Waterlogging, flooding and overwatering

Flooding in the home has a devastating effect on the building, its contents and the wellbeing of its owners. While nowhere near as heart-breaking, flooding and waterlogging in the garden can have a devastating effect on the plants.

Nearly all plants need a constant and regular supply of water for their roots to absorb and so ensure strong, healthy growth. But very few plants – apart from plants like willows, bog plants and those that naturally grow in water – will survive if they’re roots are growing in constantly and overly wet, flooded or waterlogged soil or compost for prolonged periods.

Roots need air to breath, and they get this air from tiny air pockets between the soil particles. During flooding, waterlogging and overwatering, these air pockets are filled with water, driving out the air. The roots literally suffocate, start to rot away and die.

As a result, the plant cannot absorb water from its growing medium and so the plant starts to wilt, showing more-or-less identical symptoms to drought and underwatering.

Waterlogging is always worse on very heavy clay soils, which haven’t been improved to help prevent waterlogging and to ensure they drain as quickly as possible. But flooding will affect all soils, including light sandy soils.

Overwatering is unlikely to happen in the garden – unless a hosepipe is left on for far too long – but is common in containers – especially those with no, few, small or blocked drainage holes. It is very common with houseplants – the commonest reason for houseplants to wither away and die – as many of us tend to overwater our houseplants – “killing them with kindness”!

New and recently plants are usually more susceptible to waterlogging and flooding than well-established ones, since they haven’t built up a fully developed root system.

Overwatering and irregular watering – cycles of drying out and flooding – can cause other, more specific problems in some plants. This includes blossom end rot in tomatoes.

• All plants can be affected
• Young plants are more vulnerable
• Plants in containers are vulnerable to overwatering
• Lawns

While it is impossible to do anything about the risks of flooding – except move house! – waterlogging to some extent can be prevented.

Root death will be limited providing the soil or compost dries out fairly quickly. So, try and alleviate the problem causing the waterlogging as soon as possible.

As it is always worse on very heavy clay soils [LINK: Plants For A Purpose Clay Soil], make sure such poor-draining soils are well dug over with lots of added bulky organic matter (such as well-rotted manure, compost, composted bark or tree and shrub planting compost) and even sharp sand and/or horticultural grit. Make sure any pans of compacted clay soil are thoroughly broken up, as these prevent water draining away. Good soil improvement is also important for lawns.

If this doesn’t cure the problem, dig a sump(s) at the lowest point in the garden. These should be about 60-90cm (2-3ft) square and as deep as need be. Then fill it with brick rubble, stones etc. Top off with gravel. You can then lay a planting membrane on top and cover over with soil if you need to plant there – but this is often best avoided, as the soil will drain very quickly. Instead stand planted containers on top.

If this doesn’t work, then you will probably have to lay land drains, which will need to be connected to the main sewerage system. This is something that is best done by a professional.

Keep the compost in containers evenly moist, without periods of overwatering followed by drying out. The majority of houseplants grow better if the top of the compost is allowed to dry out slightly (turns a lighter brown) between watering.

Short-term recovery solutions

Once the water dissipates, plants will start to revive, providing the soil or compost hasn’t been sodden for prolonged periods.

Don’t walk on waterlogged or flooded soil until it has dried out and becomes workable, to avoid compacting it and making root growth conditions worse. If you have to walk on the soil, use a plank of wood, board or invest in a portable path to help spread your weight and so help avoid compacting the soil.

Apply foliar feeds as soon as new growth starts and throughout the growing season to help encourage new root growth and improve leaf growth.

Feed all plants with a controlled-release fertiliser. These release nutrients into the soil as and when the plants need them to help build up plant strength.

After feeding, mulch the soil with a 5-10cm (2-4in) layer of organic matter, such as bark or compost. This will give some protection to damaged roots.

You may need to prune out badly damaged shoots, but wait until May or June at the earliest, as it can take plants this long – or longer – to recover.

Water plants thoroughly during dry spells following waterlogging, as they will be more susceptible to drought stress due to their reduced root system.

Lawns should be kept off until they have dried out. Then thoroughly spike all over with a garden fork or a hollow-tine aerator. Brushing sharp sand into the holes will further improve drainage. Feed with a lawn fertiliser to aid rapid recovery.

Edible crops are probably best discarded and not eaten if the garden has been flooded, as various pollutants and even raw sewage may have affected them.

• Plant fertilisers
• Mulches