Raised beds

Raised beds can be built against a wall or fence being accessible from one side or can be 'free-standing' giving access from all sides. The latter style will enable the usable area to be twice the size. However before any work begins it is emphasised that the utmost care and time must be taken to ensure that the beds are well constructed to avoid collapse when pressures are exerted by the internal soil.

Concrete Paving Slabs

One of the simplest and quickest ways of building a raised bed. These need to have about one third of the length buried into the ground to ensure the stability and safety of the finished bed. Remember, concrete paving slabs are very heavy and difficult to manoeuvre, so great care must be taken when handling the slabs from storage to their eventual resting place.

Make sure that each side of the slab buried in the ground has large grade hardcore thoroughly tamped down to provide a secure and firm fixture. The inside of the bed can initially be filled with hardcore but a layer of permeable material should be used on the top and up the sides before being filled with soil to prevent the loss of soil to the hardcore below. The depth of the topsoil should be approximately 400mm deep.

Pressure Treated Softwood

Use 50mm x 50mm x 1000mm upright stakes driven into the ground. The amount of stake above ground must be equal to that below in firm ground conditions, but in softer ground the ratio is more likely to be 40% above and 60% below.

Pressure treated softwood board, 150mm width x 25mm thickness can be fastened by galvanised nails or screws. As the boards are butted up to each other make sure that they are sealed with heavy-duty waterproof self-adhesive tape on the inside to prevent water loss.

Machined Timber Palisades

These can be obtained in round or square cross section from 75mm to 200mm in thickness and in lengths varying from 900mm to 3,600mm. The versatility of this product will allow curves to be included and bed height varied to give extra interest. At least half the height of the bed must be buried below ground level, which means that the maximum possible length of palisade required for the construction of a raised bed would be about 2000mm.

It will be necessary to fix these into the ground and secured with approximately 200mm layer of concrete on 200mm gravel base. The internal surface of the palisades need to be protected, by a waterproof membrane, from breakdown by soil borne chemicals and organisms.

Railway Sleepers

These form an excellent construction material and are relatively cheap. They are of a standard size of 250mm x 200mm x 2400mm. The easiest way to form a bed is to lay them horizontally, although this means that you are limited to either squares or rectangles, but useful if the garden is projecting out from an existing wall. They are difficult to cut to a particular size.

Remember, sleepers are very heavy and need care when handling.

Usually, when in place they will provide a firm structure. These are pressure-treated with a very heavy preservative which gives them a very long, useful lifespan. Sleepers can be cut in half and fixed vertically.

Make sure that they are sunk into the ground to about half their depth to provide a firm and secure fixing.

When construction of the bed is complete, a layer of polythene needs to be placed on the inner surface the prevent water loss through joins and also to prevent the toxic chemicals contained in the sleeper preservative getting into the soil.

Timber Interlocking Sections

These provide a very quick and easy assembly. They are very versatile and will adjust to any form or contour. They can be obtained in ready-cut fully treated interlocking sections. There is no need for nails or screws as they are secured by timber or galvanised steel joint pins. No lengthy preparation is necessary and they can be assembled directly onto a solid base if necessary. Should a change of design be required the structure can be dismantled easily and re-formed in another position or location.

Brick or Re-constituted Stone Blocks

Brick is a versatile material to use to construct beds. They are available in a range of colours and styles and very durable. They can be built to any shape or size to marry-in with existing space available. As a general rule, walls above a height of 600mm and length of 600mm need consideration given to lateral stability i.e. the forces acting on them from the soil contained in the bed.

It is useful to have drainage of accumulated soil water by means of 75mm diameter weep holes located in the base of the wall at 2000mm centres. The walls must be built on a suitable strong foundation of approximately 300mm wide and 150mm deep to prevent subsidence and cracking of the brickwork. It is not the purpose of this article to go into depth of construction.

However, once the brick wall has been built, it can be filled with a hardcore drainage layer, water permeable membrane and finally the growing medium.

Soil Media

When the retaining structure has been completed it needs to be filled with a suitable growing medium. This medium needs to be able to sustain the growth of all plants in the bed, particularly in that crucial period of establishment. In larger beds the problems of nutrition are not so crucial but in small beds that have a limited volume of growing medium, nutritional deficiencies will soon show up.

The bed can be filled with soil after the layers of hardcore and permeable membrane have been put in place. A mixture of 50% sterilised garden soil or loam, 50% peat based proprietary compost (by volume) and proportional amount of 12 month formulation slow release fertiliser should give good results in the first year.

The bed may need topping up again after 12 months, due to settlement, so it might be a good idea just to plant with a temporary crop of annuals in this period until the soil surface has stabilised, when more permanent species can be planted. After 12 months, supplementary top dressings of fertiliser or liquid feeds can be used.