Wednesday 11 December 2019

Diarmuid Gavin: It's a washout - ways to protect your plot from flooding

Garden pond
Garden pond

Diarmuid Gavin

I'm gazing out at the field beyond my garden at pools of water which have appeared as if by magic. The relentless rain has fallen pretty consistently for weeks. The fields in my part of Wicklow are completely waterlogged and can't take anymore. On the news we see water at its most devastating, destroying cities such as Venice as well as villages built on flood plains in the UK. Communities in vulnerable parts of our island are praying that the flooding of the past does not return. It can seem trite to discuss gardens in this context, however gardeners are concerned.

A concerned gardener at a Q&A session in Cork recently asked me if a bed of shrubs he had planted in the spring will survive being submerged this autumn. Would the roots of his plants survive the waterlogged ground or should they be dug up and replanted in March?

I understand the concern about the plants as roots sitting in water can drown due to lack of air, however, on balance I think potentially more damage could occur by interfering. When plants are left standing in water for long periods, their roots can suffocate. Poisonous gaseous compounds can build up in waterlogged soils. Also photosynthesis, which is the process where sunlight is turned into energy by the plant, can stop.

However, our plants are likely to be dormant at this time of the year and this should make them more tolerant to temporary aquatic conditions. Some planting will suffer - if you've sown seeds in an area which has been waterlogged even for a few days, they may have washed away. But in general, don't worry too much and let nature take its course. Wait for the rain to stop and allow the soil to dry out. As long as the water recedes within a couple of days - which it does in most cases - most shrubs and trees will rebound with little or no damage. More sustained flooding will damage vegetable crops and possibly tender herbaceous plants.

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If your garden has been affected by flood water, be aware that this can contain pollutants so always use gloves and in the case of vegetable plots, it's not advisable to eat any of the crops that have been contaminated.

If the flooding is of the more normal temporary type, what do you do? Despite the fields beyond my house being sodden, my adjoining garden is relatively dry. That's because I installed some French drains, a very simple form of drainage that involves trenches dug out and filled with gravel or rock leading to a gravel soakaway pit. Other drainage methods often involve laying out a system of perforated pipes that redirects surface water and groundwater away from an area. In some cases, if you've a very large garden, it may even be worthwhile digging a pond to collect excess water.

Raised beds are always a good option, and planting trees and shrubs on raised mounds will help. Improve soil structure with the addition of organic material that will help drainage such as garden compost or well-rotted manure. You don't even have to dig this in - earthworms will happily do the work for you. You can also add silver (river) sand to help.

When installing paving such as patios, make sure the fall of the patio is away from the house and include some borders near the house to help absorb excess water. As we know, nature has a great way of levelling out, righting what is wrong, unseasonal or odd. Although many of our gardens are soaked now, when the rain stops some wind or natural drainage will take care of all those issues.

Top Tip

If you suffer from flooding with sea water, there will unfortunately be permanent damage to your soil structure. In this case, you may need to dig the soil out and replace it with fresh topsoil.

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