Drainage for raised beds – Do raised beds need drainage?

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Raised beds are an excellent way to grow plants in a deep and fertile soil.

Growing in raised planters provides you more control over a number of different factors. One of these is drainage, raised beds by their very nature isolate a particular volume of soil.

Like pots, this can lead to greater volatility in seasonal ground moisture. Consequently raised bed drainage is a topic which deserves some attention for successful growing. In this article I will highlight some important factors to consider about the drainage of raised beds.

​I will also summarise some common raised bed drainage problems and solutions. 

 Do raised beds need drainage?

​All mediums intended for growing plants require a good level of drainage including raised beds. However, the level of drainage will depend upon your soil type, bed height, volume and position.

All raised beds will require drainage holes to allow excess rainfall to drain away. The size and spacing of these should reflect the size and volume of your raised bed. For large beds a 40mm diameter hole should be placed about every metre along the base.

If your raised bed is not in contact with underlying soil you will require extra drainage holes along the bottom. It is also vital that these drainage holes do not become silted and blocked.

​To avoid this you will need to back the holes with wire mesh and filtration membrane. 
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 Well drained soil for raised beds 

If you want good drainage inside your raised beds then you will need to focus on your soil. The best soil for raised beds is generally a clay based loam.

Such a loam consists of a combination of clays, fine sands and silts. The clays help to retain moisture and nutrients while the sands and silts increase drainage. However, how much drainage your raised beds need will depend upon your specific situation.

For example, if your beds soil meets the underlying ground it will require less drainage. Some raised beds are installed with solid bases which can lead to water logging. In such cases drainage holes or ‘weep holes’ should be added to your raised bed.

How much drainage you include within your raised bed will also depend upon your existing soil. You may already have very well drained soil that dries out in summer.

​If so, your raised beds will require less drainage, if you have boggy soil you will need more drainage. 

​Drainage for brick and concrete raised beds

Brick and concrete raised beds are extremely popular as they are robust and durable. These raised beds are easy to waterproof and style with a broad variety of finishes.

​The great advantage of building raised beds with masonry is it doesn’t degrade like timber raised beds.  However brick and concrete raised beds can be more difficult to drain especially if they are waterproofed. 
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In such cases you should install weep holes into the lower sections of the bed. These can be added as you build the beds by laying around sections of 50mm plastic pipe. 

​Alternatively circular holes can be drilled out by using a circular diamond coring bit. Each hole should include a filtration net to prevent it from becoming blocked. This normally includes some wire mesh and a layer of filtration fabric. 

​Drainage for wooden raised beds

​Good drainage for wooden raised beds is somewhat more important due to their construction.

Firstly, even pressure treated timber will eventually decompose leading to failure. Even very well waterproofed timber beds with plastic and bitumen can succumb to water damage.

​Normally dampness around screw fixings leads to corrosion and eventually failure. On the plus side timber will shrink and expand depending upon its internal moisture content. 
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This can create thin crevices and drainage gaps within the bed to improve drainage. Such shrinkage and expansion can easily be seen within the joins of timber sleeper raised beds.

As timber beds will naturally create drainage gaps there is less need for substantial weep holes. However the more plastic and paint waterproofing you apply inside the bed the more drainage you should add.

​All raised beds should have at least some drainage holes for excess water to escape. 

​Gravel bases for raised beds

​If you have particular concerns about water logging inside a raised bed you can add a gravel base. This involves spreading a 100mm layer of free draining gravel to the base of the bed.

The gravel layer should then be capped completely with a layer of landscape filtration fabric. This will help prevent the gravel from silting over time.

​The gravels will help to draw out excess water from saturated soil in times of heavy rain. 

Thank you for reading our article about raised bed drainage. If you require raised bed services including concrete or sleeper raised bed construction and drainage do not hesitate to contact us. 

​Based in Amersham we undertake a broad range of landscaping services throughout Buckinghamshire. 

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