Best Building Material for Raised Garden Beds

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Robert Pavlis

In a previous post I discussed the pros and cons of using raised garden beds. If you have decided to make raised garden beds there are many options for making the walls, including concrete blocks, different types of lumber, galvanized metal and even old branches.

Each option will make good walls for the bed but some of them have health or environmental concerns that you should be aware of. Price may also be a consideration.

At the end of this post I will also have a quick look at materials you might consider placing at the bottom of the bed and the soil you should use inside the bed.

raised beds made with wood
Raised beds made with wood

How Permanent Do You Want Them?

Someone new to gardening decided to use raised beds and asked me about the best size, location, building materials etc. My response might surprise you. I suggested that they don’t build them. Building raised beds is quite a commitment in both time and money. If you are not sure how interested you will be in the hobby and don’t have specific plans for your garden design it is better to just make a simple garden bed and start with that. Raised beds will not make or break your ability to grow food.

Soil Science for Gardeners book by Robert Pavlis

If you are a new gardener I strongly suggest that you start by growing in the ground. The beds may not look as nice, but they cost a lot less. If in a few years you decide this is not for you, you have spent no money and there are no walls and soil to remove from your yard.

If you still plan to go ahead with walls, then build then with cheap material – probably regular pine. They will last many years and during that time you will get a better understanding of your preferred size and location. With this experience you can then make your next set of raised beds out longer lasting material.

Concrete Blocks

Concrete blocks are very popular for making raised beds. They produce a very solid wall, require no building experience, and are readily available. The one limitation is the height of the walls. Unless you start adding concrete you can’t make the walls too high. A double row of bricks works well, and three high is probably the limit since taller walls are less stable. But a two brick wall is more than adequate for most users.

There are two concerns about concrete blocks for raised beds; pH and heavy metal seepage.

The pH of concrete is alkaline, and it is known that the soil around concrete foundations becomes alkaline as lime slowly seeps out. This might be a concern in very alkaline soil, but in most soil this is not a significant problem. In fact many people add lime to acidic soil on a regular basis to raise the pH and sweeten the soil.

The second concern stems from the fact that fly ash is used to make concrete. Fly ash is the byproduct of the coal industry and can contain heavy metals. Research has shown that the leaching of heavy metals from concrete is insignificant and growing vegetables near concrete blocks is safe (ref 1).

Untreated Wood

Wood is a good choice for raised beds. It is relatively inexpensive and can be used to make beds of various design, even tall walls and raised beds are possible. They can also be fitted with sitting areas to make weeding easier.

Cheaper wood like pine will last for 6-8 years depending on climate. I use pine for my raised beds. I figure that in 6 years I will probably want to change things anyway.

Cedar is much more expensive but lasts longer. Is this extra expense worth it? I am not convinced it is, but that probably depends on the price of cedar where you live – it is quite expensive in Eastern Canada.

Treated Wood

Cheap pine and other woods can be treated with chemicals to make them last as long as cedar. Prior to 2004 the chemicals used to make pressure treated lumber included a process call CCA which contained arsenic. Small amounts of arsenic can leach from this wood and be absorbed by plants. For this reason CCA pressure treated lumber is not recommended for raised beds.

Almost all of the pressure treated lumber available today in North America and Europe uses a process called ACQ which does not contain arsenic or other heavy metals, but it does contain copper. ACQ treated wood is considered safe for use in the garden.

If you are planning simple, low beds, I would stick to untreated wood, knowing that it needs to be replaced more frequently. However, if you are building high beds and using a substantial amount of wood – I would use ACQ treated wood.

Certified organic production facilities are not allowed to use treated wood, but they are allowed to spray plants with copper fungicides – which are much more toxic than treated wood.

Sticks and Logs

In this picture you can see raised beds made from sticks. This works quite well and is perfectly safe. It can be difficult to keep the soil from running out of the cracks between the sticks. One solution is to use larger logs, but they can be difficult to get and move.

Raised bed made with sticks
Raised bed made with sticks

Corrugated Metal

Raised bed corrugated metal
Raised bed corrugated metal

In the last couple of years I am seeing more designs that use corrugated metal for the sides. This material will last longer than wood, but to be honest, I think it looks ugly. It might be a good choice for high beds since it can be purchased in wide widths and it is quite strong.

