How Much Wood Do I Need To Build An Above Ground Garden

How to make a raised garden bed including guidance on the best sizes, types of wood, and what to fill them with to grow vegetables. Includes an instructional video

There are several ways to go about creating a vegetable garden, but if you have challenging ground or mobility issues, raised garden beds are the best. That's because they create an elevated space above your existing ground. A box that you don't have to lean down as far to tend, and in which you can control the growing medium. Since you have to fill them, you get to choose what type of soil, compost, and aerating materials go into them. With that choice, you can grow vegetables in a medium that's better drained and more fertile than what the soil on your property gives you.

Last year my boyfriend and I built four raised garden beds in our small back garden. It was the best choice we could have made and they've given us incredible harvests both the first year and this. In this piece, I'll share how and why we built them and other information you may need to create your own.

How to build a raised garden bed including guidance on the best sizes, types of wood, and what to fill them with to grow vegetables #gardeningtips #diygarden #raisedbeds — The raised garden beds in their second year, and surrounded with pea gravel pathways

Elevated gardening in raised garden beds

Most of the positives of growing in raised garden beds come down to their being elevated. Giving the soil a bit of height can help improve drainage, and can also thaw out a bit quicker in spring. Very handy if you live in a colder region. The height of raised garden beds tends to be anywhere from six inches to waist-high. When you make a raised garden bed, the height is purely down to your and your garden's needs. For example, if you're in a wheelchair or have mobility issues then a taller structure will help you to reach into beds. If you have poor soil under the raised garden beds, then you'll want to plan your beds to be deep enough to accommodate the longest roots. This can be up to two feet for vegetables like carrots and parsnips.

Another positive thing about raised garden beds is that they help stop erosion. My home garden is on a slight slope and I know from experience at my allotment garden, that mulch and soil will erode downhill over time. Boxed sides will help keep it in, and stop erosion from happening. If you're on a slope too, you could go the full gamut, and terrace your raised garden beds so that they were completely level. We built mine directly on the slope though, and they've worked out just fine.

Place raised garden beds away from trees

In my case, I've designed beds that are about a foot high. I'm planning on using them to mainly grow herbs and shallow-rooted leafy greens. I've also lined their bottoms and sides with landscaping fabric to help stop roots from a nearby hedge and trees getting in. Those roots were one of the main reasons that I chose raised garden beds. Tree roots tend to hang out in the top 18″ of soil so the beds should be free of root-invasion.

Vegetables and fruit crops will suffer if trees rob their growing spaces. If you're planning to make raised garden beds, try to situate yours at a good distance from them. Everything from shrubs to giant redwoods will creep their way near those fertile boxes of nutrients and water and can suck them dry. When planning the best placement for your raised garden beds, try to put them at a distance from trees that's a minimum of around two to three times the reach of the tree canopy. That general rule mirrors the extent of the root system under the ground.

Lining raised garden beds

Sometimes it's very difficult to place beds away from trees, hedges, and large shrubs. You could have a small growing space, a shared hedge, or trees that you are reluctant to take down. In that case, you can still build raised garden beds in their vicinity but you should line the bottoms of your beds. Very small gauge galvanized steel mesh is a long term option that will keep large roots and moles or gophers out of your beds.

I have hedges on three sides of my garden and some fruit trees at the back. The neighbor has a large oak tree too and some of its roots come into my garden too. Instead of steel, I've opted to line my beds in landscaping fabric. It should keep the bulk of the roots out for a good long while. It's also water-permeable so will ensure the beds don't get waterlogged. For that reason, it's probably not the best idea to line raised garden beds in plastic sheeting. It could turn your beds into little swamps.

The best placement and sizes for raised garden beds

You create raised garden beds by building a large container and filling it with soil, compost, and aerating materials. You can use anything from wood planks to logs, metal panels, purpose-made plastic, and other materials. The most common type is made with wood and that's what I chose for mine too.

