Min soil depth 30 cm (most veg)
Max bed width 1.2 m (both sides)
Soil mix 60% topsoil / 30% compost / 10% grit
Key risk Dries out faster than ground
75%

The reduction in water loss that mulching can achieve in a raised bed, according to RHS research. Good drainage is the main advantage of raised beds over ground-level growing; it is also the main risk. Mulching addresses both at once by keeping moisture in while protecting the soil surface.

Raised beds are not always the best option. On good loamy soil with no drainage problems, growing directly in the ground costs less, requires no bought-in soil, and the greater soil volume buffers against drought more effectively. Raised beds make the most sense where the existing soil is waterlogged, heavily compacted, or so poor that improvement would take years, and where the additional setup cost is justified by the result. On a shared plot or community growing site they are particularly valuable because each bed can be managed independently with different soil conditions, no one needs to step on the growing area, and they remain accessible to growers who cannot work at ground level.

What raised beds offer

Raising the growing surface above the surrounding ground improves drainage immediately. A bed filled with well-structured soil above waterlogged clay will drain freely even when the clay below is saturated. The RHS identifies waterlogged soil as the primary situation in which raised beds are genuinely necessary, particularly for permanent crops like rhubarb and asparagus that would rot in prolonged winter wet.

The elevated soil also warms faster in spring. Air circulates around all four sides of the bed, sunlight reaches the soil at more angles, and the looser fill heats through more quickly than compacted ground. In practice this can mean starting to sow and plant two to four weeks earlier than an equivalent ground-level bed, which is a useful advantage on Suffolk’s heavier clay soils, where the ground can stay cold and wet well into spring.

Because you never stand in the bed, the soil remains uncompacted regardless of weather conditions. This matters over years: the structure built up by earthworms and organic matter is preserved rather than repeatedly destroyed. It also means the bed can be worked immediately after heavy rain without risk of damage.

Finally, the soil quality is entirely under your control. On a site where the underlying ground is poor, filled with builders’ rubble, or too shallow for root crops, a raised bed filled with good topsoil and compost bypasses all of those constraints in a single step.

Choosing size and position

Width

The width of a raised bed is the most important dimension to get right. The whole point is that you should never need to step into the bed, and you should be able to reach every part of it comfortably from the path on either side. The maximum practical width for a bed accessible from both sides is 1.2 m (4 ft). For a bed accessible from one side only, reduce this to around 60 cm. A bed wider than these limits forces you to overreach or step in, which defeats the purpose.

For accessibility at raised height, the maximum reach from a seated or standing-but-limited position is around 50 cm from the near edge. Taller beds for wheelchair users are typically built no wider than 80–90 cm total.

Height

A standard raised bed 25–30 cm tall provides enough soil depth for most vegetable crops and is straightforward to build. Beds as shallow as 15 cm work for salads, radishes, and herbs. For root crops or squash that develop extensive root systems, 30–40 cm is preferable.

Beds built for wheelchair access or for growers who cannot bend low need a working height of 60–65 cm. At that height a wheelchair user can work from a seated position without bending or overextending. The paths alongside these beds need to be at least 90 cm wide for wheelchair clearance, and a firm, level, non-slip surface. A U-shaped or H-shaped layout allows the gardener to reach into the bed from multiple angles without changing position.

Length and position

Length is largely a matter of available space. Orienting the long axis of the bed roughly east to west means neither side permanently shades the other, and maximises sun exposure across the whole surface. South-facing slopes or aspects speed up warming in spring. Avoid positions shaded by walls, fences, or large trees for more than half the day.

What to build with

The safest and most practical materials for food-growing raised beds are:

Avoid old railway sleepers, which were treated with creosote and can leach compounds into soil that are toxic to plants and unsuitable for food growing. Also avoid any timber treated before 2003, which may contain Chromated Copper Arsenate (CCA), a treatment that includes arsenic. Modern pressure-treated softwood uses safer alternatives (ACQ), but some caution is still appropriate for direct food-growing contact; untreated wood or the options above are preferable.

Avoid plastic and plastic composite boards, including those marketed as recycled or eco-friendly. Plastic degrades over time when exposed to UV light, heat, and moisture, releasing microplastics into the soil around and below the bed. The rate of degradation is difficult for a buyer to assess from manufacturer claims. Untreated timber and galvanised steel are both safer and more appropriate choices for a food-growing context.

The base layer

Before filling the bed, lay a base layer over the ground to suppress any weeds or grass growing up through the bottom. Flattened cardboard boxes are the best option for this: they smother existing growth, break down within one to two seasons into organic matter that feeds soil organisms below the bed, and encourage earthworms to move through from the native soil into the fill above. Use plain cardboard without glossy coating or staples, and overlap the sheets generously to leave no gaps.

Woven weed fabric is often suggested as an alternative but is generally less suitable for food-growing beds. While it prevents weeds, it also blocks earthworms and other soil organisms from moving freely between the fill and the ground below, and it does not improve the soil over time. Cardboard is both more effective biologically and free.

