At a glance
The depth of a raised bed is one of the most important decisions you make before building, and one of the most frequently underestimated. A bed that is too shallow for its intended crops will restrict root growth, cause the soil to dry out too quickly, and produce disappointing yields regardless of how well everything else is managed. A bed built to the right depth for its crops, filled with the right soil mix, will consistently outperform a deep-rooted vegetable planted directly into compacted UK garden soil.
The depth you need depends almost entirely on what you plan to grow. Salad leaves, herbs, radishes and strawberries thrive in as little as 15-20cm of good growing medium. Courgettes, tomatoes, beans and brassicas need 30cm or more to establish properly. Parsnips, carrots and other long root vegetables need 40-60cm of loose, stone-free soil to produce straight, full-length roots. Planning the crops first and building to suit them is far better than building a standard depth bed and then discovering that your intended crops cannot perform in it.
Why depth matters
Roots do not grow downward because they are programmed to do so – they grow downward because that is where moisture, nutrients and cooler temperatures are found. In a shallow raised bed the root zone is restricted in all three dimensions, and in a UK summer even a well-watered shallow bed will warm up and dry out far faster than a deeper one. Shallow roots in warm, dry soil produce plants that are under constant stress – they grow more slowly, produce fewer fruits or leaves, and are more susceptible to pest and disease attack.
Depth also affects drainage. A bed with less than 15cm of growing medium has almost no drainage buffer – rain saturates the root zone immediately and there is nowhere for excess water to go. A bed of 30cm or more allows a proper drainage layer at the base and a buffer of moist but not waterlogged growing medium above it. This distinction matters particularly in the wet UK climate where waterlogging is often as much of a problem as drought.
Finally, depth affects temperature regulation. Deeper soil masses take longer to heat up in spring and longer to cool down in autumn, extending the effective growing season at both ends compared to a shallow bed that swings rapidly between temperature extremes.
Depth guide by vegetable type
The figures below are minimum depths of growing medium – the actual bed sides can be the same depth or slightly deeper to allow for the soil settling over time and to leave a small gap at the top to prevent compost washing out when watering. Where a range is given, the lower figure will produce acceptable results; the higher figure will produce noticeably better yields.
The 30cm bed is the most practical all-round depth for a UK kitchen garden. It accommodates the widest range of crops, is achievable at a reasonable cost, and does not require an unusually deep frame. If you can only build one depth of bed, 30cm with a good soil mix will grow the majority of popular vegetables well, with the exception of full-length root crops which will need a dedicated deeper bed or a shorter variety.
Soil mix for different depths
The depth of the bed affects not just what you can grow but how the soil mix should be constructed. A shallow bed needs a higher proportion of water-retentive material to compensate for its limited moisture reservoir. A deep bed can afford a more open, free-draining mix as its greater volume retains moisture more effectively.
For root vegetables in a deep bed, the soil mix must be completely stone-free. Any stones or hard lumps cause roots to fork and produce stunted, misshapen crops that are difficult to harvest and unpleasant to eat. Before filling a deep bed intended for carrots or parsnips, sieve the compost and topsoil thoroughly to remove any debris. Sharp sand rather than coarse grit is preferable in this mix – it creates a fine, friable texture that long roots can penetrate smoothly rather than having to navigate around larger particles.
Does the ground underneath matter?
Yes – more than most people realise. A raised bed placed directly on compacted clay, concrete or paving restricts drainage severely and limits the effective root zone to the depth of the bed alone, with no ability for deeper roots to extend downward into the ground. On good garden soil, roots from a 30cm bed will often extend 20-30cm further into the ground beneath, effectively giving you a 50-60cm root zone from a 30cm bed. This matters significantly for the performance of deeper-rooted crops.
Before constructing a raised bed on grass or soil, remove any perennial weeds from the ground beneath and loosen the top 15-20cm with a fork. Do not rotovate or dig deeply as this can bring up weed seeds – a single pass with a fork to open the top layer is sufficient. Line the base with a single layer of cardboard if grass suppression is needed; it will rot down within a season leaving the soil permeable. On concrete or paving, restrict crops to those with genuinely shallow root requirements and be prepared to water more frequently, as drainage will be entirely dependent on the bed depth alone.
Use the bottom 10cm as a drainage layer. In any bed 25cm or deeper, fill the lowest 10cm with coarse material – gravel, broken crocks, bark chip or even torn-up cardboard – before adding your growing mix. This prevents the base of the growing medium from sitting in standing water during wet periods and significantly improves overall drainage without reducing the effective growing depth.
Choosing a bed depth before you build
The most practical approach for a new kitchen garden is to build beds in two depths: a shallower bed of 20-25cm for salad crops, herbs, strawberries and radishes that will be cropped and replaced regularly, and a deeper bed of 35-40cm for permanent crops, brassicas, legumes and fruiting vegetables. This avoids the cost of filling deep beds with expensive compost where shallow-rooted crops would perform equally well, while ensuring the crops that genuinely need depth have it.
If budget is a constraint, prioritise depth over width. A deep narrow bed performs better than a wide shallow one. A 1.2m wide bed at 40cm depth will outperform a 1.8m wide bed at 20cm depth for most productive crops, and it uses the same volume of timber for the frame. The extra cost is almost entirely in the filling – and a deep bed filled with a good mix of topsoil, compost and grit will establish a productive growing environment that improves year on year as organic matter breaks down and soil structure develops.
Common mistakes to avoid
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