How to Design and create a Rain garden in the UK.

This article will explain

  • What is a rain garden
  • what is a rain garden used for?
  • Where to site it.
  • Suitable plants to plant in a residential rain garden.
  • How it is different from a bog garden?
  • How to build/create a rain garden.
Description of a rain garden
Description of a rain garden

What is a Rain garden?

A rain garden is a low-lying, planted area of garden that catches and slowly releases rainwater.

A rain garden is a low-lying area of ground or dip in your garden built to catch and slowly release rainwater thereby, slowing down run-off from roofs and other hard impervious surfaces, like patios, concrete paths and tarmac parking areas and from higher ground. Lessening the effect of flash-flooding.

With selective planting it is a creative and attractive way of making the most of rainfall and helping the environment.

Rain gardens were first developed in the USA around twenty years ago. As a way for gardeners to contribute to reducing the growing problem of severe flash flooding in towns and cities.

The idea is that the garden soaks up the rainfall and the runoff coming from roofs, paths, pavements, roads, and all the other sealed surfaces we keep on installing. Preventing it overwhelming the drainage system. The idea quickly caught on and is also catching on in Australia and New Zealand.

 

We're becoming all too familiar with this issue in the UK and rain gardens are catching on here too.

 

What is the function of a Rain garden?

Rain gardens offer these important benefits to the environment

Apart from creating an interesting and colourful garden, rain gardens offer important benefits to the environment! They reduce water pollution and improve local water quality by absorbing and filtering storm-water runoff.

Runoff carries pollutants from sewers, our household rubbish, chemicals and pesticide and poses many health hazards for people and pets. In addition, storm water runoff can flow into ponds and streams, resulting in water pollution and erosion of the banks of streams.

 

This pollution can contaminate drinking water supplies, endangering humans and natural aquatic habitats, affecting fish and other aquatic life. So instead of polluting our space. The absorbed groundwater provides nutrients for plants and improves soil health. Reducing the need for artificial plant feed and fertilisers and importantly avoid the need for wasteful irrigation systems and hosepipe watering. Although there are various strategies to help mitigate runoff pollution, rain gardens offer a simple-to-build, cost-effective and low-maintenance approach to beautifying your lawn and garden while helping to minimize ecological pollution.

 

One more important benefit of planting rain gardens is guess what! A positive effect on global warming through the decrease in ambient air and water temperature, especially effective in urban areas containing too many impervious surfaces that absorb heat. It is the phenomenon known as the heat-island effect. One of the reasons London and other large cities are  two degrees hotter than the average Rain gardens also reduce energy consumption by decreasing the load on conventional stormwater infrastructure.

So in addition to looking good in your garden, helping reduce flash flooding, encouraging wildlife and biodiversity. Rain gardens are a type of green infrastructure system with a proven positive effect on global warming.

The best position and shape for a Rain garden

A rain garden will not work on heavy clay soil or where there is a high water table.

A rain garden will not work on waterlogged gardens built on heavy clay or where there is a high water table. The water will not drain away. So before starting to dig, try this simple test to see if your garden soil will drain quickly enough.

  • Dig a hole about 10in deep and a spade width wide in the middle of the proposed area and fill it with water.
  • Wait for it to empty then refill it and time how long it takes to drain.
  • If it drains within about 24 to 48 hours. Tick-off the ten points below and then you can begin creating a rain garden. 
  1. Check whether water, gas electricity services run through the proposed site.
  2. Avoid planning a rain garden in the vicinity of tree roots.
  3. For the rain garden to collect the optimum volume of rain, the longer side should face up the slope.
  4. Make sure the rain garden area is lower than where the water is being collected from.
  5. If you only have space for a smallish rain garden and plan to direct a lot of water from roofs and slopes and the soil is less well draining. Adding gravel or grit into the soil will help faster draining.
  6. Ensure that the lowest point of the rain garden allows overflow water to access a drain, or soak-away or other garden drainage element.
  7. Make sure the bottom of the rain garden is level.
  8. If after you have completed the rain garden you find soil washes away, add some more stones at inflow and outflow points.
  9. The nearest edge to the house should be at least 16ft away from the property.
  10. Site the rain garden in full sun or partial shade. Most plants will not thrive in full shade. If you don't have a choice there are plants that will tolerate shade listed here. Plants for shade.

How big does a rain garden need to be?

Depends on the soil drainage rate, the volume draining into the rain garden and available space.

 

This depends on the type of soil you have, the size of the area draining into the rain garden and the drainage rate of your soil. The space available will also govern the size. Even a small rain garden will control run off. It will just overflow more often.

Area. As a general rule the area of the rain garden should be approximately 20% of the roof surface, slope or paved area you intend to direct rain water from.

Clay soil drains slowly if at all, so a rain garden built on clay would need to be extra large and need a lot of grit or gravel mixed into the soil

Sandy soil drains fast. So a rain garden built on Sandy soil could be a lot smaller to be effective. Compost would need to be added to provide nutrients plants need.

Loamy soil retains water and drains fairly well so rain gardens built on loamy soil can be of average size.

How to work out the type of soil in your garden can be found here. What type of soil have I got in my garden?

 

 

Depth

On Sandy soil that drains well the rain garden would need to be 6in deep, with an extra 2-5in below the lowest point where excess water would leave.

On loamy soil a rain garden would need to be about 10in deep.