If the bed is being placed on hard standing (paving, concrete, or compacted hardcore), there is no weed risk and no base layer is needed. Ensure the soil depth is generous, at least 30–40 cm, as the plants have no native subsoil to reach into. On very hard surfaces, placing the bed directly on the ground without gaps is unnecessary; leave the base open so any excess water can drain freely.

Filling the bed

The standard filling mix that works consistently across most vegetable crops is: 60% quality topsoil, 30% peat-free compost, 10% horticultural grit. The topsoil provides mineral structure and weight that compost alone cannot supply. The compost adds fertility, organic matter, and biological life. The grit keeps the mix open so water moves through freely and the surface does not compact after heavy rain. A mix filled with this ratio should produce consistently without major amendment for five to seven years, refreshed only by an annual surface mulch of compost.

When sourcing topsoil in quantity, specify grade A or B to BS3882 and buy from a reputable UK supplier who can provide a specification sheet. Cheap topsoil of unknown origin is one of the most reliable ways to introduce persistent weed seeds, soil-borne diseases, or even contamination into an otherwise clean growing space.

After filling, the soil will settle by 10–20% over the first season as it consolidates and organic matter breaks down. Top up with compost in the first autumn and maintain with a 3–5 cm annual surface mulch thereafter.

The watering challenge

The same drainage that makes raised beds good for waterlogged sites also means they dry out faster than ground-level beds, particularly during Suffolk’s dry summers. In hot dry spells, a shallow bed in full sun may need watering every day or two. Deeper beds hold more moisture and buffer better against short dry spells; a bed 40 cm deep will retain moisture noticeably longer than one 20 cm deep.

The most effective single intervention is mulching. A 5 cm layer of compost or wood chip applied over the soil surface reduces evaporation sharply, moderates temperature extremes at the surface, and feeds earthworms and soil organisms at the same time. Mulch after planting or sowing once plants are established, keeping it back from direct contact with stems. Do not wait until the bed shows signs of stress before mulching; apply it as standard practice from the start.

To check whether watering is needed, push a finger 3 cm into the compost. If the soil feels dry at that depth, water thoroughly until water runs freely from the base of the bed. Frequent light watering wets only the surface and encourages shallow rooting; less frequent but thorough watering is more effective.

Growing in containers

Containers are the right choice where there is no space for a raised bed: a balcony, a paved courtyard, a doorstep, or a community site with limited ground. They also suit crops that benefit from being moved (tender plants that need bringing in for winter, or crops you want to reposition to chase the sun). The trade-off is that containers dry out faster than raised beds, need feeding more regularly because nutrients wash out with each watering, and limit root development more strictly.

Pot size

Bigger is almost always better. The RHS recommends a minimum depth and width of 45 cm for most vegetable crops; smaller pots dry out too quickly and need watering too frequently to be practical. Exceptions:

Every container must have drainage holes in the base. Without them the compost becomes waterlogged quickly and roots rot. For containers larger than 30 cm diameter, two to four drainage holes are needed.

Compost for containers

Fill containers with peat-free multi-purpose compost rather than a topsoil mix. Pure compost holds moisture well at container scale while remaining light enough to lift and move. Unlike in raised beds, you do not need grit for drainage because the limited volume and drainage holes manage this already; adding grit makes the container heavier without benefit. Replace the compost or top up generously each season: the material breaks down quickly and loses structure within one to two years.

Container-grown crops need more regular liquid feeding than raised beds because nutrients wash out with every thorough watering. Begin feeding with a balanced liquid feed from around four to six weeks after planting (or from when the pot-grown compost becomes nutrient-depleted). Switch to a high-potassium feed such as comfrey liquid once fruiting crops begin to flower.

Pot material

Terracotta is porous and allows air to reach roots, which suits Mediterranean herbs and reduces the risk of overwatering. The trade-off is that it dries out faster than plastic and is heavier. Plastic retains moisture longer and is lighter, which matters on balconies where load-bearing capacity is limited. Glazed ceramic sits between the two. All work for food growing; the choice depends on what you are growing and how often you can water.

Which crops suit which approach

Most vegetable crops do well in raised beds. Root crops (carrots, parsnips, beetroot), brassicas, legumes, and perennial crops like asparagus, strawberries, and herbs all perform as well in a raised bed as in open ground, often better on difficult sites. Climbing crops like runner beans and cucumbers suit raised beds well because you can position the frame or support structure at the edge of the bed.

Containers work best for crops that either benefit from a controlled environment or are highly productive relative to the space they occupy. Tomatoes, peppers, chillies, and aubergines all perform well in large containers in a sheltered sunny spot. Cut-and-come-again salad leaves, herbs, and spring onions produce heavily from a small container relative to the investment. Potatoes can be grown successfully in large bags.

Container growing is less well suited to crops that need a large undisturbed root run (large squash and pumpkins), crops that produce over a long season and deplete the compost quickly (brassicas in standard pots), and crops that benefit from direct association with soil organisms (the root nodule nitrogen fixation of legumes is less effective in peat-free compost than in soil, though they will still produce a crop).

On shared growing sites, labelling each raised bed or container with its owner and any soil amendments applied helps new volunteers pick up where others left off. A simple laminated card noting the season’s crops, compost added, and any pH adjustment saves time at the start of the next season.

Further reading

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