How to Plant a Rain Garden to Help Manage Flooding

Posted on by HedgesDirect

As our towns and cities grow, so do the impermeable surfaces they are constructed out of, and the natural water cycle is increasingly interrupted. The more roofs, roads, pavements, car parks, driveways, and patios we have, the less vegetation and open ground there is to intercept, absorb and filter stormwater (i.e., rain and snow melt). This results in what can be problematic levels of run-off, which either gathers somewhere inconvenient (hello, sodden patch in the lowest part of the garden!) or is channelled into drainage systems leading to rivers and lakes in the surrounding countryside.  

On this journey it gathers pace, being contaminated as it goes with the variety of pollutants you might expect – petrol, oil, litter, animal faeces, debris, dust, bacteria, chemicals, fertilisers etc. The surge of this contaminated water into our rivers and lakes causes untold damage, eroding natural habitats as well as poisoning and suffocating wildlife.  

So far so dismal… but all hope is not lost. There is an exciting and increasingly adopted approach to managing stormwater which is not only effective and beneficial to wildlife, but also attractive. And what’s more – it can be done on a variety of scales from large municipal landscaping to an individual, domestic garden. So, whether you’re thinking about the environmental impact of runoff from your property, or simply wanting to address a flood prone area of the garden, creating a rain garden may be just the solution for you.

What is a rain garden? 

Put simply, a rain garden is a planted shallow basin or depression which uses the natural properties of plants and soil to manage the quantity and quality of stormwater – a fantastically simple concept, designed to mimic the natural water cycle. Think of it as a living sponge, sustainably dealing with the runoff caused by a property.

Quantity 

Water runoff from buildings and hard landscaping is captured, soaking into the soil until it is saturated, at which point water begins to pool on the surface. The water in the soil is either taken up by plants (and then either lost through transpiration, or held in plant cells), or slowly released into the surrounding ground water table. As this process occurs, the water pooling on the surface either evaporates or gradually soaks into the soil as well.  

Quality 

Rain gardens are known to have purifying effects on water, removing up to 80-90% of contaminants. This is thought to be due to several processes:  

  • Settling: when the water pools on the surface, suspended solids and particles settle out.  
  • Filtration: dust and debris particles are left behind by the water as it moves through the soil and roots.  
  • Phytoremediation: some contaminants can be used by the plants as nutrients and are subsequently removed when growth is cut back and taken away each year.  
  • Adsorption: contaminants stick to the surface of roots, organic matter, and soil particles.  
  • Decomposition: other contaminants are broken down by the soil micro-organisms encouraged by the presence of plant roots. 

What are the benefits of rain gardens? 

  1. Absorb up to 30% more water than a lawn.  
  1. Keep rain out of the main drainage system, reducing the pressure on our countryside caused by harmful surges. 
  1. Help prevent localised erosion and flooding caused by a heavy downpour.  
  1. Naturally filter runoff before returning it to the water cycle. 
  1. Provide a habitat and food source for wildlife. (See our How to Create a Wildlife-Friendly Garden blog post for more ideas on this).  
  1. Low maintenance once established. 
  1. Can make beautiful garden features.  

What types of gardens can benefit from rain gardens?

A sustainable approach to managing runoff from our properties is something all of us ought to be thinking about, though is particularly relevant if any of the following apply:  

  • You live in a built-up area. 
  • Have lots of impermeable surfaces (tarmac, paving, sheds etc.). 
  • Regularly experience flooding somewhere on or around your property. 
  • Live in a region with high rainfall.

How to tell if a rain garden is suitable for your garden 

For a rain garden to function successfully, it needs to be sited on well-draining soil (rather than on a naturally waterlogged spot, as may initially seem most logical…!). To find out if your soil fits the bill, a recommended test is to dig a hole around 25cm deep and fill it with water. Once this has drained away, refill it, and it is the rate at which this second dose drains away that will give you an accurate picture of what you’re dealing with. 

