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Wet & waterlogged soil: improving drainage & choosing plants

Val Bourne / 23 April 2014

Gardening expert Val Bourne advises on ways to improve soil drainage and recommends plants that thrive in damp conditions.

Astrantia claret
Astrantias with red flowers, like this Astrantia Claret, will tolerate a damp site with some shade

The British climate, widely influenced by the Atlantic, throws a lot of weather at us and climate change will almost certainly produce extremes.

Many British gardeners have seen their plants lying under water at the worst, or thoroughly sodden at the best. However, plants are resilient and will normally survive. If they do succumb, take an optimistic stance and see it as an opportunity to plant something else.

Visit our Home and Garden section for gardening guides, home improvement tips and much more.

How to improve soil drainage

You can improve your soil’s drainage by adding coarse grit and this is highly desirable on heavy, sticky soil - but counter productive on lighter soil. 

However adding well-rotted, home-made garden compost on a yearly basis will helps to aerate any soil, thereby aiding drainage.

If you haven’t got a compost heap, build one. The other thing is to light titillate the soil surface, using a small hand fork, without treading on the soil. There are long-handled versions available. This will break up the pan, and allow further rain to sink in better.

Find out how to cope with clay soil

Other ways of draining soil

The other great way of draining soil is to create a woody canopy using shrubs and trees. If the soil is very wet throughout the year consider the European dogwoods and willows.

The red-stemmed dogwood Cornus alba ‘Sibirica’ and the green-stemmed Cornus sericea ‘Flaviramea’ will both grow in very damp conditions. These produce pencil-thick stems that pop up every now again, so they are not suitable for the small garden.

The technique is to cut out the old stems every April to encourage new, colourful growth, but do allow the plant to get established for two or three years first. The cream and green leaved ‘Elegantissima’ is pretty throughout the year and still has red stems, although not as bright as ‘Sibirica’.

Willows are notoriously thirsty, but they can be pollarded back to the base every year - again, once established. The orange-stemmed Salix alba var. vitellina ‘Britzensis’ glows in winter light.

Find out about growing plants in raised beds

Develop a greener approach

Buying less manufactured garden products will cut global warming in two ways. Less energy will be expended in factories and less petrol will be spent either going to collect, or deliver sundries. Sustainability is highly desirable in all walks of life - including gardening.  Re-use your pots, consider making comfrey tea,  and recycle everything.

Find out how to make comfrey tea

Heavy rain and its effect on the soil structure

Heavy rain compacts and pans the soil surface. Keep off until it’s drier. Use a plank to stand on when digging and try to aerate the soil surface with a fork to prevent panning, but without setting foot on it.  

Excessive rainfall tends to wash out nutrients and you will need to feed your plants. Vitax Q4 is an potash-rich slow release fertiliser that’s excellent for roses, peonies and fruit trees, or you may prefer 6X, an easily applied powdered chicken/animal manure. All feeds are best applied to damp soil on still days when your plants are dry.

Yearly applications of garden compost will also feed your plants and then you will not need to purchase manufactured fertilizers which take a great deal of fossil fuel to produce. 

It will also save plant material from being transported by fossil fuel and either composted under heat for green waste, or put into landfill.

Find out how green manures can improve your soil

Making compost.

It’s all about decomposition by bacteria that multiply in moist, warm conditions. Water your compost heap in dry weather and cover completed heaps with cardboard. Cover all heaps, when warm, in the autumn before cooler weather arrives.

Find out how to make a compost heap.

Which plants will survive in wet conditions?

Most plants resent winter wet. It’s akin to a human being standing in a cold bath, but dormant plants that have retreated underground stand more chance of survival. All the prairie plants from North America will probably shrug this winter off and monardas, American asters, rudbeckias, sanguisorbas and heleniums will return unscathed.

Find out the best plants for clay soil.

Which plants will suffer in wet conditions?

Alpine plants need very sharp drainage during winter because they’ve evolved on bare mountainsides where snow and frost desiccate the ground.