Before construction, you'll need to work out a few things: the situation, bed sizes, number of beds, and the building materials. Choose a sunny spot and if it's well-drained and had good soil that's a bonus. One of my challenges is that my beds are near trees and a hedge. If you can, situate yours at least

The best width for a raised garden bed is four feet (1.2m). Beds this wide are easy to reach into from all sides which is what you want. The best length is debatable. I think that if you're tempted to hop over your beds regularly then they're too long. Eight feet (2.4m) is standard but 12′ (3.7m) is common too. These measurements are based on the beds being rectangular but you can build your beds whatever shape you choose.

Raised Garden Bed layout

Choose a sunny spot
If possible, situate beds away from trees and hedges
Build beds to be 4′ (1.2m) wide or less
Give space between beds to walk, mow, or push a wheelbarrow through. Mine are 20″ (51cm) wide
Use a grid layout for rectangular beds. This makes access easier.

Raised garden bed materials

Once you know the size of beds you'd like and the layout, you'll be able to work out your materials. In my case, I bought 16′ planks (4.8m) that we cut down into 8′ (2.4m) and 4′ (1.2m) lengths. The planks are 1.85″ (4.7cm) thick and 6″ (15cm) wide.

For the corner posts, I chose 2×2″ (5x5cm) stakes that are 2′ (61cm) long. My posts are long because my beds are on a slope and I don't want them to move at all. If you're on a flat surface, you don't need to drive the stakes in if you don't wish. That means they can be as short as the height of your finished beds.

To make wooden raised garden beds you'll also need long stainless steel screws meant for outdoor use. They're usually called exterior or decking screws.

The best wood for raised garden beds

There are various materials you can use to build the sides of your raised garden beds. I've seen bricks, cinder blocks, corrugated roofing sheets, tires, and even wood-effect plastic. Some of these are expensive and some may leak toxins into the soil. That's one of the reasons I chose wood for my own beds. That, and I like how they look.

When considering wood, you could use whole logs, or more commonly, planks. Planks over an inch (3cm) in thickness will last longer, and the best wood for raised garden beds is hard-wood. The best of the best is cedar since it's naturally rot-resistant, doesn't require chemical treatments, looks great, and will last for 10-20 years. It's also expensive so could be cost-prohibitive. In my case, it's not even available since I live on a small, isolated, island and my choices are limited.

The next best wood for raised garden beds is soft-wood like pine and spruce. It's a cheaper option but beds made of soft-wood only last 7-10 years. It's also a lot more susceptible to fungi and pests like termites which is why it's usually always pressure treated.

Pressure-treated wood for raised garden beds

The wood I've used for my new raised garden beds is spruce and it's pressure treated with Tanalith E[1]. It's a compound made up of copper and organic biocides that slow the wood's natural rotting process and defend it from fungus and insects. If it's been pressure treated onto the wood, it's accepted by the organic organization, the Soil Association, for use in building organic vegetable beds[2].

I spoke with the timber merchant and the sawmill that cut and treated the wood I purchased. It turns out that before 2006 in the UK and Europe, and 2003 in the USA, most wood was treated with an unsafe preservative. It included arsenic and high amounts of chromium that could leach into the soil. These days that's no longer the case but older reclaimed wood could still have that preservative. Also, some commercial wood like that used for telephone poles is likely treated with toxic preservatives.

If you find soft-wood that has not been treated at all then you can use that too. Its lifespan can be very short though so plan to replace the planks every five years.

Using pallets for raised garden beds

I built my very first raised garden beds using wood reclaimed from pallets. It was a nightmare getting the planks off undamaged but we got there eventually. They worked a treat at first but after two years the wood already looked ready to be replaced. It was an inexpensive option though — practically free — so if your budget is limited, go for it.

When using pallets please be aware that there's a type you should avoid. On the side on any pallet is a stamp with various letters and symbols. It will always include either the initials 'HT' or 'MB'. HT means the wood was heat-treated to kill any pests and it also means that it's safe to use. MB means that the wood has been treated with the pesticide Methyl bromide and is unsafe for your garden or home. Often times these initials appear alongside 'DB'. That just means that the wood has been debarked.