The ideal soil for a rain garden is where the water drains between 1.25cm and 5cm per hour. Any less than this and the soil is unsuitable for a rain garden (you may find this on heavy clay or where the water table is high).  

When is the best time to plant a rain garden? 

The basic structure of a rain garden can be made at any time of year, providing the soil is dry enough to work on. For planting, early autumn or early spring will yield best results.  

How big should a rain garden be? 

To accommodate the typical UK rainfall, a rain garden should be 20% of the size of the impermeable surface (roof, patio etc.) from which it will be collecting water.  

This can be calculated by measuring the area then multiplying by 0.2.  

Using a standard shed of 3.2m x 5.97m (10’ x 20’) as an example:  

Total roof area: 3.2 x 5.97 = ​19.1㎡​ 

Size of rain garden required: 19.1 x 0.2 = ​3.8㎡​ 

In terms of depth, the faster draining the soil, the shallower the rain garden may be. For soil that drains at a rate of 5cm per hour, a depth of 15cm will work. The slower the rate of drainage, the greater the depth required.  

How to make a rain garden 

  1. Mark out the shape of your rain garden – any shape you like! Ovals or kidney shapes are common choices.  
  1. Remove existing plants and vegetation. 
  1. Dig out the soil to the required depth (see above), aiming for a saucer-like profile with a flat base.  
  1. Use the excavated soil to make a compacted lip around the rain garden, leaving a notch at the lowest part (if there is one) for overflow.   
  1. Create a gravel-filled channel from this notch, directing overflow to the conventional drainage system.  
  1. Dig in organic matter to improve the soil. 
  1. If you’re collecting runoff from a roof, install a downpipe to divert it to your rain garden. For patios, a rill or brick-lined channel can work well.   
  1. Gravel or pebbles at the point where water enters the rain garden can help prevent soil being washed away.  
  1. Plant the most wet-tolerant plants towards the mouth of the pipe. 
  1. Water the plants in well – check out our guide to watering here. 

You may also wish to use a water butt to harvest rainwater for use elsewhere in the garden. To combine this with a rain garden:   

  1. Have the downpipe feeding directly into the top of the water butt (rather than using a rain diverter kit).  
  1. Install a water overflow kit, to connect the water butt to the rain garden.  

Things to bear in mind when planting a rain garden

  • Check for any utility pipes that may run through your garden. 
  • Site away from any tree roots. 
  • Always make sure the rain garden is lower than wherever it is collecting water from.  
  • Choose a spot in full sun or partial shade, avoiding deep shade.  
  • Consider sowing an annual flower mix for the first two or three years, to fill the gaps between perennials as they establish (otherwise you may find yourself having to weed frequently!).  

The best rain garden plants for water absorption

Clearly, plant choice is key when it comes to planting up a rain garden. The nature of these features means there will be short periods of saturation and pooling, but the well-draining properties of the soil also mean that this is not a permanent state. Plants able to cope with this fluctuation are often those found naturally growing on the edges of lakes and rivers, or in low lying prairies and meadows which receive high levels of water for some of the year.  

For the wettest end of the rain garden where water first enters, good choices include:  

For the rest of the rain garden where moisture levels can vary more, success is likely to be had with:  

We hope you’ve found this introduction to rain gardens interesting and inspiring. With our increasingly erratic weather patterns, it is more important than ever to manage water in an intelligent way. As we are all starting to realise, the more our land management practices resemble nature, the more sustainable they often prove to be. Mother nature knows best!  

Before we go, we can’t miss the opportunity to point out the similarities between the arguments for rain gardens and hedging. Yes, we know we never stop harping on about this, but using plants rather than fence panels and walls has huge environmental benefits. Like rain gardens, hedges slow and reduce the flow of rainwater, anchor soil and prevent erosion, provide habitats and food for wildlife, and filter contaminants in the water. Visit our All Hedging page to browse sustainable solutions for your screening and boundary marking needs.

The best rain garden plants for wetter soils 

If you are looking for something more structural then our best hedging plants for wetter soils are: