Raised bed depth: match vegetables and roots

Illustration of vegetable raised beds with clear access paths

Affiliate disclosure: As an Amazon Associate I earn from qualifying purchases. This article contains affiliate links.

Raised bed depth is not a one-size-fits-all decision. The RHS guidance is clear: 30cm (1ft) is enough for dwarf bulbs, salad leaves and strawberries, but most plants, including many vegetables, fruit bushes, shrubs and small trees, need 45cm (18in) or more. These figures describe useful growing depth, not a minimum frame height on open, rootable soil. This article explains how to match bed depth to the vegetables and roots you actually want to grow, and what to check before you build or buy.

Table of Contents

What depth changes

A raised bed is simply an enclosed, freestanding bed constructed above ground level. The total depth of usable soil, including suitable ground below an open-bottomed bed, determines root room, how quickly the soil drains, and how much good soil you need to fill it. A bed that is too shallow will restrict root development, dry out faster in summer, and limit you to a narrow range of crops. A bed that is deeper than necessary costs more to fill and may need extra drainage work at the base.

The RHS notes that raised beds are particularly useful where garden soil is waterlogged, because lifting the soil above ground level improves drainage. The soil surface needs to be raised, with an outlet for excess water; the frame itself can sit on the existing ground.

The three practical depth bands

Use these bands as a starting point, based on RHS advice.

30cm (1ft): shallow-rooted crops

This depth is enough for dwarf bulbs, salad leaves and strawberries. It suits plants with small, fibrous root systems and short growing seasons. If you only want cut-and-come-again lettuce, radish, spinach or alpine strawberries, a 30cm bed can work well, especially on open ground where roots can also penetrate the soil below. On a hard surface, around 30cm is a useful starting point for these crops, and drainage holes are essential.

45cm (18in): the workhorse depth for most vegetables

The RHS states that most plants, especially fruit bushes, shrubs and small trees, need 45cm or more. For a general vegetable bed, this gives flexibility for many annual crops, including beans, peas, brassicas, courgettes, tomatoes and many root vegetables. Long-rooted crops such as carrots and parsnips will usually do better at this depth or more, because they have room to extend without hitting a hard base. Exact root length varies by variety and soil conditions, so treat 45cm as a safe starting point rather than a guarantee of straight roots.

50cm (20in) or more: deep beds for permanent and deep-rooted plants

For permanent crops such as rhubarb, asparagus and fruit bushes, assess the total rootable profile and drainage rather than treating 50cm as a universal threshold. A deep frame over compacted or waterlogged ground does not automatically solve the underlying problem. Preserve useful topsoil and avoid putting unknown rubble beneath the roots merely to save on fill.

Matching common vegetables to depth

Here is a practical way to group crops by the depth band they are most likely to need. These groupings follow RHS depth guidance and general root habit; they are not exact measurements for every variety.

30cm band: salad leaves, spinach, radish, spring onions, garlic, dwarf bulbs, strawberries.
45cm band: most annual vegetables, including beans, peas, brassicas, courgettes, tomatoes, beetroot, carrots, parsnips, leeks, onions grown to full size.
50cm+ band: rhubarb, asparagus, globe artichokes, fruit bushes, small trees and shrubs.

Where the bed sits on a hard base, about 45cm of suitable growing medium gives useful flexibility for many vegetables. On open, well-prepared ground, a much lower edging may be enough because roots can continue below it.

What changes on a hard surface?

If you are building on concrete or paving, depth alone is not enough. The RHS advises drilling plenty of drainage holes in the sides of timber beds, or leaving gaps in the first two courses of masonry beds. Fill the bed with a mix of three parts organic matter, four parts sharp sand and seven parts topsoil, and be prepared to refresh the soil regularly. A hard surface removes downward drainage, so confirm a working outlet before deciding on depth; extra depth alone cannot prevent waterlogging.

Practical checks before you build or buy

Measure internal soil depth, not external height. A bed with 45cm sides but a 10cm drainage layer of rubble has only 35cm of root room. Check the manufacturer’s stated internal depth or calculate it yourself.
Keep the required root zone usable. Bulk base materials reduce growing depth, and a coarse layer does not create an outlet. Use only a designed, clean fill arrangement appropriate to the site.
Check drainage holes if the bed sits on a hard surface. Without them, even a deep bed can become waterlogged.
Plan for watering. Raised beds dry out more quickly than open ground, so deeper beds hold more moisture but still need regular checking, especially in warm or windy weather.
Match the soil mix to the depth. For shallow beds, a standard mix of topsoil, organic matter and sharp sand works. For deep beds, specify a stable soil profile rather than filling with material that will slump or restrict roots.

Choosing a raised bed for the right depth

When buying a ready-made kit or materials, look for these features:

Modular timber kits that can be stacked. These let you start at 30cm and add a second or third layer later to reach 45cm or 60cm. Check that the corner joints are designed for stacking and that you can screw layers together securely.
Deep raised bed planters with a stated internal soil depth of at least 45cm. Some planters look tall but have a false bottom or built-in water reservoir that reduces root room. Ask the seller for the internal soil depth, not just the external height.
Recycled plastic sleepers or pressure-treated timber. Both are durable options. If using treated timber for edible crops, check that the treatment is suitable for food-growing areas; use material with a known treatment and suitable intended use. A side lining does not make unknown or contaminated reclaimed timber suitable, and it must not block drainage.

Before buying, confirm the internal dimensions, the number of drainage holes (if for a hard surface), and whether the material is suitable for growing edible crops. A bed that is 45cm deep internally will cover almost all vegetable growing, while a 30cm bed suits salads and strawberries; other possibilities depend on the soil beneath it.

Raised bed depth is a decision you make once, but it affects every season after. Start with what you want to grow, choose the shallowest depth that will support those crops, and add a little extra if you think your plans might expand. The RHS thresholds of 30cm and 45cm are the most useful guide, but total rootable depth, crop choice and drainage should decide the final design.

Useful products for this job

The links below open Amazon UK product searches. Check the exact item, seller, current price and compatibility before buying.

Modular timber raised bed kit (stackable): Allows you to start at 30cm and add layers later to reach 45cm or more as your cropping plans change. Confirm the internal soil depth per layer, that corner joints are designed for secure stacking, and that the timber treatment is suitable for edible crops. Search Amazon for Modular timber raised bed kit (stackable).
Deep raised bed planter with minimum 45cm internal soil depth: Ready-made option for most vegetables and root crops without needing to build from scratch. Verify the internal soil depth, not just external height; check drainage holes if placing on a hard surface; confirm material durability for outdoor use. Search Amazon for Deep raised bed planter with minimum 45cm internal soil depth.

Related garden guides

Sources and further guidance

Guidance researched in September 2026. The feature image is an AI-generated illustration. Product guidance is based on published information, not hands-on testing.