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Building a Raised Garden Bed
March 30, 2009

Asparagus sproutsThe two vigorous asparagus sprouts at right got me started building my second permanent raised bed in a year. I've used raised beds in the garden for a long time for growing intensive plantings of onions, carrots, beets, and such. But I had never put in any permanent raised beds, always just building a plateau of raised soil for the plantings.

The reasons for building a permanent raised bed are varied. Some folks just like the appearance and order of a garden or flower bed neatly contained with landscape timbers.

A raised bed may also be necessary to control soil erosion. Our first raised bed (shown below), more of a terrace until completed this spring, was built for that purpose. I really don't recommend building a raised bed this large, but this one fits our current needs.

Main garden

Flooded gardenRaised beds generally dry out and warm up more quickly than the surrounding terrain. This drying can allow the gardener to get back into the garden more quickly after heavy rains. It is especially valuable in the spring when frequent rains make getting cool weather crops into the soil difficult. The photo at right is of our big raised bed and our non-raised, older garden patch after a heavy, spring rain.

Narrow raised bedI guess I've saved what I think is the best reason for building a permanent raised bed for last in my list. Long, narrow raised beds allow the gardener access to the full bed in any weather.

The raised bed at left is our asparagus patch. It's inside width is 42 5/8", which is actually a little wide for easy access. But even with that dimension, I can reach in from the ends or the sides and work any part of the raised bed without stepping into it.

Intensive plantingWith a larger raised bed such as our main garden plot, one still has all-weather access along the sides of the raised bed, but "walking boards" are necessary to reach the interior of the bed in wet conditions.

The section shown at right from our main garden plot produced a succession of good crops last summer. The spring intensive planting of onions, carrots, beets, and more onions was easily accessible from the side and via walking boards on the interior. Compost on areaWe followed the spring planting with a crop of lettuce that lasted until Thanksgiving with the aid of a portable cold frame. Of course, that part of the garden plot had served for several years as our composting area, as the slope and subsequent erosion discouraged other efforts there. It didn't require a lot of fertilizer!

I didn't get around to fully enclosing the main garden until this spring. Once done, I still had several of the 6" x 6" timbers left and put them around the our asparagus patch.

Finished main bed

Since our main plot in the Senior Garden suffered both from soil erosion and standing water at times, I also added a dry sump near the middle of the patch. A dry sump is simply a homemade drain where you dig as deep as you can with a post hole digger and fill the hole with coarse builders' sand. It isn't fancy, and it certainly isn't a perfect drain, but it helps dry things out in the middle of the plot.

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plot in 2004Our asparagus bed is located on some gently sloping ground at the corner of our property that in the past has grown acceptable crops of tomatoes and broccoli. I deep dug the plot in 2006, mixing copious amounts of composted cow manure, peat moss, bone meal, and lime into the soil before transplanting asparagus plants I'd grown from seed into it.

With all the soil amendments added, including eight, forty pound bags of composted cow manure added last fall, the soil level of the plot was well above the surrounding area. I'd used a couple of old boards to hold most of the soil in place, but it was obvious this spring that something better was needed.

Asparagus 2008

TwineI already had some of the required treated timbers on hand, so I decided to go ahead and build a permanent raised bed for our asparagus. My first step in building the bed was to extend the true north-south line I'd built our main raised bed on. I was fortunate that I wanted the end of the new bed to be along the same line as our main raised bed. I used stakes and twine to extend the north-south line to and past the site for the new raised bed.

Then I scraped away the topsoil with a garden spade to create a level surface for the first landscape timber. I first only dug down about an inch or so, but later went down about 2 1/2" for the first timber. I originally thought I could use just one level of 6"x6" treated timbers (actual size 5 1/2"x5 1/2") to enclose the area. Lacking surveyors tools (or skills) to accurately survey the slope involved, I later realized a second layer of timbers would be required on the high side, as the base would disappear into the soil.

Slope

I was able to cut the cost of the project a little by using 4"x6" treated timbers for the second level of boards.

