building raised beds: getting started
although raised beds can be built at any time, most gardeners find it convenient to build them in winter, as long as the soil is not too wet or frozen. where winter waterlogging is a problem, build raised beds in late summer. when building raised beds, the following points need to be considered:
- firstly, define how big your raised bed needs to be, and where you need it
- walking or stepping on raised beds is best avoided, so go for widths of less than 1.5m to allow access from the sides
- avoid long runs of beds so that people are not tempted to step on them to get to the other side
- pathways should be wide enough to wheel a barrow or accommodate special needs such as wheelchairs; 30cm (1ft) is the minimum width for walking and 45cm (18in) the minimum width for wheelbarrows
- consider the materials: timber is cheap, but even when treated is the least long-lasting; sleepers are long-lasting but costly, bulky and difficult to cut; masonry (for example, brick, stone or paving slabs) is costly but permanent. alternatives include recycled plastic ‘timber’ lookalikes
- small scale projects might be accomplished using a ready-made kit
- if your garden lies within a conservation area, or is part of a listed building, local planning regulations may limit choice of materials. additionally, if the bed is to be situated near a boundary or roadside, height limitations may be enforced. if in any doubt regarding these regulations contact your local planning authority.
commonly used materials
stone: both natural stone and stone prepared for wall construction can be purchased. skilled labour is required for construction and footings are nearly always required. generally the most expensive material.
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brick: strong and durable. curves can easily be incorporated into the design. skills are needed for construction, and footings are generally required. engineering bricks are most suitable as they are weather resistant. domestic bricks are much cheaper but porous and so much less durable.
pre-cast units: constructed from concrete or reconstituted stone, these materials are cheaper but less adaptable.
timber: very versatile. pressure-treated (also called ‘tanalised’) wood is available. as a shorter-term alternative, untreated wood can be painted with a preservative. to prevent wood preservative leaching into the soil, line wood exposed to soil within the bed with black plastic sheeting. untreated wood will have a shorter life than treated, although untreated hardwoods such as oak and western red cedar will still last many years. gravel boards are generally sold only as pressure-treated timber.
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railway sleepers: it is no longer permitted to use railway sleepers impregnated with creosote in garden, due to the risk associated with frequent (daily) skin contact. if you already have raised beds made from old railway sleepers and have this level of contact, then protective clothing (gloves etc.) should be worn. for new beds, use sleepers treated with other preservatives, or untreated hardwood sleepers. note that this material requires heavy lifting.
paving slabs: can be inserted on their side. at least 15cm (6in) of slab needs to be buried in the ground for stability, leaving only 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. relatively inexpensive, but heavy to lift.
building raised beds: construction
- clear the site of existing vegetation and level as required
- mark out the beds with stakes and string, and check the levels
- for all but masonry walls, insert retaining stakes (5 x 5cm or 2 x 2in timber is suitable) at the corners and then at every 1.5m (5ft), sunk 30-45cm (12-18in) into the soil to support the sides
- attach the sides to the retaining stakes with nails or screws (screws are better for avoiding splitting)
- for masonry walls, any wall higher than 20cm (8in) should be laid on concrete footings on 15cm (6in) deep hardcore foundations, and should be bonded with mortar. footings should be approximately 15cm (6in) deep and 30cm (1ft) wide
- where a masonry bed is to be built on a hard surface such as a patio, leave gaps at 45cm intervals between the joints in the first two courses to act as drainage holes. cover these holes inside the wall with fine wire mesh, and heap gravel against the mesh when filling to help prevent clogging
- sleepers and logs can be laid directly on level, firm soil. fix into place by hammering metal rods into the ground through holes drilled vertically in each corner. alternatively, use heavy-duty staples and wire
- sink sawn logs or log rolls partially into the soil, and then ram them hard to keep the logs upright
filling the bed
- once the sides are in place, cultivate and enrich the underlying soil with organic matter
- although raised beds constructed on free-draining soils drain naturally and permit deep rooting those on poorly-drained soils, or on a solid base such as concrete or paving should be filled with open textured soil, adding sand if necessary, and making sure there are adequate holes in the sides allow water to drain away. most plants need about 25cm of top soil and beneath the top soil sharp sand can be used which will drain freely yet hold some water to sustain plants and into which plants can root for stability. think of raised beds on impermeable surfaces, especially shallow ones, as being similar to pots needing careful and frequent watering and replacement of the soil at intervals of several years when drainage becomes impaired.
- for beds deeper than 50cm (20in), remove the underlying topsoil (for use later in filling up the bed) and replace with subsoil, rubble or old inverted turves. then replace the topsoil, enriched with organic matter (such as well rotted compost or manure) and fertiliser, firming in layers to prevent undue settling
- where the topsoil is unsuitable for the crops or plants to be grown, leave it in place, but simply loosen it and fill up the bed with new soil. small beds can be filled with john innes no 2 or 3. a good general soil mix can be made by combining 3 parts organic matter and 2 parts sharp sand to 7 parts topsoil
- once filled, allow beds to settle for two weeks before planting
six steps to making a raised bed out of gravel boards
- roughly dig over and level the whole area, removing any perennials weeds (unless grass paths are to be left, in which case individual bed
zones should be dug)
- mark the beds out using cane and string
- hammer corner posts into the soil. these should be constructed from 45cm (18in) lengths of 7.5cm x 7.5cm (3in x 3in) timber, inserted so that at least 30cm (1ft) is below soil level. check that posts are square and tops level, and add extra posts for support if the bed is longer than 3 metres (10ft)
- nail or screw the boards to the posts, checking levels as you work
- top up and firm soil levels on the paths to make a 3cm (1¼in) overlap of soil on the outside edge of the beds. this will help prevent soil from within the bed falling through the base of the gravel board onto the pathway
- dig the bed thoroughly, adding plenty of organic matter. leave ground to settle for two weeks before planting
specialist ornamental beds
beds for rhododendrons and azaleas should have a minimum rooting depth of 45cm (18in). brick or stone constructions should be lined with polythene to separate any lime in the mortar from plant roots. a good soil mix consists of three parts neutral-acid ph topsoil, five parts ericaceous compost and two parts composted bark or fine grade chipped bark. a surface mulch of acid leaf mould, or composted pine or spruce needles is also beneficial.
provide a rooting depth of 45cm (18in) to prevent rapid drying out. a suitable soil mix is four parts horticultural grit, two parts leaf mould or fine grade chipped bark and two parts topsoil.
dwarf bulbs beds
provide a rooting depth of 30cm (1ft). the compost mix should drain freely and dry out in the summer months to give the bulbs a good resting period. an ideal mix is three parts topsoil, one part sharp sand or horticultural grit and one part fine chipped bark.
for squirrel and rodent control, insert a layer of fine mesh wire netting just beneath the soil surface.
raised beds for vegetables
growing in raised beds brings additional advantages for vegetable growers:
- plants can be grown in blocks, encouraging plants to grow more uniformly as the competition between them is minimal, and suppressing weeds more effectively
- raised beds can be more easily covered than in a larger vegetable patch or allotment
- the higher level of the soil encourages beds to warm up faster in the spring, allowing the season to start earlier
- fertilisers, organic matter and manures are concentrated on
zones where plants grow, rather than being wasted on pathways
- rotations are simple to practice with several narrow beds rather than one large plot
long vegetable beds should be orientated to run north to south so that they receive even sunlight levels.