How To Master The Art Of Designing Urban Rain Gardens

Rain garden design is a true art. If you’re looking to build your very own urban rain garden, be it in your backyard, front yard or city garden, you can succeed if you follow the instructions in this article. From site selection to the best rain garden plants, to watering and maintenance – designing urban rain gardens properly is essential for success!

Designing Urban Rain Gardens: All you Need to Know

Urban rain garden design principles consider site selection, water flow, soil type and plant selection, to ensure the rain garden’s longevity and effectiveness. Proper construction and installation techniques are also vital to make sure the rain garden is durable and effective.

In this blog, I’m going to talk you through the best way to design your urban rain garden, so it’s not only functional, but also aesthetically pleasing! 

Ready to delve into how to master the art of designing rain gardens? Let’s go!

Table of Contents

Understanding Urban Rain Gardens

Before I go into further details of designing rain gardens, I will explain what rain gardens are, their many benefits, key components and environmental impact.

Purpose and Benefits of Rain Gardens

Rain gardens have a number of benefits, with the main one being to capture and treat rainwater runoff from impervious surfaces such as roofs and driveways.

They are shallow, landscaped depressions, that are ideally planted with native plants that can survive in both wet and dry conditions.

Rain gardens work really well in urban areas, as long as you are realistic about space and have a small garden or backyard you can put to good use!

Some of you may even consider hosting a rain garden on your rooftop, which requires extra consideration such as the weight your roof can handle.

You can also put that collected rainwater to good use in your home collecting it before you direct the rainwater to your rain garden, or devising irrigation techniques to water your plants and other parts of your garden.

Some other benefits of rain gardens include:

• Recharging groundwater.
• Keeping your rain garden plants hydrated.
• Providing habitat for wildlife.
• And last but definitely not least making your garden look beautiful!

Key Components of Rain Gardens

Rain gardens consist of several key components, including:

• Inflow: this is the area where stormwater enters the rain garden.
• Ponding area: the shallow depression where stormwater is temporarily held.
• Soil mix: a well-draining soil mix ensures infiltration and plant growth.
• Vegetation: native plants that can tolerate both wet and dry conditions.
• Overflow: the outlet where excess stormwater leaves the rain garden.

Designing Urban Rain Gardens: Key Design Principles

When designing urban rain gardens, there are several key principles to keep in mind to ensure your garden functions well.

Site Selection

The first major point to keep in mind is selecting the right location for your rain garden.

Remember that:

• The site should be in a low-lying area where water naturally flows when there’s heavy rain.
• The area should be at least 10 feet away from any building foundation or deep root systems.
• The site should not be located over any underground utilities or septic systems.

Size and Shape

Your rain garden size and shape will depend on the amount of rainfall your area tends to receive. On average, it should be around 10% of the size of the impervious surface area.

• A good rule of thumb is to design the rain garden to capture the first one inch of rainfall.
• The shape of your garden should be elongated and follow the natural contours of the land to encourage water to flow through it.
• Your garden should be at least six inches deep and have a slope of around 10%.

Soil and Drainage Considerations

Your rain garden needs the right soil to thrive. The best soil for your garden is a well-draining mixture of sand, compost, and topsoil.

But, sometimes the soil type in our outdoor space makes it difficult to design rain gardens.

For example, if you live in an area where you have clay soil, you may have some problems with your rain garden as your soil won’t drain properly.

To further encourage infiltration and prevent erosion, a two-to-three-inch layer of mulch can also be placed on top of the soil.

Designing Urban Rain Gardens: Plant Selection and Biodiversity

Plant selection and biodiversity are crucial factors to consider for your rain garden. After all, if you select plants that work well, you can improve your garden’s performance and make it more aesthetically pleasing.

Let’s check some key considerations for plant selection and biodiversity in urban rain gardens:

Native Plant Advantages

Native plants have several advantages in urban rain gardens because they are adapted to the local climate and soil conditions, making them more resilient to environmental stresses.

They also provide habitat for local wildlife, such as birds and pollinators.

Native plants tend to require less maintenance and water than non-native plants, which can save time and resources.

Here are some of the plants that suit the UK and are good in wet and dry conditions:

1. Bulrushes: these perennial, grass-like plants belong to the Scirpus genus in the sedge family and thrive in wet soil.
2. Coneflowers: with their vibrant colours and drought tolerance, coneflowers flourish in rich soil but adapt well to various soil types, making them a good choice for rain gardens.
3. Daylilies: with prolonged blooms and trumpet-shaped flowers, daylilies are an excellent addition to rain gardens.
4. Switchgrass: a versatile grass variety, switchgrass produces tall, airy flower spikes and can thrive in rain gardens with a range of soil types.

