As our weather becomes increasingly erratic and extreme, it is not just heatwaves breaking records. This year the UK had its sixth wettest July on record, and the wettest since 2009, with 70 per cent more rainfall than average, according to Met Office data. Northern Ireland experienced its wettest July since 1836. Six of the 10 wettest years in the UK since 1862 have occurred since 1998.
According to the Met Office, it will get wetter. It forecasts that, compared with 1990, by 2070 UK winters could be up to 4.5C warmer with 30 per cent more rain. Summers may be up to 6C warmer and 60 per cent drier, but downpours will be up to 20 per cent heavier in the summer and 25 per cent in the winter.
Significantly heavier rainfall is not good for our gardens. In addition to overwhelming our sewerage system, flooding erodes and damages the soil structure and drowns plants.
Designers are adapting in a variety of ways: reducing run-off by building rain gardens and stormwater planters, and reducing hard landscaping, including incorporating plantings in patios and terracing. On July 4 2021, the Edinburgh site of Royal Botanic Garden Edinburgh had a month’s worth of rain in less than an hour. Paths were eroded, compost was washed out of flower beds and plants were flattened.
All four RBGE gardens are having to adapt to the changing climate. All are in the process of replacing paths with porous materials, such as gravel and open aggregate, and have expanded their drainage systems to slow run-off emptying into drains. All are having to control species that thrive in the wetter conditions. For example, Logan Botanic Garden in Dumfries and Galloway is seeing the spread of Epilobium brunnescens and Dicksonia antarctica.
The Edinburgh site has built an experimental rain garden. A rain garden is a basin-shaped depression that sits below the level of its surroundings, designed to intercept and slow down rain through a mixture of improved soil and plantings that can cope with both waterlogging and drought. There are two aims: to combat persistent flooding, in particular on its Birch Lawn, and to gather scientific data for a growing number of organisations, including Scottish Water and local councils, that are seeking nature-based methods to prevent flooding.
“You couldn’t walk on our Birch Lawn at all in the winter months because it was so wet and over the past 10 to 15 years it was getting noticeably worse,” says David Knott, curator of living collections at RBGE. “Even by Scotland’s standards, we’re getting wetter.”
RBGE teamed up with the local Heriot-Watt University to consider the options. They chose a rain garden because its plantings make it more attractive than a soakaway, a hole in the ground filled with stone to allow water to filter through it into the ground. They also boost biodiversity by attracting pollinators and providing food and shelter for insects and birds, says Knott.
A rain garden typically absorbs 30 per cent more rain than a lawn. The RBGE rain garden measures 20m x 7m and has a depth of 450mm at its centre. Run-off pools in the middle then soaks slowly into soil that has been improved to enhance drainage. Plant roots take up some of the water and leaves intercept and slow rainfall. Plantings are a mixture of perennials and grasses that can cope with extreme conditions.
“In the past, gardeners would have just put in bog plants but that’s not the case any more because these plants have to sit there either saturated or dry, so we’re asking a lot of them,” says Kirsty Wilson, garden manager at RBGE and a presenter on BBC’s Beechgrove Garden.
At Edinburgh, the planting is layered, with species capable of surviving for prolonged periods in pooled water sitting at the centre and those preferring slightly drier conditions at the edge. Plantings such as Filipendula ulmaria, Cicerbita alpina, Ligularia fischeri and Aruncus gombalanus have been selected for the wettest areas. Species including Aquilegia formosa and Anthyllis vulneraria were picked for the drier edges.
The team improved the drainage of the site’s mixed, claylike soil by using a formula recommended by the Ciria SuDS (Sustainable Drainage System) manual used by water management professionals. The soil mix is 30 per cent existing soil, 45 per cent fine sand, 10 per cent fine gravel and 15 per cent compost.
The rain garden is a success.
“We were getting a rate of 20mm per hour of rain infiltration in the lawn before the rain garden. With it we’re getting 200mm to 300mm per hour, which is a substantial difference,” says David Kelly, associate professor, public health and environmental engineering group at Heriot-Watt. “Since we built the rain garden, we’ve had no flooding.”
The garden also shows visitors to RBGE what they can do at home.
“Whenever we’re working on it, people come up to ask what we’re doing. They always seem to know someone who has been affected by localised flooding, or have been themselves. It’s an effective way of communicating with people,” says Kelly.
Gloucestershire Wildlife Trust is creating urban rain gardens to combat flooding caused by concrete and tarmac run-off, including from homeowners converting greenery into driveways. Financial backers include the Environment Agency, Gloucester City Council, Severn Trent Water and insurer More Than. A rain garden was built to stop flooding in a paved area of Matson Baptist Church in Gloucester. Measuring 6m x 3m and 2m deep, downpipes direct roof water into the garden and any excess empties via an overflow pipe into a drain.
“But it flows much more slowly, so it isn’t gushing down there,” says Nicola Simpson, GWT’s head of engagement. The water is also cleaner by the time it reaches groundwater and rivers. This is because a rain garden’s plantings and soil mix filter toxic materials from run-off polluted by hard surfaces. According to the Wildfowl & Wetlands Trust, rain gardens remove up to 90 per cent of nutrients and chemicals and up to 80 per cent of sediments from run-off.
In a domestic setting, a rain garden should be about 20 per cent of the size of the run-off area, whether it is a roof, lawn or hard paving, says Adrian Thorne, horticultural adviser at RHS Wisley. An average-sized rain garden would take an afternoon to build and, once the plantings are established, it would be low-maintenance.
“Bombproof” plants recommended by the Royal Horticultural Society to tolerate temporarily wet soil as well as dry include Sambucus nigra cultivars, Cornus sanguinea “Midwinter Fire”, Hydrangea “Annabelle”, Crocosmia “Lucifer”, Iris sibirica, Calamagrostis brachytricha, Deschampsia cespitosa and Miscanthus sinensis cultivars.
An alternative for a garden with a high water table or limited space is a stormwater planter connected to a downpipe to collect run-off, says Thorne. A planter follows the same principles as a rain garden but on a smaller scale.
In 2022, GWT installed three rainwater planters behind Gloucester Rugby Club’s stadium to combat flooding on tarmac. Downpipes empty directly into the planters. Plantings include Salvia “Hot Lips” and Tulipa “Kingsblood” to match the club’s colours.
“We want to show people what they can do at their homes, workplaces and schools. It’s working, we’ve had feedback from people building their own rain gardens,” says Simpson.
Creating green roofs is another way to deal with run-off says Simon Rose, head of experience development at WWT. The trust was in the vanguard of creating rain gardens with its first project, designed by Neil Dunnett in 2010 at its London Wetland Centre. Any excess water not soaked up by a green roof empties into linked pools that spill over into plantings.
For Kelly, the more of us who get digging the better. “If a few people did this in their street or area, it would have a big impact on flooding,” he says. “As an individual it’s difficult to know what to do about climate change. Creating a rain garden is something you can do.”
In the US, the Philadelphia Water Department offers financial incentives to property owners to increase stormwater management, including by building rain gardens.
“That’s what we need our government to do over here,” says Wilson.
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