Rain Gardens and Designing around Rainwater

The garden design process can often start with a Big Idea. This could be something as obvious as the architectural features of the house that inspires a style of garden or perhaps something within the customer’s home such as a picture which kick starts the creative flow. Sometimes a problem that needs solving by the design can drive the whole process.

When a customer wanted the rainwater drainage from an old extension of the house to be included in the design this gave us the opportunity to introduce green systems to deal with the water run off. This rainfall management is part of a Rain Garden system. In this case the down pipes were not directly connected to the surface water drainage system in the back yard and rain water would flow across the flagstones and onto the highway.

There are now planning rules for front gardens, namely that if the surface to be covered is more than five square metres, planning permission will be needed. This applies to the laying of traditional, impermeable driveways such as block paving etc.

If the water that falls onto your plot of land can be held up, the flow of water into the storm water drainage system and sewage systems is reduced. The torrential downpours and the consequences we see nowadays are made worse by all the loss of green space in our towns and cities. 

So what can we do in our own gardens and yards?

Green Roofs

A green roof is one intervention that has many advantages over a traditional roof.

Obviously the shed, garage, bike store or garden room has to be able to take the weight of the various layers that make up a planted green roof. The green layer protects the waterproofing from UV light, preventing its deterioration. A green roof can improve the insulation of the building, cooling the room in the summer. Sound insulation is also improved. In a city many green roofs have the potential to reduce the ‘heat island effect’, reduce pollution and dust and improve air quality. These are all energy saving measures.

The type of plants that can be grown on a roof are almost infinite but we often see small sedums because there is minimal depth of soil needed. The depth of soil or substrate, determines what can be grown. All the usual factors such as sun, shade, aspect rain fall etc contribute to the choice of plants available to us.

In a domestic setting what we see is the immediate impact on wildlife and birds. The green roof acts like a small extra garden space at a higher level. Of course the aesthetic value is also increased and in large public buildings there is potential to increase the amenity value to the occupants.

DIY versions of green roofs are certainly possible as long as the building has the necessary structural requirements to support the extra weight. Smaller versions such as bin stores can also make a feature out of rather unsightly objects.

green roof on shed

Downspout or Drainpipe

When following the rainwater in its progress to the drains it is usually channeled in a downspout. This too can be made more exciting and interesting. 

Japanese styled rain chains are an unusual, more aesthetic feature.

Drainpipes may lead to a Water butt and there are plenty of alternatives to the usual plastic versions.   

The energy of a torrent discharging from a pipe or rain chain can be dissipated with large paddle stones or pebbles. This is particularly useful if the down pipe or rain chain is directed into a rain garden planter. 

Rain Garden Planter

A rain garden planter is a waterproof container filled with layers of mulch, soil and sand which all filter the water before it passes to the next stage of rainwater management. Plants for rain planters have to be able to tolerate some wetness at the root zone as well as grow in more normal conditions. Here Iris pseudocorus is the main plant.

rainwater planter

Other suitable plants range from perennials such as Geraniums , Persicarias or Hemerocallis and grasses such as Deschampsia or Miscanthus. Some shrubs such as Cornus sanguinea would be suitable for a large planter. 

Water may discharge from a planter into the normal drains or it could be diverted further into a Rain Garden. The water could travel along a beautifully constructed rill or channel into a garden bed. 

Rain Garden

A Rain Garden, like the planter, is made up of layers of mulch, soil and a sandy drainage layer with a perforated drain at the lowest level. It is built as a shallow bowl but could be any shape or size. It can be planted with a range of plants and shrubs which again are able to tolerate water for a while before it drains away. There are many possible plants to chose from.

In the urban setting these  rain garden are occasionally found in car parks to deal with the large run off that can occur from the impervious surfaces surrounding them. The soil and vegetation helps clean up the run off before it enters our drainage systems. They should be routine for any new development and particularly in flood prone areas.

rain garden

For the DIY gardener and in the domestic situation, rain gardens should always be positioned well away from any buildings or walls to prevent the undermining of foundations. 

All these features demonstrate that a potential problem in a garden can be addressed in creative and beautiful ways to benefit the home owner as well as the local and wider environment.

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