Plant Arrangement

Plant arrangement is another major consideration when designing an urban rain garden. The plants should be arranged in a way that maximises their performance and minimises maintenance.

For example, taller plants should be placed at the back of the garden, while shorter plants should be placed at the front.

This arrangement will ensure that all plants receive adequate sunlight and water. Grouping plants with similar water requirements can also help to reduce water usage and make it easier to look after your rain garden.

Seasonal Considerations

It’s important to choose plants based on their ability to thrive in different seasons.

For example, spring-blooming plants can add colour and interest to the garden during the early growing season, while autumn-blooming plants can provide late-season colour and support pollinators.

Additionally, evergreen plants can provide year-round interest and help stabilise the soil.

You may also need to pay extra attention to plants’ ability to thrive in heatwaves, even if you live in a temperate climate due to unpredictable weather conditions.

And, in urban spaces, plants may need to be especially resilient because of all of the pollution in the air.

Designing Urban Rain Gardens: Water Flow and Management

In this section, I’ll delve into inflow and outflow design, overflow strategies, irrigation and maintenance of urban rain gardens.

Inflow and Outflow Design

The inflow and outflow design of urban rain gardens is critical to their effectiveness.

The garden must be designed to receive water from the surrounding area, typically via a downspout or other drainage area.

The inflow area should be designed to slow down the water flow to prevent erosion and allow the water to percolate into the soil.

The outflow design is equally important. The garden should be designed to allow water to drain slowly, so the water can infiltrate the soil.

Overflow Strategies

Rain gardens are designed to hold water, but when there’s a serious downpour, they can overflow. To prevent flooding and damage to the garden, you should think carefully about overflow strategies.

One common strategy is to design the rain garden with an overflow pipe that directs excess water to a nearby storm drain or other drainage system.

Another option is to design an overflow area, that’s an area adjacent to the garden that can accommodate excess water.

Irrigation

Rain gardens are designed to be self-sustaining, but they may need additional irrigation during dry periods, which are becoming only too frequent!

Ideally, your garden should be designed to drain within four hours of a one-inch rainfall to protect the health of the plants while absorbing the water.

Designing Urban Rain Gardens: Construction and Installation

So, you’ve considered site selection, plant selection and water flow and management. Now you’re ready to install your rain garden.

Preparation and Excavation

First you need to mark out your area to prepare for the installation. The depth of the excavation will depend on the size of the rain garden and the type of soil in your area.

But, generally you’ll want to stick to the minimum six-inch design depth with a 10% gentle slope as mentioned earlier.

Materials and Layers

Next, your rain garden needs to be lined with a geotextile fabric to prevent soil erosion and to keep the soil separate from the drainage layer.

After the fabric is in place, a layer of coarse gravel or crushed stone can be added to the bottom of the rain garden to improve drainage.

Next, you can add a layer of sand or loamy soil on top of the gravel layer. This will create a suitable growing medium for the plants.

Also, remember that layer of mulch. You can add it on top of the soil to help keep retain moisture and suppress weed growth.

Edging and Aesthetics

Finally, edging can be placed around the perimetre of the rain garden to stop erosion and give the garden a finished look.

Your edging can be made from a variety of materials, including stone, brick, or plastic.

After all, it’s not all about functionality, you’ll also want your rain garden to look visually appealing!

Designing Urban Rain Gardens: Policy and Regulation

When designing a rain garden, remember that you’ll have to follow local guidelines. These rules exist for a reason and you don’t want to get into trouble if you fail to meet them.

Local Guidelines

In the UK, the government has introduced a range of policies and strategies aimed at promoting sustainable urban drainage systems (SuDS), including rain gardens.

Local councils may also have guidelines along with compliance and enforcement measures in place to ensure that rain gardens are designed and constructed to the required standards, so make sure you check them carefully!

Designing Urban Rain Gardens: Monitoring and Maintenance

Rain gardens should be regularly inspected. This includes checking for signs of erosion, sediment build-up, and plant health.

It’s also really important to check your rain gardens as the seasons change, so you can adapt to the changing weather by extra watering and pruning.

Rain gardens also require long-term care, such as replacing plants as necessary, removing accumulated sediment, and replenishing mulch.

Designing Urban Rain Gardens: Final Thoughts

To build an effective rain garden you will need to rely on careful planning and adherence to key design principles.

Selecting the right site, considering size and shape, soil composition and plant diversity are fundamental aspects of successful rain gardens.

If you live in a city and only have a small space to work with, you’ll need to take extra care when it comes to site selection because you may be closer to building foundations.

You’ll also need to take more caution when choosing your plants to ensure they’ll thrive in urban centres.

But, with a bit of time and patience and by following best practices, you can construct your rain garden up so it works as a beautiful, eco-friendly and sustainable solution to your stormwater management process.