 

Digging out and shaping a Rain garden

Design the rain garden to be functional and attractive from all angles.

  • Dig out a saucer shape, with a flat base, checking the levels are correct
  • Use the surplus soil you dig out to make the Bund (earth barrier) around three sides to keep the water in the rain garden. The Bund should be well-compacted, around 1ft wide and 6in high.
  • If the area you are converting into a rain garden is on a slope, the top end will require more digging out then the lower end to get a level base. Use the soil from the top end to fill in the lower end to make the rain garden level
  • Fork organic matter. (Leaf mould, homemade compost or well-rotted manure) into the excavated soil to improve soil structure and then back-fill to the original level. At this stage you can mix in some grit or gravel to help drainage. 
  • Do not remove stones, stony soil drains quicker.
  • The soil should be not be compacted. Compacted soil will not drain. 

How water gets into the rain garden. 

Roof water. Fit an elbow' to the drainpipe leading from the roof gutter. If required dig a shallow channel filled with gravel leading to the rain garden.

Garden slope. At the point where the water enters the rain garden, cobbles or gravel will help prevent soil washing away

What is the difference between a rain garden and a bog garden?

A rain garden is different from a bog garden in the following important ways;

Rain gardens are simply slightly sunken areas where rain water can be collected.  The whole intention is that water is allowed to drain away into the soil. In dry periods  will dry out, and look no different from any other area of the garden. However, in wetter times, water may puddle temporarily on the surface.

Plants to be used in rain gardens must  be able to withstand some wetness, but be able to grow in normal conditions for most of the time.

A bog garden soil is permanently moist to create a habitat for special bog plants and creatures which thrive in boggy conditions. It should never dry-out. 

So the simple answer is; rain gardens can dry-out, whereas bog gardens never do.

Planting a rain garden in the UK.

Plants for planting in a rain garden in the UK.

 

Suitable plants for planting in a UK rain garden.

Rain gardens are planted with plants that can tolerate water-logging for up to 48 hours at a time and commonly include native wetland edge plants such as wildflowers, Sedges, Rushes, Ferns, shrubs and small trees. Deep rooting plants create additional channels for water to filter into the ground. The more drought-tolerant plants are planted towards the edges.

 

A big plus with rain-gardens is that it is about the house, the garden and the environment. So, you will need to put your gardeners and garden designer's hats on because this will only work with planting. You need the plants to soak up that rain water and help tackle climate change. However, even more than this, rain gardens with a lot of good planting are better for wildlife, and people too!

The plants take up nutrients and water that flow into the rain garden. Then, release water vapor back to the atmosphere and sustain Microbes that help break down organic compounds (including some pollutants) and remove nitrogen.

Planting tips,

  • Plant the more wet-tolerant plants near the rainwater inlet  and towards the base.
  • Use plants that attract bees, butterflies and other pollinators and have all year round interest. Plant in groups of three to seven plants per square yard.
  • Grass provides welcome cover for insects, especially over winter.
  • Lay the plants out, in their containers before actually planting them, stand back and move them around if necessary.
  • Take care to plant the plants at the same height they were in the containers.
  • Even though it is a rain garden, in dry weather you might need to water for the first two summers.

 

Maintenance of a rain garden.

Rain gardens using native plants are low-maintenance. During the first two years water in dry spells and remove weeds weekly to prevent invasive plants from overtaking the garden. After the native species in your garden have been established, they will grow and crowd out the weeds, reducing the need for weeding.  If you want to check out my basic gardening tips go to this page on my site

Suitable plants for a rain garden in the UK.

list of plants tolerant to some water in winter and dry spells in summer.

 

 This is a list of plants tolerant to sitting in water for at least a week in winter and then drying out in summer. It is worth checking other conditions such as tolerance of shade and frost. Also, space, planting large trees may overwhelm a small rain garden.   

 

Trees. Amelanchier 'Robin Hill’-Betula utilis var. -jacquemontii 'Silver Shadow' -Cratageus laevigata ‘Paul’s Scarlet' -Magnolia grandiflora

Malus (crab apple) 

Shrubs. Buddleja davidii ‘Nanho Purple’ -Cornus alba ‘Sibirica -Fuchsia magellanica ‘Riccartonii’ -Hydrangea arborescens ‘Annabelle

Physocarpus ‘Diablo-Rosa rugosa ‘Roseraie de l’Hay-Viburnum opulus ‘Roseum

Herbaceous Perennials. Alchemilla mollis-Astrantia ‘Ruby Wedding’-Geranium Rozanne = 'Gerwat'-Geum ‘Mrs J. Bradshaw’-Hemerocallis ‘Burning Daylight’-Hosta (Tardiana Group) ‘Halcyon’-Iris sibirica ‘Tropic Night’-Iris unguicularis-‘Robert’-Persicaria affinis ‘Darjeeling Red’-Phlox paniculata ‘White Admiral’-glow in the dark House number 4-Verbena.

Grasses. Calamagrostis brachytricha-Miscanthus sinensis ‘Kleine Fontäine’  

Annuals and short-lived perennials that tolerate wet and dry soils due to their capacity to self-seed:

Aquilegia vulgaris-Borago officinalis-Digitalis purpurea-Knautia macedonica-Verbena bonariensis 

Native wild flowers and plants.  If they have survived thousands of years of our British weather come rain and shine, they should be a safe bet.

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