced drainage flow, rivers will shrink, reservoirs

fail to fill and the level of groundwater can fall. This

causes the water table (the level where soil and underlying geology

is saturated) to fall, too. Trees may depend on a high water table, and can suffer in the following summer.

In wet winters, by contrast, soils can stay saturated for long periods. Although roots rarely suffer in wet winters (the soil is cold so they do not respire much), gardening activity is limited. If the soil remains wet as it warms up in spring, however, severe plant damage may result, as respiring roots lack oxygen so they literally drown. With spring and summer waterlogging, only especially tolerant plants such as willows will survive, unless improved drainage can remove water and allow air to enter the soil.

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Encouraging SUDSWith increases in built-up areas and changes in agriculture (reduced autumn ploughing, for example), rain reaches rivers more quickly and in larger volumes, potentially causing flooding. Rainfall events sufficient to wet the soil (more than 5mm / ¼in of precipitation) currently account for about half of annual precipitation but, as our climate changes, such events are expected to be more frequent and severe, and floods more common. Rapid runoff from impermeable areas can also wash pollutants into rivers.

Sustainable Urban Drainage Systems (SUDS) are water-management regimes designed to slow down or prevent the flow of runoff water from urban landscapes into low-lying areas and rivers. Many methods can be used, including replacing impermeable concrete and tarmac with porous paving, grassy areas and trees; and inserting low-lying areas, ditches, swales, ponds or lakes designed to hold runoff back until it can soak or trickle away. ‘Green’ roofs and walls also intercept water, which can be diverted into tanks or reservoirs to be saved for irrigation in dry spells.

Wetted surfaces allow evaporation, and so cool urban areas with the urban ‘heat island’ effect in summer: much of the rain falling on trees wets foliage and evaporates before it reaches the soil.

Case studies appear to suggest SUDS are effective, and cheaper to install than conventional drainage. They do use more land, but the land used often has good recreational value (albeit also needing more ongoing maintenance).

FURthER InFoRMAtIon✤ To check your flood risk, sign up for flood alerts

and find advice on minimising flood impacts, visit www.gov.uk/prepare-for-a-flood

✤ For more on groundwater, see: www.ground wateruk.org/Groundwater-Basics.aspx

For more information, search the RHS website for the following terms: ✤ ‘Drainage: installing’ and ‘Improving drainage’✤ ‘Waterlogging and flooding’✤ ‘Front gardens: permeable paving’✤ ‘Harvesting rainwater’

www.rhs.org.uk

RHS Garden Hyde Hall’s rain-fed irrigation

reservoir.

A dam across a planted swale established at London’s Olympic Park holds back runoff.

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Flash-flood running into

a storm drain.

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seamains water

percolation

precipitation

evaporation

condensation

reservoir

water treatment

works