Raised beds - frequently asked questions
What other materials can be used to build a raised bed?
Timber is the most popular material for constructing raised beds, but there are several other options:
Stone: both natural stone and stone prepared for wall construction can be used. Skilled labour is usually required for construction and footings are nearly always required (see 'Top tip' below).Generally the most expensive material.
Brick: strong, durable and can be used to create curved beds. Skills are needed for construction and footings are generally required (see 'Top tip' below). Engineering bricks are the most suitable as they are weather resistant. Domestic bricks are much cheaper but porous and therefore less durable.
Top Tip
Raised beds built with masonry walls (stone or bricks) higher than 20cm (8in) should be bonded with mortar and laid on concrete footings approximately 15cm (6in) deep and 30cm (1ft) wide. These footings should sit atop hardcore foundations 15cm (6in) deep.
If you plan to grow ericaceous (acid-loving) plants in a masonrybed, make sure to line it with recycled plastic to separate any lime in the mortar from plant roots.
Paving slabs: can be inserted on their side to create a reasonably deep and inexpensivebed with one layer. At least 15cm (6in) of slab needs to be buried in the ground for stability, leaving 45cm (18in) above soil level. As paving slabs often move over time, 30cm (1ft) deep concrete haunchings can be laid for extra stability, and metal plates fixed at each vertical joint.
Recycled plastic: several companies now offer 'sleepers' made from recycled plastic bottles. Different colours are available and some are finished to look like timber. Extremely durable and maintenance free, but fairly expensive and only available in certain sizes.
Can I build a raised bed on a hard surface?
Yes, but it's important to adjust the bed design and soil mixture to ensure good drainage and avoid problems with waterlogging. If constructing the bed from timber, drill plenty of holes in the sides of the bed to allow excess water to drain away. For beds made of stone or brick, leave gaps at 45cm (18in) intervals between the joints in the first two courses to act as drainage holes. Cover these on the inside of the bed with fine wire mesh, and heap gravel against the mesh when filling the bed, to prevent soil clogging them up.
Fill beds constructed on a hard surface with amixture of three parts organic matter, four parts sharp sand and seven parts topsoil. Be prepared to refresh and replace the soil regularly, as you would when repottinga large container, to maintain good drainage.
Will I have to water more?
Plants in raised beds tend to suffer more quickly and more severely from drought than plants growing in the garden soil, owing to improved drainage, so it's important to keep an eye on watering needs. As with watering containers, it's a good idea to regularly check the moisture level about a finger's depth below the soil surface to see if additional watering is needed, and be prepared to water more following warm and windy weather when plants lose water more quickly.
If you're finding the soil is often dry, or your plants are showing signs of stress, add a layer of mulch to the surface of the beds to help conserve moisture.
Will fungi grow on my raised bed?
Saprotrophicfungi can colonise and rot down untreated timber, which softens it over time, so you may need to undertake occassional repairs to keep beds structurally sound if you use this material. However, spongy wood rotted by fungi creates valuable habitats for garden wildlifeand retains water, helping to regulate the moisture content of the bed. If you're happy to undertake repairs every few years, then using untreated wood, whole logs or even fungi logs is a great way to create little wildlife havens in your garden, and may even help keep plant health problems away.
However, if you are struggling to control honey fungus in your garden, it may be best to steer clear of using timber to construct your raised beds and choose masonry or recycled plastic instead, in case the wood acts as an additional food source for the fungus.
