Raised beds

Raised beds are a great way of growing a wide range of plants, and are particularly popular for growing fruit and vegetables. They are a good way of boosting drainage and can be used to introduce a different soil type to your garden. Raised beds are also a useful way to garden if you have restricted mobility, as they reduce the need to bend.

Raised vegetable beds. Credit: RHS Advisory.

Quick facts

Raised beds are good for:

  • Improving drainage
  • Increasing soil temperature
  • Improving access
  • Growing plants in a different soil type

Suitable for...

You can grow almost any plants in raised beds. Try the following:

  • Soft fruits: Such as strawberries, currants, raspberries and blackberries
  • Vegetables: Almost any vegetable can be grown in raised beds
  • Herbaceous perennials: Raised beds are a good idea for establishing a cutting garden for cut flowers
  • Alpines: Ideal for alpines that relish good drainage
  • Small trees and shrubs: Depending on how big your beds are, you may be able to grow some smaller trees and shrubs
  • Ericaceous or lime-hating plants: By filling beds with acid soil, lime-hating plants such as heathers and rhododendrons may be grown in areas of alkaline soil

Raised beds can be used to:

  • Improve drainage: Soil is raised above the surrounding ground level. However, this can be disadvantage in droughts as more watering may be needed
  • Increase soil temperature: Soil in raised beds is better drained, so warms up faster in spring
  • Enhance root health: Filling the beds with good topsoil enriched with fertiliser and organic matter gives excellent root zone conditions
  • Improve ease of management: Raised beds have a bigger soil volume than containers, so are easier to manage with watering
  • Match the soil to the plant: By filling raised beds with acid soil, for example, ericaceous (lime-hating) plants can be grown even where the underlying soil is alkaline
  • Improve ease of access: Raised beds are easier to manage for gardeners with mobility problems

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.

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.

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 require no drainage layer, on poorly-drained soils, or on a solid base such as concrete or paving, raised beds should have drainage material laid in the base. This should be above ground level, and consist of at least 8cm (3in) of coarse gravel, stones or hardcore, which should be covered with a geotextile membrane before filling with soil to prevent clogging up but omit the membrane if planting deep-rooted plants such as conifers, small trees or shrubs
  • 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

  1. Roughly dig over and level the whole area, removing any perennials weeds (unless grass paths are to be left, in which case individual bed areas should be dug)
  2. Mark the beds out using cane and string
  3. 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)
  4. Nail or screw the boards to the posts, checking levels as you work
  5. 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
  6. Dig the bed thoroughly, adding plenty of organic matter. Leave ground to settle for two weeks before planting

Specialist ornamental beds

Ericaceous 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.

Alpine beds

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 areas 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.

Top tips

  • To save cost, use soil scooped from paths to fill beds, and fill paths with bark, gravel or other paving materials
  • Bury any turf removed in making the beds in the lower levels of soil in the bed to enrich the soil as it decays
  • When building raised beds on top of hard surfaces, ensure a depth of at least 45cm (18in), but ideally 60cm (2ft), so that plants can root deeply. This will reduce their watering needs

Problems

Plants in raised beds can suffer more quickly and more severely from drought due to improved drainage, so keep an eye on watering needs.

Modern wood treatments do not contain potentially harmful heavy metals, so are safe to use. If in doubt, line the inside of the bed with polythene.

New railway sleepers may contain creosote that should not be used where skin contact is a possibility. Creosote is thought to have dissipated from older sleepers, and these may be used without concern about skin contact.

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