Raised Beds

It’s time to write about raised garden beds.  First, I’ll cover a problem that I’m having; then give you my advice about raised beds.

The problem that I’m facing this year is that two of my raised garden beds are falling apart. Some of the landscape timbers that I used to frame the beds when I built them several years ago are decomposing.  In the photo below, the upper timber that I pulled off the frame is still holding together but the lower one is only a shell.

raised_beds01

Here, you can see how a timber has rotted from the inside out:

raised_beds02

I believe that there are two reasons for this rot pattern.  One is that the pressure treatment on these logs did not penetrate all the way to the center.  Pressure-treated dimensional lumber that I’ve bought (e.g. 2 by 4 boards), on the other hand, was clearly treated throughout (and seems to be lasting longer).  The other reason is that, unlike dimensional lumber, these timbers are made from small logs and contain pith from the center of the tree.  Pith is not as durable as heartwood.

Here are two photos of timbers that have split open:

raised_beds03

raised_beds04

Contrast what you’ve seen above with the following bed frame that is still sound:

raised_beds05

And two more photos of timbers that are in trouble:

raised_beds06

raised_beds07

When I started prepping the garden this spring, I discovered how bad some of the timbers and indeed the entire frames are.  Unfortunately, the timbers that I used were unusually large and I can no longer purchase them.  This eliminates the option of simply replacing the worst timbers.  I could rebuild the two bad beds with all new timbers, but that will be difficult.  What do I do with the soil that the frames are currently holding in place?  Perhaps more importantly, how long would the new timbers last?  Another option would be to build a frame using dimensional lumber, which I think would last longer.  That might allow me to build the frame a side at a time, as the ends don’t have to interlock like the timbers (see the fifth photo above).  It would still be time-consuming and expensive.

I think that I will leave the beds as they are this year and hope that they last.  I have some other, non-gardening projects that are taking time this spring.  Procrastination will also allow me to think more about the best course of action.  Having decided to leave this problem for the future, I can get back to preparing the garden.

If I had it to do all over again (well, it seems like I do…), what would I do?  Here are my suggestions for readers considering building raised garden beds.

You should ask yourself, “What do I get with raised beds?”  They allow you to work in the garden a little more comfortably.  With the wide timbers that I used in my frames, I can sit on the edge of the bed and plant seeds, pull weeds, or harvest vegetables.  It is also claimed that raised beds are ready to plant earlier in the spring because the soil warms up quickly and it drains more readily if it is soggy.  On the other hand, raised beds probably require more water during hot, dry weather.  Because I filled my beds with good soil removed from areas where I was doing other projects, they have deeper topsoil than a regular garden would.  Raised garden beds also look nice.

If you decide to build raised beds, you have to consider the frame materials carefully.  As you can learn from my experience, landscape timbers might not be the best choice.  Pressure-treated dimensional lumber should perform better, but nothing made of wood will last forever.  I should mention that, in the USA, pressure treated lumber was reformulated several years ago so it now uses only a copper complex.  You no longer have to worry about the arsenic or chromium that were previously used in pressure treatment of lumber.  All the pressure treated wood that I’ve used was of this new type.

To secure my landscape timbers, I used 18 inch (1/2 meter) long pieces of reinforcing bar.  These are steel rods about a half inch (12 mm) in diameter that are used to strengthen concrete structures.  I drilled holes in the timbers that are on the bottom layer and pounded the bars through those holes into the ground.  The timbers are fastened to each other with special-purpose screws.  The ones that I used are the TimberLok brand.  They come in various lengths (4, 6, 10 inches; 10, 15, 25 cm) and are designed to be drilled directly into the timbers without a pilot hole.  They worked well but the longest screws were too much for my old drill — I had to purchase a larger drill to drive them.

Another option for building the frame might be the concrete blocks that are used to build retaining walls.  This option would be expensive (and heavy!) but should last forever.  Any other material suitable for building an outdoor wall should work.

You also need to consider what size to make your raised beds.  Mine are six feet (a little less than two meters) across the outside, which I now believe is too wide — it makes it difficult to reach the middle of the beds.  I would recommend making your beds a little less than three feet across the inside.  Why?  Because I like to put landscaping fabric on some of my beds (I’ll cover that in a future post) and it typically comes in three foot widths.  I also recommend lining the inside of the frames with landscaping fabric — a friend did this and I wish I had — to keep soil from coming into direct contact with the timbers and to keep soil from leaking through any cracks.

Finally, you have to consider what to put around your raised beds.  I don’t recommend leaving lawn around them as that makes it easier for grass and weeds to grow into your garden.  As you can see in some of the photos above, I have crushed rock around my raised beds.  The rock is on landscape fabric, which reduces (but does not eliminate) weeds.  The problem with crushed rock is that it accumulates dirt, leaves, etc.  This makes it easier for weeds to grow.  It is difficult to clean the crushed rock so this problem will just get worse.  Concrete pavers or bricks might be a better choice, but again you have to weigh the expense and the effort needed to install them.

I hope you can learn from my mistakes and that any raised garden beds you decide to build will reward you with years of enjoyable gardening.  More on raised beds in this post.

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About brianbreczinski

work: chemist, NMR manager; hobbies: gardening, reading, photography, electronics, biking, woodworking
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