Constructing a Raised Bed for your Garden

Constructing a Raised Bed

The same construction principles as for building a rock garden apply. The corner ends of a raised bed can be squared off or curved. Squared ends, interlocked at the sides like brickwork, are stronger but perhaps less smooth and polished looking. Curved ends look much better and gentle sweeps are very much easier to construct.

raised beds

As the work proceeds, inspect the site from different angles – not after laying every stone, but often enough to make corrections simple. This applies to a flat or sloping site.

Building Materials

The simplest construction is made from railway sleepers (beware of soft tar in the summer) sunk about 3-5cm (1-2 inch) below the ground level and leaning gently back towards the site, with the broadest surface to the ground (see figure 20). Additional layers can be added to the base using the bricklaying technique of overlapping the joints.

Bricks and artificial stone blocks may be laid the same way as sleepers but the degree of leaning needs to be greater to resist the outward pressure of the soil onto their individually smaller surface areas. The height to which such a bed can be raised is limited – because of that pressure – to a maximum of five convexly sloping layers of bricks or blocks. This shape is stronger than any other.

Another possible building material is limestone. Tufa stone, an expensive form of limestone, is unusual in that pieces can be laid to form a surface almost completely composed of stone, with a number of small areas of exposed soil for planting. It looks glaringly white when purchased, yet within two years, it takes on first a pink tinge, then becomes grey tinged with pink, making it look old and weathered in a very short time. It is particularly effective on awkward sloping sites in small gardens. Since it is a relatively light material, use pieces as large as possible placed on their edges with a slight tilt towards the slope. They may be fitted together to form any height up to 1-1/2 metres (4-1/2 ft). Because of the nature of the stone, do not try to build it further into the slope than can be reached from any natural ground level. It will not take kindly to being trodden on regularly. Read details for planting in tufa stone. Avoid planting any thick, fleshy rooted plants in the stone itself; these will do very much better if planted in the soil pockets between.

raised gardens figure 20

raised gardens figure 22

The stepping stones for any of the sites must be carefully considered. Railway sleepers will make a very stable structure, but a bed made from bricks, blocks and small stones if more than three layers high, will have stability problems. Every step made on the site will increase the outward pressure and to reduce that as much as possible, choose stones weighing not less than 11 kg (25 lb). Their surface areas should be large enough to accommodate two feet on one stone at a time and they should be close enough together to prevent you from having to leap when moving from one stone to another!

Stone-built raised beds are made by laying one stone upon another, starting just below ground level. There is no keystone so they may be built in any sequence, to left, right or upwards, to a predetermined level. Flat stones are easier to lay than rounded ones, but it is worthwhile making trial constructions to discover the possibilities of your own materials. If you have any smaller stones do not use them for the base or the top; these should be held in place by the larger stones. Short, rounded stones will dislodge easily, even when wedged between larger ones, as a trial run will bear out. The only real limit to the size of stone you can use is the size of the project itself.

Difficult joins between the stones can be overcome by placing stone wedges between the soil and the wall, but this should not be done too often or there will be a bulk of stone and not much room for the plants. An alternative solution is to use the spaces for planting.

Walls over 2 metres (6 ft) long and over 45cm (18inch) high can be strengthened by building buttresses. To do this, simply turn some of the stones at approximately 1 metre (3 ft) intervals along the length of the wall and at varying heights, so that they lie in an opposite plane to the line of the building. With soil above and below these buttresses, some of the pressure will be eased off the wall.

The height of the completed bed must be the same to within 5cm (2 inches) throughout its length, to keep the soil within the site and not lose any through gaps on the top layer. Thus the soil is being positioned whilst building is done. This strengthens the walls and enables you to firm round the buttresses and to plant between the retaining materials. Planning and execution of planting should follow the same principles as for the rock garden.

I have left the use of concrete until last because as a material it is the exception to the rule. There are no layers, and no holes to plant in; instead, obtaining a natural effect relies entirely upon trailing and mat plants falling over the surface to reduce its harsh appearance and to break up its flatness. I would never recommend building a concrete wall specifically for this purpose, only utilizing an existing one, simply because other materials are so much better. But never use a wall that is cracked and broken, or made up of a hotch-potch of bits and pieces, because walls of this kind harbour weeds including perennials and make maintenance a drudgery. It is much better to break the wall down and start from scratch, using one of the other materials.

The drainage at the base of a concrete wall will have to be good because there is no way out through the sides. The soil should, ideally, be entirely renewed to improve further the drainage quality of soil for the alpines. There will probably be concrete foundations below ground level, often thicker than the walls they support; if the soil is clay it is best to fill the base with rubble to ensure good drainage below these foundations.

Sloping Sites

Building a raised bed on a sloping site can be compared with the more formal construction of a rock garden on a slope. The retaining materials are laid from the lowest level upwards, so that the height of the wall at the top of the slope is about 15cm (6 inches). The layers should be horizontal at each level. You can make a continuous run of beds if you have a large area available.

You can build on a north facing slope so as to make the flat surface of the soil face the sunlight and hence can support a greater range of plants but you must anticipate obstructions to the light in winter, when the soil lies cold and damp and the greatest loss of plants is likely to occur. Planting woodland and semi-woodland varieties where sunlight is poor can be very effective if done well.

Flat Sites

Flat sites are more common than slopes and allow alpines to be grown in most gardens. If you have a sandy soil the bed should be raised by at least 22cm (9 inch) above the surrounding ground level. If you have heavier soils raise the bed higher still. In both cases, follow the principles for construction given earlier. The soil mixture can be made to suit whatever type of growing conditions the plants you want will need.

The possibilities of a raised bed are so varied and attractive as to fire the imagination beyond the squares of grass and narrow borders so often seen and all the work is above the mud and mire of winter.

The combination of a sink garden and a pavement surrounding the raised beds, opens up a new world with plenty to do but not necessarily in long stints. Twenty minutes a day in summertime is all that is needed to maintain such a combination of sites covering a total area of 200 sq metres (650 sq ft). Construction will of course take a lot longer, but if you do it in simple stages, spread over, say five years, it will give a return on the investment of your time equal to few other forms of gardening.

22. September 2010 by Dave Pinkney
Categories: Gardening Ideas, Raised Beds | Tags: | Comments Off on Constructing a Raised Bed for your Garden


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