LevelIt's really, really important to make sure that first board or timber goes in level (both ways). On projects such as this one, you definitely need a good level. Mine wasn't terribly expensive, as I picked it up several years ago off the sale table at a local hardware store. It also helps to have both a 48" and a 12" level, although one can do the job.

I generally don't get my trench level on the first try and end up rolling the timber in and out of the trench as I add or remove soil to level it. I also keep a bag of builders' sand around to make filling the low spots a bit easier.

Having been humbled a bit (and made a good bit poorer by the acquisition of the second level of timbers) by my mistake on the slope of the bed, I used my carpenter's square to set my string at a right angle for the first perpendicular side before anchoring the first timber. I dug and leveled the trench for the timbers and placed the base and one top timber to be sure it was all going to work out right.

I read somewhere online that a good way to anchor landscape timbers was to use rebar. Rebar are iron rods used to reinforce concrete. One of our local hardware stores carries precut sections of rebar that measure 3/8"x22". That proved to be an ideal size for firmly anchoring the timbers on our raised beds.

Rebar diagramThe first board was anchored with three angled pieces of rebar. I angled two of the pieces against the expected force of the soil inside the new bed with the other piece angled the opposite way just for good measure. I use a long, 3/8"x16" spade wood bit to drill through the timber before hammering in the rebar with a two handed short handled sledge.

TwineLet me get back to the string a bit here. The sides of my raised bed were approximately two, eight foot timbers long. To get the side straight, it was necessary to use stakes and twine to keep my sides straight.

Once I had the first timber in level, I actually laid two layers of timbers, the 6"x6" as the base topped by a 4"x6" that overlapped the end piece, and anchored them both in with rebar hammered straight down. On our first raised bed, I anchored the base pieces first and ended up using a lot more of the rebar than may have been necessary.

The photos below are from our main plot raised bed construction, but give a good look at rebar going in both at an angle (inset) and straight down.

Rebar going in

I'll add here that while my DeWALT cordless drill is a dandy, I did string heavy duty extension cords to the building site for the asparagus bed. I found that I exhausted the battery pack on the drill after about 4 holes through the doubled timbers. Since I had to use my circular saw to start my cuts on the timbers, I needed power at the site anyway. Using an old corded drill speeded my progress considerably.

I really didn't have to do much cutting for this project. When I did, I used the circular saw to cut on each side of the 6" timbers and finished the cut with my chain saw. I would have never thought of that on my own, but years and years ago, I worked as a carpenter's assistant for one summer. We were cutting heavy rough cut ceiling beams and lacked a saw that could make the cut. So my carpenter boss asked, "Do you have a chain saw?"

Since the 6"x6"x8' timbers were actually 96 5/8" long, I cut one timber for the end pieces. That's how I came up with a 42 13/16" inside dimension for the bed. If I were building a raised bed just for vegetables or flowers, I'd make the inside width an even three feet to make everything easy to reach. As it turned out, 42" was just barely enough, as my original planting of asparagus roots had spread beyond the original 3' wide bed, and I ended up cutting roots as I dug out to lay the timbers on each side!

Overlapped cornersOverlapping side timbersThe rest of my cutting was just a matter of trimming the final timbers laid for length. Do note that I overlapped my corners for added strength. Overlapping the corners also made the side pieces overlap, adding strength to the sides and making it much easier to keep the sides straight.

That's about all there was to it, although it took considerable time to get the rest of the raised bed done. The finished product is something that will serve our purposes well. I ran outside as I was finishing up this feature and snapped a shot of the Senior Garden, showing the asparagus bed in the foreground, the main Senior Garden, and our original garden plot at the back.

The Senior Garden

Materials:

  • 5 - 6"x6"x8' Treated (non-arsenic) timbers
  • 5 - 4"x6"x8' Treated (non-arsenic) timbers
  • 27 pieces of 3/8"x22" precut rebar
Tools Used:
  • Spirit level(s)
  • Hand sledge
  • Tape measure
  • Circular saw
  • Power drills (cordless and corded)
  • Chain saw
  • Carpenter's right angle
  • Combination Square
  • Garden spade
  • Garden shovel (for backfilling)
  • Wheelbarrow and garden cart
  • Twine and wood stakes

I looked around the web just a little bit after completing this feature to see what else was out there about building raised beds. Not everyone will want to invest as much as I have in my raised beds, and some may want circular or irregular raised beds for landscaping purposes.

