Flooding is a much more serious and unpredictable problem that we are increasingly having to deal with as our climate becomes more uncertain. The amount of damage caused by flood water depends on the intensity of the flood itself and on how long it lasts.
– Flash floods are common in summer when the ground is dry and hard and unable to absorb the water from a sudden heavy downpour. They are becoming more common as front gardens are paved over to make way for cars and the water has nowhere to go. This sort of flooding rarely lasts more than a few hours, and although your plants may not like it, they are unlikely to suffer any lasting harm.
– Floods which occur because the ground is already saturated may last considerably longer. The main danger to plants here is that the air pockets in the soil will fill with water. Annuals, Mediterranean plants and most herbs are particularly vulnerable if the flooding lasts more than a couple of days.
– The deeper the water the more harmful it is as more leaves are submerged, effectively cutting off the plant’s air supply.
– Fast-flowing water may undermine the roots of plants of all sizes. Wet soils lose the ability to hold together and may also be swept away by passing water.
– Moving flood water will probably deposit silt and other debris in your garden. The silt from the River Nile may have kept Egypt fertile but most of the stuff dumped on your plants by passing flood water will probably do much more harm than good.
If you know your garden, or part of it, floods regularly, it is worth taking a certain amount of evasive action to minimize any damage.
– Raised beds are a useful way of protecting tender plants if your flood water isn’t too deep. They must have solid sides so the soil does not wash away, and be robust enough to withstand any flowing water. Bricks or thick planks are usually the best options.
– Berms or long earth mounds can be used to divert water on a large scale. It is usually worth getting professional advice for the exact size and position as you may make matters worse by channelling the water and giving it extra strength. You also need to give careful consideration regarding where the diverted water goes; you will not be popular if you simply divert it into your neighbour’s garden.
– Dry stream beds can be used to accommodate extra water and carry it away. Again be careful of the siting so the water is carried away harmlessly.
– Water butts can hold a large amount of water that would otherwise drain into your garden. Try and position one at the base of every downpipe. These will reduce the risk of flooding and enable you to save the water for dry periods.
– Make the soil structure as good as possible so it drains well. Mulch every year to maintain this structure. In this way, the soil will hold together better even when wet. This will also help with alternating periods of wet and dry weather.
– Choose plants that are more tolerant of flooding.
If your garden is flooded there are a number of things you can do to minimize the damage and help it recover as quickly as possible.
– At first do nothing. You will cause more harm than good walking on flooded soil and until the water subsides there is no action you can take.
– Once the water has receded, put planks down to walk on so you do not compact the soil.
– Clear away any mud and debris as this will smother the plants. Rain may wash some of this away, but if it does not it is worth clearing the area with a hose. A little more water will do less harm than a thick layer of sludge.
– Do not walk on lawns until they have dried out. If you need to cross them, put down planks and remove them as soon as possible to allow air to reach the grass. Once it has dried, rake to remove any dead grass.
– Check any plants for structural damage and stake if necessary. With large trees it is worth asking a qualified tree surgeon to look at them as the plant may seem fine but it could be weakened to the point where it could fall at a later date.
– Be patient. Many plants may look dead but they will recover with time.