Raised Garden Beds – What Are They?
A Raised Garden Bed is an elevated garden bed that sits higher than the surrounding soil or the ground that it sits on, and is usually supported by some sort of frame or enclosure, though this is not always the case.
The optimum size of a raised bed is 4 feet wide or less, at any length. The reason for a maximum width of 4 feet is a matter of ergonomics, this is the maximum width that a person can reach across to access the area efficiently from either side. They 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 constantly.
I have found that 4’ x 8’ (1.2 x 2.4m) is an ideal length not only to work around, but to construct from 8’standard-length pieces of timber or railway sleepers if using these materials.
The benefit of breaking up a long continuous 4’ wide bed into shorter sections is that of increasing the amount of “edge” available. To understand the benefit, I’ll need to introduce the permaculture concept of the “edge effect” – where two different environments meet, we have an “edge”, this interface between two different area creates a broader range of favourable environmental conditions or ecological niches, which support an increased variety of plants and animals, and are therefore more highly productive areas.
For example a 4’ x 100’ bed has (2×4) + (2×100) = 208’ of edge
If we now divide the same bed into 5 sections, we now have (5x2x4) + (5x2x20) = 240’ of edge.
Through this simple change we now gain an additional 32’ of edge.
The only thing to remember is that you will need an additional 32’ of edging material (if you are using edging material!) to build this, but rest assured that the benefit of increasing edge will outweigh the initial outlay in materials.
What Are the Benefits of Using Raised Garden Beds?
The benefits of raised beds are many and varied, as listed below:
- Increased Productivity According to Ohio State University, I quote, “In a traditional home garden, good management may yield about .6 pounds of vegetables per square foot. Records of production over three years in a raised bed at Dawes Arboretum near Newark, Ohio, indicate an average of 1.24 pounds per square foot, more than double the conventional yield.”Raised beds are more productive per square foot because plants can be spaced more closely together. This is because you don’t need to leave spaces to walk through, because you don’t need to ever step on the garden bed, ever. Having a higher density planting also has the advantage that the plants growing there will shade out bare soil, making it harder for weeds to grow there.There most important benefit of raised garden beds is productivity. Raised garden beds are at least twice as productive as a conventional garden.
- Better Soil Conditions Since there is no soil compaction, there is no need to plow, till, fork or dig the soil to loosen it up, traditional practices which destroy the soil structure and do more harm than good. Therefore, raised bed designs also lend themselves very well to the gardening technique of “no-dig gardening”.You can use raised garden beds to overcome issues of poor drainage, poor soil, or even no soil, such as gardening on top of straight concrete or asphalt, because you create the garden bed and fill it with the type of soil you require, and build it by adding organic matter. You could even create a number of beds with different soil blends for a variety of different growing environments.By not having to step on the soil, you avoid soil compaction. When soil is compacted, water and air do not move as easily through the soil to the roots of plants. Even the plant roots themselves have trouble growing through compacted soil, limiting the plant’s access to water and nutrients. It has been suggested that soil compaction can lead to a 50% loss in productivity.
- Increased Flexibility You can also attach trellises, supports, fences, frames, or shade cloth over/around the bed much more easily, or permanently as part of the structure.For people with physical limitations, such as those unable to bend over, or who are confined to a wheelchair, waist high raised beds are the answer. A bed of this height will enable the person to partake in their interest in gardening without hindrance.Not having to step in the garden bed has the benefit of being able to tend to the garden, that is, sow, plant and harvest, whenever you want, even when the ground is wet because you won’t be stepping in mud!
- More Efficient Irrigation Additionally, raised beds can support very thick layers of mulch above the soil, which will not slide off, or be blown or washed away. This not only aids in water conservation, but allows you to enrich and build the soil through the constant addition of organic matter.The dimensions of raised beds lend themselves to the installation of drip irrigation, which is an efficient way to water the garden, minimising loss by evaporation, and reducing disease by not wetting the plant’s leaves.
Raised Garden Bed Construction
Raised beds can be constructed from a variety of materials, and in many different heights, depending on your requirements.
Basically, any materials can be used to create an enclosure of the height of your choosing to hold the soil.
