10 ways your garden can help reduce flash flooding

06 July 2016

With periods of drought followed by heavy rain now a common problem in many parts of the country, it's more important than ever to look at ways your garden can store water and help reduce the risk of flash floods.

With periods of drought followed by sudden downpours causing flash flooding across the country, the RHS is calling on British gardeners to look at what changes they can make to their garden to handle the increasing amounts of rain.

Water gardens have been making a splash at this year’s Hampton Court Palace Flower Show (5-10 July 2016), including Perennial’s Immerse and the Gold medal-winning HSBC WWT Working Wetland Garden.

Designed by Peterborough-based garden designer Jeni Cairns, the Working Water Garden allows water to run off the roof and through a water feature where plants and soil slow down the water before it ends up in a wildlife-friendly pond. During heavy rainfall the pond overflows into a mini wetland that releases the water slowly.

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Working Wetland Garden

The WWT Working Wetland Garden at RHS Hampton Court Palace Flower Show

Clever garden planning like this can be used in urban areas where paving and tarmac increase the amount of rainwater run-off by as much as 50%, but you don’t need to completely revamp your garden to make a difference. Here, Guy Barter, the RHS Chief Horticulturalist, shares his top tips for making your garden work with the changing climate.

Every garden helps

You might be thinking that one garden can’t make a difference, but if every garden on a street was optimised to retain rainwater the chance of a sudden flood would be greatly reduced, so try to do what you can and spread the word. "Some people don’t realise the massive role that gardens play in reducing the risk of flooding," says Barter. "Rainwater needs somewhere to go, so if it’s not soaked up or captured in some way, it’ll usually flow into street drains which can’t always cope with the thousands of extra litres in a storm so may result in flooding."

“Greening grey Britain is our call on the nation to plant up grey spaces, and by conserving rainwater in our gardens and green spaces we can help reduce flash flooding. On its own one garden with opportunities for rain to soak in or be temporarily held rather than flowing into the street might not make much difference but together many gardens optimised to retain rain water will greatly reduce sudden floods.”

Make sure you have gutters

“Rainwater can be collected from the roof of homes, garages, greenhouses and other garden structures as long as they have gutters and a drainpipe,” says Barter. In fact, even in dry areas, it is estimated that around 24,000 litres fall on the roof of an average home every year, so make sure you collect it.

Use water butts

“When watering the garden use water butts first so there is capacity to collect water in summer downpours,” says Barter. After all, there is much point in having a water butt if it is constantly full and overflowing.

Local councils, water companies and DIY stores are often good places to purchase basic affordable plastic water butts.

Find out how to install a water butt

Plant smartly

Plant plenty of tall vegetation. Hedges and trees catch water on the leaves from which much evaporates before it even reaches the soil, preventing the soil from becoming waterlogged.

After a period of drought the soil can become hard and baked, making it harder for sudden rainfall to penetrate the ground. This can cause the rain to run downhill and cause flooding. Trees can help dry the soil because they can gulp up heavy rain, particularly in clay soils where drought can cause fissures that soon fill with water.

Don’t leave bare patches

Avoid leaving bare patches of soil in your garden. Bare soil compacts under heavy rain and run-off occurs, often taking your valuable top soil with it.

If you have an area of garden that you don’t use then instead of leaving it bare cover with mulch or sow with a speedy green manure crop, such as mustard.

Find out about green manures

Even patios can collect rain

Don’t want to give your patio up? That’s fine, but it can still help collect water. “Containers, even hanging baskets, absorb water each time it rains,” says Barter. “Covering as much as is convenient of the patio with container plants will go far to reduce run-off.”

Even a few buckets dotted around the patio when you know wet weather is on the way can be useful – simply empty them into water butts, ponds or flower beds once it has passed.

Use porous paving

Where water could run off into the street use porous paving wherever possible. If using gravel, use plastic or concrete reinforcing cell units to keep the gravel in place and spread the load, as seen in the WWT Working Wetland Garden above.

Use your pond as a reservoir

Allow pond levels to fall in dry periods (making sure any fish in the pond still have enough water) so they can fill up again during sudden downpours.

Find out how to create a wildlife-friendly pond

Consider a green roof

A planted or green roof can also absorb water, and it isn’t as complicated to do as you might think as outbuildings can be used. "Sheds and other small structures are relatively cheap and practical  and easier to ‘green roof’ than bigger buildings," explains Barter. What’s more, they look fantastic and are good for wildlife as they provide a source of nectar and a place for insects to hibernate.

Pay attention to the weather forecast

“Monitor weather forecasts and avoid watering before rains,” advises Barter. “Rain won’t soak into saturated soils and is likely to run off, possibly in damaging ways. And you might be able to save a job if the rain is heavy enough to do your watering for you.”

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