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.
Timber Raised Garden Bed Construction
Timber raised garden beds are built using planks of wood fastened to corner posts. The corner posts can be actual wooden posts, or short lengths of the same planks used for the sides, as shown below.
The wooden planks are screwed into the posts using long galvanised screws. It’s better to use hot-dipped galvanised screws, as these have a thicker coating of protective zinc (compared to zinc-plated screws) which prevents them rusting.
There are two steps required to build a timber raised garden bed.
- Assembly of the long side panels of the garden bed
- Attaching the short side planks to the two long side panels
The first step is to assemble the long two long side panels. Screw the timber posts to the long side planks, using two screws at each point, enuring that the edges of the post are flush with the ends of the planks. The planks should not overhang past the edge of the corner posts, nor should they be short, they should be perfectly level with each other, as shown in the diagram below.
The second step is to attach the short side planks to the long side panels to make a large box shape. It’s easier to carry out this part of the construction on a flat, level, surface if possible, to ensure that all the corners are square.
A carpenters square can be used to check that the corners are actually at 90 degrees, as they should be. Once again, use two screws at each point on each plank.
How to Attach the Sides to the Corner Posts of a Timber Raised Garden Bed
NOTE: When fastening the planks to a post, the positions of the screws must be offset on either side, otherwise they will hit each other inside the post and won’t be able to screw all the way in!
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 (railroad tie, crosstie, railway tie) 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 math, 20/5 = 4cm
So, on the left hand piece of timber in the diagram, 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.
It’s just a matter of marking where the holes go, drilling the holes, and fastening it all together with screws, coach screws or coach bolts.
- Screws can be screwed in using a cordless drill fitted with a screwdriver bit.
- Coach screws and bolts both have hexagonal heads, and can be tightened with a spanner, socket set, or socket fitted to a cordless drill.
The reason for using cordless drills is because they have a screwdriver function, which allows the user to set the amount of torque that is applied to the screw by a variable clutch. By setting the level of torque, this determines how hard or deep the screw is screwed in, without shearing the head off the screw.
If using screws or coach screws to fasten the planks to the posts:
- As mentioned earlier, use hot-dipped galvanised screws, as these have a thicker coating of protective zinc (compared to zinc-plated screws) which prevents them rusting.
- Use the right length of screws. The rule for using screws is that they should be long enough to go right through the first piece of timber (the side plank), and approximately 2/3 of the way into the second piece of timber (the corner post).
- In hard wood, it’s not possible to screw straight into the timber without either splitting the wood or breaking the screws. Test on a scrap piece of the same wood if unsure. Drill pilot holes smaller than the thickness of the screws, so they can bite into the wood. Drill only part of the way through the post, so the screws can bite more into the wood. If the pilot holes are drilled too small, 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 they screw into the wood more easily, this also prevents the heads of the screws sheared off.
If using coach bolts to fasten the planks to the posts::
- Use hot-dipped galvanised bolts, nuts and washers for corrosion resistance.
- Drill a hole slightly larger than the bolt, and drill all the way through, both the plank and the post.
- Use a washer on the inside, then screw on the nut, and tighten using a spanner or socket on both sides.
Single Level Timber Raised Garden Bed Construction Using Steel Angle Brackets
Another way is to fasten the sides of a timber raised garden bed together, if it only is one plank high, is with steel brackets.
Steel brackets attached to the outside corners can be fastened using screws, while the brackets that are used on the inside are fastened using bolts, with nuts and washers.
The diagram below shows a steel bracket used on the inside of each corner, fastened using nuts and bolts. The heads go on the outside, and the bolts extend into the inside of the bed, out of sight, preventing a snagging or tripping hazard.
Another popular type of steel bracket used in landscaping is one which has a long stem with a pointed end, shaped like the letter “T”, 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 stabilises the structure. This is particularly important when joining two sleepers end to end.
Considerations When Measuring and Laying Out Timber Garden Beds
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.
This point is illustrated in the diagram below:
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.
Toxic Treated Timber to Avoid for Raised Garden Beds Used for Food Growing
When building timber raised garden beds, there are two considerations when it comes to the choice of timber:
- 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
Recycled Railway Sleepers
A durable material for raised garden bed construction are railway sleepers (railroad ties, crossties, railway ties), but don’t use recycled ones. Use railway sleepers are normally treated with creosote, a highly toxic and carcinogenic mixture of chemicals, oily or tar-like in consistency, most often derived from coal tar, and used for preserving wood.
Creosote will contaminate soil, and is best avoided for all garden beds. Researchers have found that children who play with creosote-contaminated soil tend to get more skin rashes than other children, so its presence is harmful.
I’ll focus mainly on timber raised bed construction here, though I’ll cover a few other methods too.
