Raised garden beds have many advantages over traditional in-ground beds. However there are some mistakes that gardeners keep making with raised beds. How can you avoid making these mistakes yourself?
Raised garden beds are preferable to simply creating a bed in the ground for the following reasons:
- better drainage
- easier access
- control over quality of the soil
- warms up faster
- more aesthetically pleasing if made from the right materials
So what do you have to keep in mind when designing and building a raised garden bed or two? Are there some mistakes you might make that you will regret later? And how do you avoid those mistakes in the first place?
Using pressure-treated lumber
This is a bit of a controversial topic. The manufacturers of pressure-treated (PT) lumber want you to think that today’s PT lumber is safe, even for raised garden beds that grow edibles. They say that the chemicals in the lumber are locked in and don’t leach out even in very wet soil.
However do you really want to take a chance with your family’s health? It just takes one family member who has a repressed immune system or a young child to fall sick. There are so many other alternatives that in my opinion using PT lumber for raised garden beds that will grow food crops just isn’t worth the risk.
I would still use PT lumber for fence posts and other posts that will be set in concrete in the ground. And PT lumber should be used for deck framing as it is stronger than cedar or redwood. However you need to make sure that any edible plants are planted at least a few feet away.
So what are the alternatives? Check out this blog post on The Best Lumber To Use In The Garden
Building with concrete
Concrete is relatively permanent. That’s why some gardeners think that it is the perfect material to use for a raised garden bed. After all there is nothing to rot away so these beds will last a long time.
However that reason alone makes this not a good idea. While you hope that your raised beds will never need to be moved or changed, there is a possibility that your needs change and you do decide to move a bed or change the orientation.
With concrete you are going to have to break it up with a jackhammer and sledgehammer. Back-breaking work! And what do you do with the broken up concrete? It is not that easy to recycle or reuse.
Modular raised garden beds made out of wood, bricks, cinder blocks or stone are easier to break up and you can use the materials again for new raised beds in a different location or shape. Or reuse them in another project in your garden.
Concrete also can leach a lot of lime into the ground. If you have plants that prefer acidic soil, they will not do very well.
Concrete beds also require proper reinforcing and are susceptible to cracking, especially in winter if any water gets into even a hairline crack. When that water freezes it can create a larger crack. Not easy to fix either once that has happened.
In my opinion concrete also is not aesthetically pleasing unless you face it with some other material such as stucco or veneer brick or stain it (and the stains do contain chemicals that you may not want to have close to edibles).
Making them too wide
You need to be able to reach all areas of your raised bed without having to step onto the soil. Stepping onto the soil will compact it and then you have drainage and other issues.
A standard maximum width is usually 4 feet (1.2m) assuming you can reach in from both sides of the bed. If the bed is against a fence and you only have access to one side, make the maximum 2-3 feet (0.5-1m).
Use multiple beds if you need more space, separated by a pathway. The pathway size will depend whether or not you need to roll a wheelbarrow or other equipment through it. I used 3 foot (1m) pathways (see photo) but you could go down to 2 feet (0.5m) if it is only for access by one person for planting, weeding and harvesting.
Exposing end grain and not capping it
I see designs all the time that call for using 2×2, 2×4 or 4×4 posts in the corners of raised beds to tie together the sides of the beds. This is a strong construction method at least when newly constructed.
However one flaw with this is that the exposed ends of the posts will absorb water from rain and irrigation. Over time these saturated posts will start to deteriorate and rot.
To solve this problem you need to cap the posts or change the construction to avoid using posts. Capping the posts with a decorative post cap like what is used on fence posts works but may look a bit odd.
The better way is to avoid the posts in the first place and tie the corners together with metal brackets or more sturdy construction.
Read more about this in 7 Essential Tips For Building Long-lasting Structures
Using the wrong soil
When I had my raised beds filled in the spring of 2017, I had to purchase some soil. I skimped and saved about $30 by not going with a higher quality soil.
I regret that now as the soil I got had a bit too much sand in it and ended up turning to concrete if not protected by mulch.
To solve this problem I have been adding compost and covering the soil with mulch in order to try and build up the soil health. Planting cover crops will also hopefully build up the soil. But I have to live with it until the soil has had a chance to improve.
I’ve also heard horror stories of gardeners using what is sometimes dubbed “killer compost” or “killer manure”. Persistent herbicides are often still used on hay fields to kill broadleaf weeds. Horses and other livestock eat the hay but the persistent herbicide does not break down during the digestive process. The resulting manure is laced with this herbicide that is still very active.
If this manure is used in the garden (even if composted first) it will stunt your plants or worst case kill your plants. The only way to get rid of it is to either remove the contaminated soil or try to amend the soil and keep turning it to expose it to sun and oxygen as Joe Lamp’l in the linked story above had to do.
So what is the secret formula then for raised garden beds?
- build them from non-PT wood (cedar or redwood are best), brick, stone or blocks
- build them no larger than 4 feet (1.25m) wide if you can access both sides, 2-3 feet wide if you can only access one side
- make sure all end grain is capped
- obtain top quality soil or have enough of your own compost on hand to amend poorer quality soil
In fact all of these criteria were used when we built my daughter’s 2’x3′ garden. I documented the build in detail in my eBook How to Build a Garden Planter Box for Children
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Tranquil Garden Urban Homestead, Victoria, BC