Gardening On Wet Soils

Guest post by Sarah Gerhardt

Is the soil in your garden always wet and a lot of plants fail to thrive? If you notice water pooling in holes and plants suffering from fungal diseases you probably have wet soil in your garden.

Today’s blog post explores how to identify wet soil, understand the reasons behind it, and discover ways to help your garden flourish despite the challenges.

1.  How can you tell if you have wet soil?

It is common to have dry soil in summer and wet soil in winter which is often the case for clay soils. Truly wet soil is wet all year round. Reasons for wet soil are usually either that the soil is compacted or because of a high water table where the upper level of the groundwater is near the surface of the soil. In an area with lots of rainfall you may find your soil to be wet or damp most of the time. However, as long as the soil has good drainage and the water table is not too high you may not encounter many of the challenges of truly wet soil.

You can do a simple soil test to determine whether you have wet soil in your garden. Dig a straight-sided pit approximately 60cm deep and leave it covered overnight so no rain can get in. If water has collected in the pit, the ground has a high water table. If there is no water, fill the pit with water and cover it again for 24 hours. If there is still standing water then drainage is poor and the soil probably compacted.

2.  What can you do if you have wet soil?

If your soil test determined that you have poor drainage but not a high water table inspect the sides of the pit you dug. It may provide you with clues on how to proceed.

Plant roots should penetrate about 50 cm down the side of the pit. If the roots do not go down that far and appear restricted or you notice a hard layer that they do not penetrate then some cultivation may relieve some of the compaction. Dig in plenty of organic matter. It will help break up the clay pan into smaller crumbs. Aim for about 100 l of organic matter per square metre. That’s about one wheelbarrow per square metre. This should help loosen the soil and improve its drainage. Once the work is done avoid walking on the soil to compact it again. Clay soils compact easily. Work from a plank if you need to step on it to cultivate or plant.

If on inspection of your pit you notice the darker topsoil sitting on a thick layer of clay or rock, deep digging in organic matter is unlikely to help. Instead you should consider installing raised beds and filling them with well-drained, nutrient-rich compost. This allows you to garden independently from the conditions of the soil in your ground. If you conclude from your soil test that a high water table is the issue then your options are raised beds, installing an artificial drainage system or making a bog garden.

There are several options for installing a drainage system, for example simple ditches, soakaways and piped drains. All of these require a slope (natural or artificial) and an outlet for the water to go. Excess surface water can sometimes be eliminated by digging a simple open ditch (about 90 cm deep) with sloping sides along the lower end of a slope.

Soakaways (sometimes called French ditches) should also be dug along the lower end of a slope. They collect water away from the bed and allow it to infiltrate the soil gradually. Soakaways can be made from concrete or plastic crates lined with geotextile sheeting. Unlike open ditches soakaways are backfilled and compacted with gravel and soil.

Pipe drains consist of special perforated pipes that are placed about 40-60 cm deep and 3-6 m apart. The pipe trenches must follow a slope with a fall of 1:40 towards the outlet for drainage water. Installing a drainage system can be complex and labour intensive. Thorough research and professional support is recommended when embarking on such a project.

3.  Which plants thrive in wet soil?

If you were lucky to find that the reason for your wet soil was not a high water table and you were able to relieve the compaction it is still a good idea to pick plants that are tolerant of permanently moist soil conditions.

As long as the soil is not waterlogged or flooded and some oxygen is present then there is still a large number of plants that will thrive. In this situation it may be enough to take a look at plants for clay soils as these soils can have similar properties.

There are many shrubs, trees and herbaceous plants that cope well with wet soil that is not waterlogged. Shrubs include Hydrangea macrophylla and H. paniculata, elder (Sambucus nigra), and many species of Spiraea and Weigela. Alder (Alnus) species are very wet tolerant, for example Alnus glutinosa and Alnus incana. Lime (Tilia spp.) and mountain ash (Sorbus aucuparia) can also tolerate wet soil that has some oxygen. The same goes for herbaceous perennials such as Astilbe spp. and the Siberian flag (Iris sibirica).

It is much more difficult to find plants that will grow in waterlogged conditions caused by heavy compaction or a high water table. If a soil is waterlogged the water displaces air from the spaces between the soil particles. This causes the soil to become deficient in oxygen. Lack of oxygen makes plants susceptible to moulds and plant roots can drown in such conditions unless they are adapted to growing in waterlogged soils.

One option to combat waterlogging is the aforementioned raised beds which work great for herbaceous plants. Trees and shrubs can be protected from waterlogged soils by planting them in shallow raised mounts, similar to a raised bed.

If you do not want to go down the route of raised beds or installing drainage then your best option is to create a bog garden. A bog garden is a patch of slow-draining, waterlogged soil that mimics natural bog conditions. It is different from a pond which is a pool of standing water. Bog gardens are transitional habitats between dry and wet areas. They offer a range of different moisture conditions within a small area. Many of the plant species that can grow in these conditions would not thrive in drier areas. As a result they attract a host of different wildlife and can greatly increase the amount of different wildlife species present in the garden.

Creating a bog garden can be a great project for a frost-free day in winter. It can then be planted up in the spring. To make a bog garden you need to dig out the desired area to a depth of about 45 cm. Normally you would put down a liner and irrigation system but you will not need them in a naturally waterlogged area. Remove any weeds and large stones from the excavated soil. Mix it with compost and leaf mould and dig it back in. Bog plants like soil that is rich in organic matter. Once the soil has settled back to its natural level you are ready to plant your bog garden.

Bog garden plants will need a certain amount of drainage and air around their roots so may not work in very extreme cases of waterlogging. They love wet soil but do not enjoy swimming in water. Check their light requirements when choosing bog plants. Examples of popular bog plants include Hosta ssp., medowsweet (Filipendila ulmaria), Rodgersia pinnata, the tufted sedge (Carex elata) and ferns such as the royal fern (Osmunda regalis).

In the case of standing water, pond marginals are a better choice. They are plants that like to grow on the edge of a pond. For example pond marginals include the marsh marigold (Caltha palustris), the Japanese water iris (Iris ensata) and the water forget-me-not (Myosotis scorpioides). They thrive in mud and water up to 5 cm deep.

The bog arum (Calla palustris) and pickerel weed (Pontederia cordata) tolerate 5-15 cm of water and the arum lily (Zantedeschia aethiopica) and boog bean (Menyanthes trifoliata) thrive in even deeper water of 15-30 cm in depth. If you have standing water of 30 cm or more you can grow plants such as the great water plantain (Alisma plantago-aquatica) and the water hawthorn (Aponogeton distachyos). Your bog garden now qualifies as a pond.

The yellow iris (Iris pseudacorus) is a great allrounder. It works as a bog plant as well as a pond marginal, tolerating anything from bog plant conditions up to 15 cm of standing water. As we have seen, successful gardening on wet or moist soil starts with building healthy soil and choosing plants that thrive in that specific soil type.

About the author: Sarah Gerhardt is a gardener, linguist and punk musician based in Edinburgh. She was head gardener at the Dean Gardens, Edinburgh for 9 years and runs her own gardening business Gerhardt’s Garden Service. Find out more via her Linktree: https://linktr.ee/gerhardtsgardenservice