Rain Gardens: A Low Maintenance Way To Manage Gardens And Water

Rain Gardens Provide a Garden Feature that Is Beautiful, Reduces Flooding, Improves Water Quality And Reduces Combined Sewer Overflow Incidents.

Rain is vital to plant growth.  In normal rain conditions it percolates in to the soil and is available to plant roots. But when the soil is already soaked the rain will run off, can erode soils, pollute drains and rivers and cause flooding. That’s where rain gardens come in.

As the planet warms we are seeing more extreme weather conditions. One result of global warming has been increased heavy storms which our ditches, streams and storm drains can no longer deal with. The result has been deep gullies eroded in arable fields, garden soils washed into drains and increased Combined Sewer Overflow (CSO) incidents where raw sewage is dumped in to rivers and out to sea.

CSO dumping of sewage has been in the news like never before and there is a need to reduce excess rainwater run-off from roads, farmland and gardens, wherever possible.

But how can we as individuals mitigate natural events such as excessive rain storms?

The principles used are variously referred to as bioretention gardens, bioswales, low impact development (LID) systems or simply rain gardens.

Excess Rain Storm Mitigation Strategies

When the soil can’t absorb any more water rain has no where to go but flow down hill. It’s simple physics and no one argues the fact. But we can limit the amount that rushes into drains and rivers.

In the following paragraphs I’ll deal with mitigation strategies that will help reduce the problems. I’ll start with farmland, then deal with publicly owned land such as the motorway network and local roads, then I’ll move into gardens and the use of rain gardens.

You may think that what farmers and councils do is irrelevant to what we as gardeners and householders need to do. But bear with me, there are similarities and we can learn from one another.

Mitigating Agricultural Flooding Issues

Where crops are permanent, for example grass, the crop itself slows the flow of water. Deep rooted plants also allow the excess rain to percolate more effectively. The problems come when the soil can absorb no more water. Then water starts to flow downhill.

Over the last year or two it’s been noticeable how fields that never experienced runoff now have seasonal streams running through them.

In the past many farms retained some rain runoff with meadows and wet areas in the low points on the farm. In winter these often flooded and were the home to species of birds that were more abundant in those days.

However over the years popular opinion was that these areas should be drained and brought into more intensive production. Governments paid farmers to drain and grow more food.

Today farmers are again being encouraged to use farmland to retain water. To act as a buffer and prevent excess rain from running into rivers. Ironically they are now being paid to of this!

Where farmland is arable erosion risks are higher than where cropping is permanent. Traditionally fields were ploughed and sown in spring so autumn and winter rains created fewer problems. However, overwintered crops were encouraged as they mature earlier and giver higher yield. Unfortunately they also mean there is an increased risk of storm events causing erosion.

Cross ploughing at risk points can help limit flooding and erosion and I’ve seen some evidence of it in action in East Devon and elsewhere. The idea is to run a single furrow across the contours of slopes. This slows down rain water that is running down the slope and will trap sediments and reduce erosion runoff.

It can’t cope with extreme events and very steep land but is useful in reducing some problems. The concept can be modified and used in gardens, but more of that later.

There are many other ways that farmers and land owners can mitigate flooding, from tree planting to encouraging beavers (topical here in Devon where we have wild beavers). But that’s enough detail for now.

Mitigating Highway and Public Area Flooding Issues

Modern roads often drain into ditches along side the roads and use buffer ponds or stormwater retention ponds. The idea is to temporarily retain stormwater and allow it to slowly flow into streams and rivers. This prevents huge quantities of water rushing into rivers and imitates the way beaver dams work!

The  retention ponds are built to limit exit flow to certain levels and are essential engineering structures on all modern roads. They also aid biodiversity by retaining some water at all times.

The contribution of motorway stormwater retention ponds to the biodiversity of aquatic macro-invertebrates was written by Le Viol, Mocq et al and is typical of the research carried out over the years. 

Again the concept makes sense if viewed from gardening perspective.  And if linked with agricultural practices are even more relevant.

Large Scale Town Flood Mitigations Schemes

In Sidmouth, in East Devon, we have just seen a flood mitigation scheme installed. In the path of seasonal floodwater huge catchment tanks have been installed underground. Above them there is an amphitheatre for public performances.

Flooding will be mitigated whilst a public area in parkland is retained.

Mitigating Garden Rain Runoff

I’ve dealt elsewhere with the management and retention of rainwater for garden irrigation and watering.

So I’ll move right on to Rain Gardens.

What Is a Rain Garden?

Rain gardens are designed to retain rain water in extreme events. They are temporary water retention features we can build into gardens to give the water time evaporate or to percolate into the soil.

Rain gardens slow water movement and lets rain slowly enter the natural systems. It prevents rain  rushing down the drains, into rivers and out via CSOs when stormwater increases the amount of sewage getting dumped.

The design of a water garden is simple.

Imagine a gently sloping garden and its house/bungalow. Excess rainwater from the properties roof is initially diverted into rain butts/tanks for later use. When the butts are full, the excess water is then allowed to flow down the slope. Left to its own devices it would then continue flowing and eventually enter the drains or enter the water catchment system. But in our imaginary garden we have engineered either a simple shallow depression in the ground or built a low retaining structure to retain the water.

Clearly we don’t want a huge dam in our imaginary garden. So, if we don’t dig a shallow depression, we have a low gentle inclined bank that retains just a few inches of water. It’s such a gentle bank that we can mow the area with no impedance. It appears like a gentle ripple in our lawn. Clearly to stop water flowing around it there is a need to shape it

But a small feature like this can retain hundreds of gallons of water that then gently soaks in to the soil over hours, days or weeks.

It might be that you actually want to make a feature of this rain garden. That’s fine. It can be planted with species that like a semi wet area. Sedges, rushes, and other moisture loving species. Alternatively, as the areas is built to quickly drain it can be planted with a much wider range of plants and appear to be just another herbaceous bed. Much depends on soil type and preferences.

Or you might decide to go the whole hog and turn the area into a more permanent pond. One that perhaps grows during wet times and contracts during dry times.

The Sidmouth project mentioned above isn’t technically a rain garden but retains many of the characteristics of one.

Other Water Retention Methods

If you don’t want a rain garden there are other ways to slow water movement. Trees and shrubs are ideal as they slow the water reaching the soil and also act as a physical barrier on slopes. They slow the movement of water as it tries to flow downhill.

Long grass has a similar action. Water flow is impeded and cannot flow downhill as fast. Neither of these methods are an ultimate solution, but both contribute a little to slowing water. Put the two together, add a rain garden retention feature and we are even closer stopping water flow.

Our roof can also play a part. Add a green living roof, complete with its plants, and water run-off is further delayed. Research is still ongoing regarding the best ways to build the perfect green roof. Here’s one green roof research paper from Url and Schiedt that discusses many features.