The choice of materials is dictated by two main factors:
- the raised bed material will last a reasonable length of time as it will be exposed to the weather – heat, cold, rain and sunlight
- the raised bed material is non-toxic
You can use recycled materials, new materials, man-made or natural, the choice is yours. Timber, bricks, concrete blocks, pavers, whatever takes your fancy.
I’ll focus mainly on timber raised bed construction here, though I’ll cover a few other methods too.
CAUTION– Health Warning – DO NOT USE TREATED OR PAINTED TIMBER
Traditional CCA treated pine (Copper Chrome Arsenate) contains arsenic, a poison which will leach into the soil and be taken up by plants, not to mention why it might do to your soil, which is a living ecosystem. It’s toxic when working with it, handling, drilling or cutting it. As is burning it. Having toxic chemicals in your soil is not the intent of organic gardening, nor is it in the interests of your health.
Due to the arsenic scare, ACQ treated pine (Copper and Quaternary Ammonium Compound) is now being offered on the market. It’s touted as being a new, safer type of treated pine. That is, safer, this does not mean safe!
There are a variety of timber treatments, all of them toxic. Treatments such as creosote (often used on old recycled railway sleepers) and other treatments such as LOSP (Light Organic Solvent Preservative) are all toxic. Using painted timber is not a safe practice either.
Yes, untreated timber will eventually rot and break down, and that’s what nature intends it to do, so if you want to use timber raised beds and wish to garden organically, it’s best to come to terms with this idea…
Now that we have the health warning out the way, lets look at how we construct timber raised beds.
Measurement and Layout
Before any construction begins, it’s best to draw out a plan or design with accurate measurements, as this will help avoid mistakes. Remember, building materials are not cheap, so use the old woodworkers maxim "measure twice, cut once" and avoid the wastage.
Before we do any cutting or drilling, we need to determine where the wooden beams will be laid on the ground, and how long they need to be. This is important if the space is limited by obstructions, if you’re making an allowance for paths of a specific size, or if the beds are to match up the space you’ve allocated on a design or plan.
The critical matter to consider here is that the thickness of the timber beams needs to be taken into consideration when determining the completed internal and external garden bed sizes.
As is clearly visible in the diagram, the layout of the timber alters the overall dimensions. Decide whether the short or long sides will be the ones overlapping the ends, and stick with this when cutting and assembling, otherwise the final sizes of your beds either will be mismatched, or won’t fit where you intend to put them.
Attaching The Sides Together
One way of attaching the sides together is with a vertical post in each corner.
It’s just a matter of marking where the holes go, drilling the holes, and fastening it all together with coach screws or coach bolts (they both have hexagonal heads so you can tighten them with a spanner or socket set).
If using coach screws:
- Make sure you use the right length. The coach screws should be long enough to go into the side and approximately 2/3 of the way into the corner post.
- Drill the pilot holes smaller than the thickness of the screws so they can bite into the wood, and drill only part of the way through the post, otherwise they won’t hold. Drill them too small and it will be hard work driving the screws in.
- Helpful tip: Rub beeswax (or a bar of soap if beeswax isn’t available) over the threads of the screw to lubricate them so it is easier to screw into the wood and prevents the both heads being sheared off.
If using coach bolts:
- Drill a hole slightly larger than the bolt, and drill all the way through, then put a washer on the inside and screw on the nut and tighten using a spanner or socket on both sides.
NOTE: It is important to realise that when fastening to a post, you will need to offset the positions of the fasteners otherwise they will hit each other!
As shown in the diagram below, offset the position of the fasteners slightly higher on one piece of wood, and slightly lower on the other, so they are all spaced evenly apart.
If using a railway sleeper that is 20cm high (which is a standard height), the way to calculate where the holes go is as follows:
Since we need 4 holes evenly space from the edges and from each other, we need to divide our given width by 5.
If we do the maths, 20/5 = 4cm
So, on the left hand piece of timber, we drill the first hole 4 cm from the top.
For the next position, we can’t go down another 8cm position, because that’s where the other piece will be fastened, so we go down another 4cm again to the 12cm position from the top.
So, the left hand piece is fastened at 4cm and 12 cm positions down from the top.
Therefore, the right hand side piece must be fastened at the 8cm and 16cm positions down from the top.
