Areas in a garden can have wet soil due to things like rain, soil composition and the water table in your garden. If you have one of these areas, you might use certain plants well suited so they have a good chance of growing well.
What causes wet soil?
Wet soil is where the soil near the surface in your garden is generally quite wet for certain seasons. If it's particularly wet year-round it might be considered waterlogged which is different. Wet soil can be caused by a lot of rain, thick/compacted soil, the water table in your soil being quite shallow due to things like nearby water sources such as ponds/rivers, or a combination of these things.
Rain can have a big impact when an area gets a lot of rain and the layout of a garden allows a lot of that to reach the soil, due to no shade directly above for example. Sun also needs to be considered, where if there's both a lot of rain and sun it can dry out quickly; so, areas without shade directly above them and also not much sun can very wet soil. In the UK, generally the south gets more sun and the south east gets less rain, so areas towards the north may get wet soil.
Soil composition also affects this, where thicker/compacted soil may not allow water to pass through it easily. Clay soil that doesn't have much sand/stones for example can cause the surface to be wetter and for longer, especially after periods of a lot of rain. But clay soil for example can also get very dry in the summer in many conditions. The drainage of thick/compacted soil can be improved by adding things like compost (adding sand for example can also work but may be less effective).
The water table in your soil is also important. This is the depth in your soil where the empty spaces are filled with water rather than air. The depth where this change happens varies depending on the other things like rain/soil composition and distance from other water sources like ponds and rivers. It also varies over different times of the year.
You may also see the water table described as having different zones, where spaces in the unsaturated zone have air and in the saturated zone have water. There would be a depth in the saturated zone where it has water year-round, often called permanent saturation, which would go down to the impermeable rock below the soil. The area above this may only have spaces with water during colder/wetter seasons.
If the saturated zone is near the surface or your drainage is particularly bad, you might get soil that's waterlogged/wet year-round, especially if the permanent saturation depth is near the surface (where it's wet year-round). In these conditions plants suited for wet soil might not be able to grow, so you might use different plants suited to a bog.
All of these things contribute to the soil being generally either dry or wet. You might find your whole garden has wet soil or just a specific area that's maybe for example lower down or nearer a water source.
Planting in wet soil
You can use plants suited well to wet soil or make some changes if you want specific plants suited to dryer soil. Clay soil can have similar properties to wet soil, so you might also use plants suited to that. Some plants grow well in soil with some moisture but only if well drained, so also be careful of that.
If you use plants suited to the wet soil, there are a few things you can do to help new plants grow. Rain can decrease nutrients in soil, so you can add some compost to improve this (it can also increase drainage so just keep that in mind).
If you're keen on plants suited to dryer soil, you could use raised beds to get dryer soil higher up or improve drainage by adding things like compost, decreasing the soil compaction by breaking up the top soil and avoiding standing on the soil too much.
Also keep in mind a plant may prefer wet soil all year or just for certain seasons. If you use plants that need wet soil all year and your soil is fairly dry in the summer, they might not grow well. You may have to water them regularly yourself or use something like sprinklers to get them the required amount of water, but consider the additional water this might use vs more suitable plants. Generally, it may be best to use better suited plants.
If you have an area with visible water during parts of the year then your soil is obviously fairly wet; but if you're not sure, there you can work this out by digging a hole a few feet deep.
The depth of the roots of nearby plants will give you an idea of the soil composition, where thicker/compacted soil would have quite shallow roots. You can also see if you have a thin layer of softer topsoil and if you're reached solid rock, you'll know that the soil is quite shallow. If water accumulates in this overnight then your water table depth is quite near the surface. If you fill it with water and it doesn't empty after a day or so, your soil doesn't let water pass through very well.
Plants to use for wet soil
There's a wide range of plants that grow well in wet soil in the UK, so if you have wet areas there's a good selection of plants you could use. Below we give some that are generally suited to wet soil in the UK. Keep in mind specific plant varieties may be more or less suited.
Astilbe
Astilbes are hardy perennial flowers. They have fern/feather-like flowers and there's a range of varieties in varying sizes and several different colours, all of them are fairly hardy. They grow well in shaded areas, in particular in dappled shade like you might get near trees. They like soil with lots of moisture, but not waterlogged, so you could grow this in a bog garden for example or near a pond.
Caltha palustris
Caltha palustris are hardy perennial flowers. Often called marsh marigold, they grow well in very wet soil like you might get in a marsh/bog or beside streams/ponds. They can tolerate waterlogged soil for certain seasons. They can help attract wildlife and they tend to like full sun or partial/dappled shade, where you'd get more flowers with more sun.
Hosta
Hostas are hardy perennial flowers. There's a range of varieties, all of them are very hardy. They grow well in soil with moisture, preferring soil that retains a lot of water but isn't waterlogged. They grow well in partial shade, preferring not to get too hot in the summer, with certain varieties suited to full sun/shade. You can plant them in your garden or in a pot. If planted in a pot make sure it's fairly large and has some holes for drainage.
Weigela
Weigelas are deciduous shrubs. They come in a range of varieties/colours/sizes and can be fairly hardy. They can grow well in soil with some moisture but well-drained and not waterlogged. They can help attract wildlife and they prefer full sun/partial shade.