These plants will struggle and many will die in very wet winters. If you love alpines, consider making a sunny scree slope, using rocks and gravel so the water can escape. Or plant on a slope, or create your own slope, to aid drainage. You could also invest in an alpine bulb frame, or glasshouse to keep your treasures dry.

Bulbous plants may also suffer, although those with large bulbs will cope far better than smaller bulbs. Fungal diseases are more of a problem, usually showing themselves as brown leaf tips or leaf margins. Also look out for botryitis, grey mould. If you see either of these remove the bulbs and examine them. Throw any infected bulbs away, or move them to a quarantine area and watch them carefully.  

Silver-leaved Mediterranean plants (sages, thymes, dianthus etc) may also suffer in wet conditions. These short-lived plants should be regularly propagated because older, leggier plants are more vulnerable. Cut all leggy Mediterranean plants back in late March: this will rejuvenate them.

Find out how to make an alpine scree garden

Which plants are adapted to wet conditions?

Foliage will explain a great deal about a plant. Go for shiny green foliage, usually an indicator of a moisture lover.  

12 easy perennials for wet conditions

Amsonia deserve a wider audience, bearing grey-blue small stars in early summer. They emerge late so mark them well

Astrantia - the red-flowered forms
Astrantias with red flowers will tolerate a damp site with some shade. Always deadhead them to prevent inferior seedlings. The paler forms though prefer a drier, sunnier position

Cardamines quinquifolia, an early-flowering Lady’s smock, is a short purple-flowered form that retreats underground by summer. Our native, Cardamine pratensis, is also a wet meadow plant.

The easiest forms of Erythroniums are the named hybrids, such as ‘White Beauty’, and they also need a shady setting.

Certain deciduous euphorbias, mainly from the Himalayas, like a damp corner. These include the may-flowering warm-orange Euphorbia griffithii and the acid-yellow summer-flowering Euphorbia sikkimensis.

Hardy geraniums
Suitable geraniums include Geranium maculatum, a North American woodlander, and Geranium phaeum,  a spring-flowering European woodlander with a tendency to self seed.

Iris sibirica
Most forms prefer damp soil and will thrive in moisture. Seek out the willowy, blue varieties bearing lots of small flowers, such as ‘Perry’s Blue’ and ‘Caesar’. These are tougher than the highly-bred shorter ones with large flowers.

The moisture lovers vary from the refined white shepherd’s crooks of Lysimachia clethroides to the yellow, rather invasive Lysimachia punctata, which is only suitable for a wilder garden.

The South African Kaffir Lilies like a high water table and they come through in very damp soil, flowering in autumn. A good tomato-red form of ‘Major’ is hard to beat.

The knotweeds have tiny tapering candles in August, but again make very large plants.

The bog primulas, or candelbra primulas will need a slope. The easiest, Primula japonica, has some good forms including the dark-red ‘Millar’s Crimson’.

Globe flowers thrive in wet meadows and the Trollius x cultorum hybrids, which come in cream, yellow and white, have great vigour. The cream ‘Alabaster’ and the bright-yellow ‘Superbus’ are both excellent.

Visit our gardening section for more great garden ideas

Chartered surveyor John Conlin advises a reader on how to install a soakaway

Reader question

We have a paved patio that borders and drains on to the lawn. After any meaningful rain the lawn becomes boggy and takes an age to dry out because the soil is mainly clay. How do I install a soakaway?

John Conlin's answer

Firstly you need to catch the water before it reaches the lawn. Do this by installing lengths of plastic drainage channel flush with edge of the paving. Because your soil is clay a traditional rubble filled hole will not be an adequate soakaway. Beg some plastic milk or beer bottle crates, wire them together to form a cube, wrap it in horticultural ground cover fabric and bury it in a pit, running a rainwater pipe from the middle of the crates to the outlet of the drainage channel.

The large empty space created by the crates will hold surges of water that will slowly be absorbed by the surrounding clay.

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The opinions expressed are those of the author and are not held by Saga unless specifically stated. The material is for general information only and does not constitute investment, tax, legal, medical or other form of advice. You should not rely on this information to make (or refrain from making) any decisions. Always obtain independent, professional advice for your own particular situation.