Should you buy a new house built on a flood plain? According to Lord Krebbs, the government’s statutory advisor on adapting to the effects of global warming, the answer is yes. That is, provided you have been made aware of the “attendant risks and the possible devastation”.
You can imagine the estate agent’s advert:
Last December 16,000 homes and businesses were hit by flooding and, according to the government’s committee on climate change, flooding in the UK is set to get more common.
And while there is plenty of advice on how to mitigate the effects of flood damage to your home – such as installing basement pumps, raising your electrics and fuse boxes, and tiling floors rather than fitting carpets – there is not a great deal of advice to gardeners about how to cope after a flood.
What happens to plants when it floods?
Plants need oxygen to live, and they take this from the soil through their roots. Waterlogging occurs when the soil is so wet there is not enough oxygen to enable the plant to respire. Waterlogging also allows other harmful gases to accumulate in the root zone.
Short term waterlogging will shut a plant down – its root tips will start to decompose and its growth will stop. If this situation continues then eventually the plant will die.
You don’t need to see water on the surface of the ground for waterlogging to cause problems.
What happens to lawns?
Grass is very resilient, but it too can be killed by waterlogging. In general terms, if the lawn is underwater for less than a week, it will usually recover. Any longer and your lawn will die and, if the flood water is 100mm deep or more, the weight of the water will have compacted the soil, making future drainage worse.
Damp conditions also allow competing algae, lichens, moss and liverworts to take hold.
Which plants tolerate waterlogged soil?
Few plants can survive waterlogging. The ones that can are plants whose natural habitat is damp – aquatic marginals like arum lillies, flag irises, marsh marigolds, forget-me-nots, rushes, loosestrife, penny royal and spearmint. Many trees and shrubs are also tolerant of wet conditions, such as willow, weigela, physocarpus and Midland hawthorn.
Isn’t it a waste of time and money to cultivate a garden which is prone to flooding?
Not at all. There are many ways to control the effects of flooding in your garden. One of the best and simplest ways is to plant in raised beds. These raise the root zone of your plants and also improve drainage.
If there is somewhere for surface water to go, install a ditch or soakaway drain at the lowest part of the garden.
Soil type is an important factor in drainage: sandy, gritty soils drain much faster than heavy clay soils. Long term, you can improve your soil’s drainage by altering its structure. This done by cultivation or, as it’s otherwise known, digging. Cultivation alleviates soil compaction and also gives you the opportunity to add grit to improve drainage.
Working with nature
We designed a garden recently (currently under construction) which slopes gently down to a river bank. As part of our scheme we included a deck area at the river edge. This is to be constructed using Ecodek composite deck material, which makes it flood-proof. A wooden deck in that situation would rot very quickly. The surrounding area is to be planted with aquatic marginals. The scheme at the lower end of the garden is designed to be as flood-resistant as possible.
And that’s the issue – you can’t make any garden totally flood-proof. But you can make it more flood-resistant. So whether you suffer unexpected flash flooding of the type we’ve seen in the last couple of weeks, or more seasonal flood plain related flooding, such as in the Somerset levels in 2014, there are things you can do to protect your garden against the “possible devastation” that flooding can cause.