Few garden plants will survive prolonged periods in ground saturated with water. However, conditions can be improved using various techniques to promote drainage
Plants in saturated ground
Water builds up in poorly-drained soils leading to waterlogging. Short-lived flash floods after a downpour seldom harm most plants, but prolonged saturation leaves no air spaces in the soil and plant roots literally drown.
In cold, winter soils, roots respire little, so waterlogging is much less damaging than during warm seasons when roots respire freely and demand more oxygen. Few plants can survive this unless they have special roots that are adapted to acquire oxygen. Willows and marginal aquatic plants such as flag irises are examples of these.
Compacted soil lacks drainage channels. With water draining out more slowly than it enters, soil remains wet for extended periods after rain.
In some areas, water in the soil is permanently close to the surface. This is known as a high water table, and will make the root zone wet and airless for long periods; sometimes lasting all year.
Is my soil waterlogged?
To test the waterlogging status of your soil, dig a straight-sided pit about 60cm (2ft) deep; most plant roots grow within 50cm (20in) of the surface. Examine the sides of the pit and if roots do not penetrate deeply into the soil, or if a hard layer is identified, then deep digging with the addition of horticultural grit and organic matter will be needed to loosen soil to allow drainage.
If the darker topsoil rests on a thick layer of impervious clay or rock, drainage channels or raised beds may need to be considered.
Leave the pit covered overnight to exclude rain. If water collects in the excavation, you have a high water table. If there is none, flood the pit and leave covered as before for 24 hours. If water remains, then the drainage is poor.
Symptoms
- The first symptoms likely to be noticed are in the foliage. This includes yellowing or decay between the veins, soft tissue at the base or in the centre of the leaf.
- A root sample will show blue-black roots; a typical sign of waterlogging, which may be accompanied by a sour rotting smell.
- Roots may rot away completely, damaged roots will be blackened and the bark may peel away.
- The plant can look like it is short of water/wilt and is likely to be due to root decay.
- There may be dark areas along the midrib, and areas within the leaf go brown, especially in evergreen leaves.
- Shoots may die back as extremities of plants fail to acquire adequate moisture. Bark peels off shoots easily.
- Herbaceous plants may fail to sprout in spring, or leaves can open and then die.
- Plant growth is stunted.
Some plants suffer from a condition called oedema, commonly found on Camellia and Eucalyptus in waterlogged conditions with high levels of humidity. Symptoms include corky patches on the undersides of the leaves.
Cause
Waterlogging limits oxygen to roots and prevents carbon dioxide from diffusing away. The functioning of the roots is reduced or stopped and they die off, allowing the invasion of decay organisms. This has a subsequent effect on the visible parts of a plant. They are unable to obtain enough water and nutrients through the roots.
Caring for plants after waterlogging and plants tolerant of wet soils