Soil can be wet because it is compacted, or because of a high water table, where the upper level of the ground water is near the soil surface. Clay soils are wet in winter and baked dry in summer. Unless you install drainage, you will need to work with your soil and choose plants adapted to wet conditions.
How do I know if I have wet soil?
Excavate a straight sided pit to about 60cm (2ft) deep. Leave the pit overnight, covered to exclude rain. If water collects in the pit you have high water table. If there is no water, fill the pit with water and leave covered as before for 24 hours. If water remains, drainage is poor.
Further information about the soil texture and structure can be gathered by examining the sides of the pit. Plant roots are mostly within the top 50cm (20in) of the soil surface. Examine the sides of your soil pit, if roots do not penetrate deeply into the soil, or if a hard layer is identified, then deep digging will be needed to loosen the soil to allow drainage. If the darker topsoil rests on a thick layer of impervious clay or rock deep digging won’t help, and raised beds may be necessary.
Gardening with wet soils
If your soil has poor drainage but not a high water table, and if inspection of your soil pit reveals soil that seems amenable to cultivation, then dig in plenty of organic matter.
Organic matter is beneficial to clay soils as it helps chemically to break up the clay into smaller crumbs. Adding sand, grit and gravel is seldom effective as it is not often practical to add enough to reduce the overwhelming influence of the clay content.
You will need one barrow load of organic matter per square metre of soil in order to make a significant difference to the soil structure and drainage.
It is still wise to select plants that are tolerant of wet soil conditions. Planting trees and shrubs on shallow raised mounds helps to protect them from waterlogging in wet soil. Many plants can be planted in raised beds to improve drainage.
Where there is a high water table, you can either decide to go with your soil type and plant a bog garden, or else consider installing artificial drainage systems.
Certain root rots and root diseases can be more common on wet soils, Phytophthora root rot is a common example.
Waterlogging can have other adverse effects on plant health and growth. In a changing climate, drought resistant Mediterranean are becoming more widely grown, but are intolerant of waterlogging. With wetter winters forecast under climate change the RHS has sponsored research into the effect of waterlogging on Mediterranean plants.