Make a raised garden bed

Each side of my raised garden beds includes two planks. They're attached together and to the other planks with a stake set at each corner. This is how we built them and the video at the end of this piece will give you a better picture.

Each bed will have eight planks and four stakes. There are two planks for each side with mine the shorter sides are 4′ and the longer ones are 8′. The stakes don't need to be driven into the ground for flat surfaces. For slopes, it's a good idea to have them 8-12″ in the ground.
To begin, place two shorter planks together on a flat surface. If they're printed, set them so that the printing is facing you.
Attach them to stakes set at the corners. A screw goes through the stake into each plank and it helps to drill a pilot hole first. This helps stop the wood from splitting. With my design, I also left space between the stake and the edge of the planks. This space is where the longer planks for the other sides would slot in. I also set the stakes slightly below the edge of the planks (about an inch) so they're not as visible.
Repeat this process for the shorter planks for the other side of the bed.
Take these finished sides to near the area you're building the beds.

Building raised garden beds

Once those two shorter sides are put together, you can finish building the beds in situ.

Lay two of the longer planks on the ground, printed sides facing you. Set one of the shorter sides upright at a 90-degree angle to the long planks. Screw through the stake into the longer planks to attach. Repeat this step and attach the other short side to the other end of the long planks.
With the help of a second person, lay the three-sided bed down flat. Attach the last two long planks to form the last side. Make sure the printed side will be on the inside of the bed once again.
Measure where your beds are to be placed and mark the areas where the four corners will be.
If you're on a flat surface, flip the bed over and set it in its position. Skip the next step.
Raised garden beds on slopes need a bit more stability. It's to stop movement and give the bed a firmer standing. Driving the stakes into the ground helps the beds and wood to stay put and reduce splitting. Dig holes deep enough for each of the stakes at the four corners and then flip the bed over and put it in its final placement. Fill in the holes and stamp down.
Screw each shorter plank into its adjoining longer plank with two screws. Drill pilot holes first.
Fill the beds with your choice of topsoil, compost, manure, and conditioning/aerating materials. Wait two weeks for the beds to settle before planting up.

How to fill raised garden beds

One of the most confusing parts of building raised garden beds is choosing what to fill them with. First of all, if you've chosen not to line your beds, you should still put down a layer of cardboard or stacks of newspaper. This will suppress the grass and weeds below. That's right, you don't have to dig the ground and you can fill them in right on top of the turf.

The general rule is to fill raised garden beds with 40% topsoil mixed with 40% compost or well-rotted manure, and 20% material that adds drainage and water-retaining properties. You'll see in the video that I've ignored that rule due to my fear of the New Zealand Flatworm being introduced via contaminated top-soil. I'm in a unique situation that shouldn't affect too many other people. The topsoil on the Isle of Man may be contaminated with this pest and its eggs and I don't want to risk it in my garden.

Mulch raised garden beds yearly

After you fill them once, you should still add a layer of organic matter, such as compost, seaweed, and rotted manure, as a yearly mulch. Just spread it an inch or two deep all over the surface of the beds and plant or sow seeds directly into the compost. A compost mulch not only suppresses weeds but maintains soil health and productivity. That leads to big, healthy harvests.

Making garden compost is easy so this need not be an expensive part of gardening. I go through the easiest way to make compost over here. There's also an excellent article that outlines how to prepare and amend garden soil that I recommend you check out. I should also say that I'm against filling raised garden beds with garden waste and sub-soil, even at the bottom. If it's not good enough to grow veggies in an open bed, then it's not going to help them grow unseen at the bottom of your raised garden beds.

Video on building raised garden beds

I go through the process of building raised garden beds and the challenges in the video just below. There showing how we built the beds and also a good discussion of the best wood for raised garden beds. If you have any questions you can leave a comment on this piece or over on YouTube.

References
[1] Tanlith E
[2] Tanalised Timber: on tanalized timber being safe for organic vegetable beds