What is the optimum size of a garden bed? What is the most efficient width and length in terms of human ergonomics? What is the perfect balance between cost of materials, gardening space and usability? These are important questions in permaculture energy-efficient design.
How Wide Should Garden Beds Be?
Accessibility is critical in garden design. If a garden bed is too narrow, useful space is lost, but if it’s too wide, then unusable space is created.
The maximum width of a garden bed is a matter of human ergonomics, it’s the distance an average adult person can reach across a garden bed from both sides to easily access all parts of it.
Ideally a garden bed should be no more than 1.2m (4‘) wide if it is accessed from both sides, as this width allows an adult to reach just past the centre from any side, giving optimum accessibility to the gardening area.
If garden beds are to be accessed from one side only, the optimum garden bed width is 60cm (2’), which is half of the width of a bed that can be accessed from both sides.
Can you easily reach past the centre of a garden bed from both sides? If you can’t it’s too wide!
Narrow beds use almost as much materials as wider beds to construct, since most of the materials in a garden bed are used in the length of the sides of the bed. Making the ends a bit wider only uses slightly more materials, at a slight increase in cost. The downside is that narrow beds offer far less usable gardening space. The optimum size for a garden bed is therefore one with the largest usable garden bed width which can be easily accessed and used most efficiently at the preferred garden bed length, whatever that may be.
How How Wide Should Garden Beds Be for Children?
When designing gardens for children, the optimum garden bed width is 90cm (36”) if it to be accessed from both sides, and 45cm (18”) if it to be accessed from one side only, such as if the garden bed is against a wall. This is an important consideration in school kitchen gardens as ease of access will better help children enjoy their first experiences of gardening.
How Long Should Garden Beds Be?
Garden beds can be made to any length, though it is more efficient to keep them reasonably short to save having to walk long distances around them. Energy efficiency and minimising unnecessary work are very important goals in permaculture gardening!
What most often happens with long beds is that people will walk through them rather than around them if they are low enough, which causes soil compaction, ruins soil structure, prevents water absorption, and makes it harder for plant roots to move through the soil! This is something any gardener would best avoid. If raised garden beds are too high to walk through and too long to walk around, gardening becomes a burdensome chore!
If garden beds are required to span long areas, it’s best to build multiple shorter beds. This method will use more materials and therefore be slightly more expensive, but the shorter garden beds will be structurally stronger as the end sections will only have to support shorter lengths of sides, making the structure more rigid. The use of shorter beds will also save a lot of effort and energy getting across the line of garden beds, which will make the experience of gardening much more enjoyable in the long run.
Usually, the length of garden beds is determined by the standard lengths of timber materials. For example, railway sleepers are often sold in 2.4m (8’) lengths, so using the whole length without cutting avoids waste, reduces costs, and makes construction much easier. Making a single cut in a 2.4m long railway sleeper gives two 1.2m sections, which are the maximum width for a garden bed as discussed above. So with just three railway sleepers and a single cut only, a 1.2m x 2.4m (4’ x 8’) garden bed can be constructed. Now that’s efficiency!
How Deep Should Raised Garden Beds Be?
The depth of a raised garden bed is determined by what will be grown in it, how often it needs to be watered, the surface it will be placed on, and height requirements for disability situations.
Most vegetables are fairly shallow-rooted, and have 80% of their roots in the first 30cm of soil, so they can be grown in a raised garden bed that contains 40cm. Culinary herbs such as thyme, oregano, marjoram, and mint also have quite shallow roots.
Deeper garden beds will hold more soil, which can get very expensive to fill, but the advantage is that they can hold much more water in the greater volume of soil, and need less watering in summer. They’re also better suited to root crops, such as carrots, or tall plants which can tip over and uproot in strong winds, such as sweetcorn.
If a garden bed will be placed over soil, then depth doesn’t really matter, as plants can root into the ground beneath if they require more depth, but on a hard, sealed surface such as concrete, pavers or asphalt, that’s not possible.
For gardeners with limited mobility, who have difficulty bending down, elevated beds are used to make access easier. These garden beds are usually are waist high. The garden bed itself doesn’t need to go all the way to the ground though, it’s the height of the top that matters in such instances. It can be a shallower bed with legs to elevate it.
Where wheelchair access is required, the garden beds can be can be raised much like a table, holding around 30-40cm of soil, with space underneath. The height must be adequate for seated access, with clearance underneath for leg room and to accommodate the wheelchair. When building such garden beds, get all the measurements correct before construction!
If a sleeper is the same as a railroad tie then I would caution you to the creosote treating of same in edible gardens. Other treated timbers can be equally as bad.
I’d only use new railway sleeper timber, the used ones are not suitable for the garden since the timber is treated with toxic preservatives!
What is sleeper timber?
Thanks for asking Tony, it looks like different countries use different names!
For my US readers, the heavy timber pieces used to span across railroad tracks, or flat heavy timber pieces cut to similar dimensions are called railway sleepers in Australia, in the US they are called rail ties or crossties. Even new pieces of timber that have never been used in railways go under that same name here in Australia.
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Oh, thank you for the explanation. We have access to old railroad ties. (New ones are made with some weird recycled plastic material.) However, there are no timbers that are comparable to them for use in raised beds. Such lumber would be very expensive. Ties are inexpensive because they would otherwise be expensive to dispose of as toxic material.
Mine are 8’x4′, mainly because the timber I used came in 8′ lengths. It also fits well in that corner of the garden. I only have space for 3 of them, unfortunately, am on a waiting list for a council allotment plot.
This is a helpful post that provides useful information on choosing the right size for raised garden beds.