We were keen to start our garden this year and begin growing some of our own produce, but wanted to keep our ambitions modest. In addition to the elevated herb planter I made a couple of weeks ago, we decided to construct a single raised bed in which we could grow some vegetables.
The first challenge was finding where to put it. Most of the level space we have needs to be kept clear for the construction work to begin in a month or so. Plus, we wanted somewhere that would get plenty of sunlight.
In the end we found a spot on the edge of the driveway, right on the edge of the building site where it won't be in the way. Long term, we have another space ear-marked for our garden (but it needs clearing first!), so this is just a temporary location for a couple of years.
There are seemingly innumerable ways to build a raised bed - including wood, plastic, metal, stone and concrete. We have a sawmill, and milling our own lumber would be one effective way to build a bed.
However we chose to try something a little different - another experiment if you like! We decided to build the raised bed using logs - much like a log cabin. We've felled almost 150 trees in the past month, the majority of which are spruce and we have plenty which are arrow-straight.
We will be experimenting with a horticultural technique called Hügelkultur - an approach to permaculture that uses decaying wood debris as the base of a mound on which you grow your plants. While Hügelkultur mounds can be built on flat ground, they can also be formed in raised beds.
The mound needs to be deep, so rather than raising the bed too high, we chose the dig down into the ground - a common approach to Hügelkultur.
Having identified an approximate location for the bed, we marked out a 4x8 foot rectangle using marker flags, trying (only somewhat successfully) to avoid large rocks. We then used a combination of the tractor with the forks and bucket, a rake, shovel, mattock and garden fork to remove the forest debris and dig down about 6 inches.
Log Wall Construction
To build the log walls, there were two different approaches we considered - stacked layers or overlapping logs.
In stacked layers, you cut lap joints in each corner to join two logs together. An advantage of this approach is that the cuts are easier to do - simple square lap joints. However, the logs aren't joined to those above or below, so you then need to use fasteners (screws or a piece of rebar are common) to join them together.
By contrast, log cabins are typically built using overlapping logs at the corners so that logs on one side are vertically offset from those on adjacent sides. Since each log is joined to the logs above and below on adjacent sides, no additional fasteners are required in the walls (although you may want to secure the very top layer).
This approach looks, I think, much better and should be much stronger too. The downside is that the cuts are much harder to perfect.
Considering our options, we decided to go all in with our experiment - we'd try and build our log walls with overlapping corners, specifically a style known as the Swedish Cope! We decided to build the raised bed using spruce logs, about 5-6" in diameter, picking the straightest logs we could find.
You want the raised bed to relatively flat and level, so we spent some time making sure it was as level as we could reasonably get on a forest floor! With the wall construction style we had chosen, we needed a half log (semi-circle profile) with the curved side up on each of two opposing sides.
This meant ripping a log in half, so we put these on the short ends. We found a log and cut a piece off about 6ft long (4ft internal bed width plus 12" for a 6" log at each end plus a 6" overhang at each end).
I've never ripped a log with a chainsaw before, but it turned out just fine. We propped the log up on some spare logs to keep it off the ground, then I scored a line down with our chainsaw. Diana held one end while I began ripping the log in half, then as I reached the midpoint she moved to the other end.
I took three passes to rip the log in half, and our Husqvarna 550XP Mark II did the job very nicely. With the logs cut, we carefully positioned them and ensured they were level.
As with building anything, if your foundations are poor then everything you build on top of them will suffer. We plan to use square foot gardening to lay out the raised bed so getting the logs perfectly spaced, parallel and square was important.
The walls are made up of 5-6" spruce logs - 6ft long on the short sides and 10ft on the long sides. Accounting for the width of the logs and a 6" overhang at each end, this gives us the internal 4x8ft bed we're after.
Each log has to be cut to fit over the perpendicular logs below it. The type of cut required is called a saddle cut - appropriate since this semi-circular cut allows the top log to sit on the log below.
We found a straight, 10ft spruce log and laid it down in position across our base logs. Then, rolling it back 180° to exposed the bottom surface, Diana held the log in position from one end while I used the chainsaw to cut the saddle cut in the other.
I am by no means an expert at this, but the technique I found worked well was as follows. First, I lightly scored two lines that would delineate the width of the saddle cut. Then, holding the chainsaw as level as possible, I did a series of cuts above a chain width apart - shallower near the edges and deeper in the middle.
I could then use the chainsaw to cut through these vertical pieces, and then hog out the remainder of the material. Periodically we'd roll the log back into position to see how the fit was, and then remove more cuts as necessary.
It's a very repetitive technique, and on average it took about 15 minutes to find a log, cut it to length, position it, and carve out the two saddle cuts to lock it to the layer below. We repeated the process until we had built up 3 full logs on each side, giving us a total bed depth (including what we had dug out) of around 24".
A few of the cuts were a little loose, and a couple of logs needed a little bit of trimming to allow them to mate with their neighbors cleanly. We weren't too worried about gaps between the log layers as this won't be an issue once the whole bed is full of soil!
Each layer is held firmly in place by its own saddle cuts and the weight of the logs above. However, the top layer is therefore liable to slide a little. If you shook them you could feel them wobble.
Rather than anything too elaborate, I just sunk a 3" nail into the join in each corner and this locked them down tight. Now, trying to shake the walls doesn't do anything - they're rock solid!
We deliberately left 6" of overhang on each log so I could come through at the end and clean them up. We didn't want to flush cut them, but we did want the overhang to be consistent so I just cut vertically down each corner to trim them all to the same length.
And with that, the bed was built!
I'll be honest, I was pretty dubious when Diana suggested we build a raised garden bed using round logs - after all, we have a sawmill so why not use it to cut some square cants or boards to build with!
But I'm really impressed with how it turned out. I took my time with each cut and they came out much better than I was expecting - I won't be building a log cabin any time soon, but for this type of project I think it worked great.
The bed measures exactly 4x8ft, giving us 32 square feet of space to grow some vegetables in. And being 2ft deep, it's enough depth to build up some good Hügelkultur layers too.
Speaking of which, our next task is to fill it with forest debris, cardboard, logs, wood chips, top soil, compost and mulch!