CCA Treated Pine Timber
Traditional CCA treated pine (Copper Chrome Arsenate) contains arsenic, a poison which will leach into the soil and be taken up by plants. Arsenic is persistent, being an element, it doesn’t break down, ever, and will also bioaccumulate, which means its concentration will build up to levels higher than the soil in living things.
CCA treated pine is toxic when handling or working with it, especially when drilling or cutting it as the toxic sawdust is hard to control. It’s also toxic when burnt, and should not be disposed of that way! Having toxic chemicals in the 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 as an alternative. It’s touted as being a new, safer type of treated pine.
Another toxic wood treatment is LOSP (Light Organic Solvent Preservative) are all toxic. Using painted timber is not a safe practice either.
Painted Timber
Paints contain a range of toxic chemicals, so painted wood should not be in contact with soil. Even boiled linseed oil contains heavy metal additives such as cobalt and manganese which are used as drying agents.
Untreated timber will eventually rot and break down, that’s what nature intends it to do. To garden organically, either preserve the timber raised beds with raw linseed oil, or use wood that has good longevity.
What is the Ideal Size for a Garden Bed?
The optimum size of a raised garden bed is 1.2m (4′) wide when accessed on both sides, or 60cm (2′) wide when accessible from one side only. The reason for a maximum width of 1.2m (4′) is a matter of ergonomics, this is the maximum width that an adult person can reach across to access the area efficiently from either side. 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 constantly.
I’ve found that 1.2 x 2.4m (4’ x 8’) is an ideal length not only to work around, but also to construct from 2.4m (8’) standard-length pieces of timber or railway sleepers, if using these materials.
Are Shorter Length Garden Beds Better?
The benefit of breaking up a long continuous 1.2m (4’) wide bed into shorter sections is that it increases the amount of “edge” available. To understand this better, we need to introduce the permaculture design principle of the “edge effect”.
This edge effect is an ecological concept, which states where two different environments meet, there is an “edge” or interface between two different area, that creates a broader range of favourable environmental conditions or ecological niches which are able to support an increased variety of plants and animals, and are therefore more highly productive areas.
For example, a 1m x 10m bed has (2×10) + (2×1) = 22m of edge.
If we now divide the same bed into 5 sections, we now have 5 x (2×2) + (2×1) = 30m of edge.
Through this simple change we now gain an additional 8m of edge.
There is a trade-off though when using shorter length garden beds. The additional edge requires additional materials, which increase cost, but the benefit of increasing edge will outweigh the initial outlay in materials.
The other consideration is space, the shorter beds require more space because of the spaces between them.
When using harder wood, it may be necessary to pre-drill the holes with a drill bit which is narrower with the screw, then screw into the holes.
When using very thick pieces of wood, it may not be possible to get screws long enough, and it may be necessary to use long coach screws with a hexagonal head (like a bolt) that are tightened with a spanner or socket set.
What Are the Benefits of Using Raised Garden Beds?
There are many benefits to using raised beds, and these include:
- increased productivity
- better soil conditions
- increased usage flexibility
- more efficient irrigation
We’ll examine each of these benefits in the following sections.
1. Raised Garden Beds Increase Productivity
The most important benefit of raised garden beds is increased productivity. Raised garden beds are at least twice as productive as a conventional garden.
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 plants growing there will shade out bare soil, making it harder for weeds to grow there.
According to Ohio State University:
“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.”
2. Raised Garden Beds Provide Better Soil Conditions
Since there is no soil compaction in raised garden beds, 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. For this reason, raised bed designs also lend themselves very well to the gardening technique of no-dig gardening.
Raised garden beds can be used to overcome issues of poor drainage, poor soil, or even no soil, such as gardening on top of concrete or asphalt, because we construct or purchase the garden bed, fill it with the type of soil we require, and build the soil further it by adding organic matter. It’s possible to create a number of garden beds with different soil blends in each for a variety of different growing environments.
By not stepping on the soil, we avoid soil compaction. When soil is compacted, water and air do not move as easily through the soil to the roots of plants. Plant roots 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.
3. Raised Garden Beds Provide Flexible Growing Options
Raised Garden beds offer a wide range of growing options. It’s much easier to attach trellises, supports, frames, fences or shade cloth over or around the bed temporarily, 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 garden beds are the answer. A bed of this height will enable the person to partake in their interest in gardening without hindrance.
Also, by not having to step into the garden bed, it’s possible to tend to the garden all year round. Gardeners can sow, plant and harvest, whenever they want, even when the ground is wet, because they won’t be stepping in mud!
4. Raised Garden Beds Can Be Irrigated More Efficiently
With their raised sides, raised garden beds can support very thick layers of mulch over the soil, which will not slide off, or be blown or washed away.
This aids in water conservation, and also makes it easier to enrich and build the soil through the constant addition of organic matter.
The dimensions of raised garden beds also 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.