Corrugated metal is usually made from galvanized steel, which is coated with a zinc alloy. The zinc can leach out into the soil, but the levels are considered to be very small. Both plants and animals require some zinc in their diet so there is no health concern (ref 2).

Material for Under the Bed

The walls of the bed are usually sitting on top of soil. Some people place materials at the point where the ground soil meets the bed soil but it is best not to put anything at this interface.

Keep Rodent Out

If you have problems with digging animals like gophers and voles you could consider placing metal hardware cloth at the base of the bed. This will prevent animals from digging up into the raised bed and works best for taller beds. For beds that are only 6-8 inches tall, it will get in the way and impede the growth of some root crops. You’ll wish you never added it.

Weed Barriers

Some people add weed barrier cloth, or even plastic, at the bottom thinking that this will keep weeds out. It does not work well for weeds – they soon find a way through or around the material. But more importantly, weed cloth will prevent the water in the bed from soaking out. The raised bed no longer drains as well and can in fact be wetter than the surrounding ground.

Don’t use weed barriers.

Soil For the Raised Bed

Many sources will tell you to use highly amended soil for your raised beds. Some people even use a soil-less mix containing artificial material such as perlite and vermiculite. These solutions are expensive and completely unwarranted.

The best material for your raised bed is regular top soil. If you really want better soil add a bit of compost.

I have discussed the selection of soil previously in Soil for Raised Beds – Which One is Best.

Building Raised Beds

Here is a good introduction video for building a raised bed.

YouTube video

If you can’t play the video try this link: Raised Bed

References:

  1. Heavy Metal Leakage From Concrete; http://www.ccsenet.org/journal/index.php/ep/article/download/18289/12097
  2. Can I Use Galvanized Metal to Build Raised Beds; http://www.rodalesorganiclife.com/garden/when-sheet-metal-meets-soil
  3. Garden Use of Treated Lumber; https://extension.psu.edu/environmental-soil-issues-garden-use-of-treated-lumber

 

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Robert Pavlis

I have been gardening my whole life and have a science background. Besides writing and speaking about gardening, I own and operate a 6 acre private garden called Aspen Grove Gardens which now has over 3,000 perennials, grasses, shrubs and trees. Yes--I am a plantaholic!

11 thoughts on “Best Building Material for Raised Garden Beds”

  1. Very informative. I notice that in the video, the sides of the wood in the raised garden is painted green, and also in another picture it is painted bright pink. What kind of stain would you recommend that would be safe and not toxic for a raised vegetable garden?

    Reply
  2. I have never been a fan of raised beds, but as age and arthritis creep in, I am having to reconsider. I want something that will last but not leach anything that would be toxic (or that we will discover to be toxic down the road). Are there any known or potential issues with composite materials such as those used for decks or siding?.

    Reply
  3. Spot-on advice, as usual. One data point in Florida, where you grow year round without significant cold: wood rots year round too. My pine boxes broke down after 3 years

    I also got the sense that the intense summer light and heat necessitated more watering in the raised beds as they seemed to dry out faster than my non-raised sites. So at my new home I’m going straight in the ground, no boxes at all.

    Reply
    • I suspect the higher humidity also helps rot wood faster in the south.

      Yes – raised beds do dry out quicker. Growing in the ground actually makes a lot more sense, even here in the north.

      Reply
  4. We use raised beds because our land is a rock pile with very shallow soil. Concrete blocks work well for us and are more durable than wood.
    Three observations: Firstly, the blocks need to have a concrete foundation as they begin to tilt over time as the ground beneath them becomes wet but they don’t need to be cemented together if only going 2 high. Secondly, the blocks and the bed absorbs more heat in our Australian Summer. Keeping moisture in the bed is an issue sometimes. Planning to try a wicking feature on a soon to be refurbished bed. The upside is that the bed is warmer in Winter. Thirdly, the raised bed is great for weeding as it provides comfortable sitting. Regards Len

    Reply
  5. Your info is always so interesting and informative. I’ve gardened for
    55 years and am always amazed at what an interesting pastime it is.
    Endlessly fascinating and always something more to learn. Cheers!

    Reply

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