Another popular way is to fasten the sides together with steel brackets.
Shown below is a popular method of using a steel bracket on the inside of each corner, drill holes in the wood to match those on the bracket, and fasten using nuts and bolts. The heads obviously go on the outside, and the bolts extend into the inside of the bed, out of sight.
Another popular type of steel bracket is one which has a stem coming off it perpendicular to the bracket, like a letter "T", with a pointed long section that is hammered into the ground. These can be attached either inside or outside of the raised bed structure, and fastened either with screws or bolts. The long pointed end sits firmly on the ground and stabilised the structure. This is particularly important when joining two sleepers end to end.
Using Other Construction Materials
You can use a variety of materials to construct raised beds. If you can, recycle any materials before you go out and buy new stuff, it’s cheaper and more sustainable.
Pavers make great edging material that lasts virtually forever, and this works great when edging up to a concrete path. In the picture below I’ve used rectangular pavers to create an edge along where the garden bed meets the concrete walkway. Rectangular pavers were used, buried lengthwise in a narrow trench dug with a small garden trowel of hand spade.
To get good alignment, get some chalk and draw a line across them to indicate how far you bury one end into the ground. If it’s not exactly half way, draw an arrow pointing down on the bottom bit so you know which end is down. You can also put a wooden stake at each end, and stretch a piece of string across the top at the right height to get the tops level if the ground is uneven and can’t be used as a reference. You can hang a small builders line level on the string to make sure your string sits level if you have one.
Thin timber strips, such as jarrah edging, makes fine and delicate garden edging for things like flower beds and paths. The timber strips are simply fastened with small steel "U" shaped clips, the ends goinhg into the ground.
You can also use a mix of materials, as shown below, as long as it works. Below is a picture that shows where I’ve used an extra thick redgum sleeper, which separated the lawn from the garden bed above it. It is anchored by a steel bracket-stake op to where the concrete begins. This meets with the paver edging, which separated the garden bed above it from the concrete below. And in the top right hand corner you can see the jarah strips that define the edges of the 45cm wide paths which are mulched with pine bark mulch.
You can even do composite bed material corners too!
I encourage you to experiment, be creative and try out your ideas. That’s how I did mine! It’s quite satisfying when you succeed in making it all work too!
Labour Only, No Materials, Raised Garden Beds
OK, so you’re probably wondering if there’s a really cheap, no-cost way to do edging. Well thankfully, there is! It’s called Botanical Edging, the reason being, if you have something as large as a Botanical garden with thousands of beds, it’s not practical to install edging, so you just dig them. Botanical edging is the neat narrow trench you see around garden beds on large properties, estates and botanical gardens that seperates the beds from the lawns.
The best tool to use is a half-moon lawn edger, which just looks like a crescent (half circle) with a long handle, as pictured below, but if you don’t have one of these amazing tools, you can just use a sharp spade (they’re the ones with the square end).
The half-moon edger is wider and shallower than a garden spade, so you cut more edge done each time, and it’s set to the right depth, so you can push it all the way down. With a spade, you will need to control the depth of cut.
How To Cut a Botanical Edge:
- Mark where you intend to cut the edge. For a straight edge, cut against a plank of wood or a string line, for a curve use a garden hose or thick rope laid flat on the ground.Stand on the side of the path to do the digging, not in the garden bed.Push your half-moon edger or spade perfectly straight down into the ground. Step on one side of your edger, then the other so that you are using your body weight to drive it into the ground.
- When the edging tool is all the way into the ground, or the spade is at the desired depth (put a mark on the spade to keep it consistent), lift it towards your garden bed to dig out the soil, and toss the soil onto the garden bed.You are aiming to create a clean vertical edge on on the path side, and a 45 degree angle on the garden bed side. The trench should be roughly around 6" wide x 6" deep (around 15cm x 15cm).
- Remove any grass on the garden bed, and even off the level of soil that has been dug in the garden bed to make it tidier.The edging is complete, and if you want to raise it higher, pile on more soil or mulch to elevate the bed. Obviously, the sides of the material you add must slope down like the sides of a pyramid so it all stays there.
Now you can go out there and build raised beds and enjoy the increased productivity of your garden!