Here's what I found:

There are also lots of commercial options for putting in a small, raised garden bed. Amazon, Plow & Hearthicon, and Garden.com all offer reasonably priced kits for 4'x4' and 3'x6' raised garden beds. Shipping lumber can get a bit expensive, though, so you might want to take a peek at the ads and then design your own raised bed from what you see in the ads.

Let me add a word or two about treated lumber. The old CCA (chromated copper arsenate) treated lumber or timbers have the danger of leaching arsenic, a carginogen, into your soil. Most treated timbers today should be ACQ labeled (usually with a note that says "no arsenic").

From the EPA site:

ACQ is a water-based wood preservative that prevents decay from fungi and insects (i.e., it is a fungicide and insecticide)...The different formulations allow flexibility in achieving compatibility with different wood species and end use applications. All ACQ types contain 2 active ingredients which may vary within the following limits: copper oxide (62%-71%), which is the primary fungicide and insecticide, and a quaternary ammonium compound (29%-38%), which provides additional fungicide and insect resistance properties.

Do note that Miles McEvoy, who works in organic certification with the Washington State Department of Agriculture, has said that no pressure-treated wood is allowed in soils used to grow organic food. Are Pressure Treated Woods Safe in Garden Beds by Phil Wood (no relation) gives some alternatives to ACQ treated timber if you're still a bit wary about using it.

Since this is a Senior Gardening feature, let me add a personal comment here. After manhandling the heavy treated timbers and then hammering in twenty-seven pieces of rebar in two days, my hands were swollen and my whole body ached for days afterward! It would be better to recruit a strong helper for such a job than doing it by oneself (as I did).

Update (5/2/2010)

HeavingAfter two winters, I noticed some minor heaving of the timbers in our large raised bed from winter freezing and thawing. The heaving only occurred where the timbers were one deep. Timbers that were stacked two high experienced no heaving at all. If I were to redo this project, I'd probably sink a short stub of timber under any junction of single deep timbers to prevent heaving.

Of course, the asparagus raised bed was all two deep, so it didn't heave at all!

Raised beds

Update (6/12/2010)

Paul Calback's raised bedsPaul Calback shared his experience in building a raised bed for asparagus with me recently. Paul used 2x6s for his raised bed. Since this feature is already way too long, I shared his email and photos on my garden blog. It looks like a viable and possibly cheaper option than what I chose.

Update (9/17/2010)

I added one more, possibly our last, raised garden bed in our main gardening area this week. It's quite similar to our asparagus bed in construction, although I did change the dimensions a bit.

Our new raised bed measures 48" wide on its outside dimension. Subtracting the width of the timbers, it leaves about 36" of usable gardening space inside. It's approximately 16' long (15' interior), consistent with our other two raised beds in the area. I went with a slightly narrower raised bed this time, as I've found it a bit difficult getting to the center of our asparagus bed from the side during the summer. The lush growth of asparagus hinders access, but a shorter reach in the new bed will be helpful. And of course, a 48" outside width allowed me to cut the end cap pieces from just one timber, helping hold the cost down just a bit.

New riased bed

The new bed also is a bit higher than our asparagus bed. As you can see in the photo above, I haven't finished backfilling around the outside as yet. But even on the one end I did backfill, I needed a couple of lumber scraps to drive the rototiller into the new bed.

Main gardenAnd for some reason, I ended up using a lot more pieces of rebar to anchor this bed. That may be because I anchored the base first and then drove more rebar through the top and anchor timbers.

The "fill" for this bed was rather deluxe. I had lots of peat moss and screened compost available, so they were mixed with the existing soil to make what I hope will be an ideal growing medium.

We still have just a bit of our main garden not in raised beds. There's actually room for a wide aisle and one more narrow raised bed, probably similar in size to the one I just put in. But that will have to wait a bit, unless the cash fairy visits soon. Timbers are really expensive!

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