Raised beds with pressure treated lumber?!

Pressure Treated?!

If you’ve ever looked into raised beds, you’ve likely looked for the materials to build them out of and, if wood, probably come across mixed advice when those discussions center around using pressure-treated (PT) wood.

Out of the abundance of caution, we’ve always steered clear of pressure treated wood – that is until recently. We decided we’d do our best to safeguard against what some worry about and see how it goes. We can’t help but believe that fresh produce grown in pressure-treated beds is any worse than most of the contexts where food comes from commercially. Furthermore, gardens near populations where pollution is a problem likely pose a higher risk of causing health issues.

To be clear, we’re NOT recommending you do this. We just decided to give it a shot and describe here how we built our beds. These instructions would be just as useful for non-treated lumber – they’d just last a lot longer.

Materials:

We started with these nifty raised bed blocks from Home Depot:

Next, we purchased our pressure-treated lumber. We were aiming to make long beds that were as wide as possible but still allow adults to reach the center of the bed from the edges. We ended up making beds that were approximately 5′ x 15′ and about 16.5″ deep (three courses of lumber) followed by another 1.5″ cap layer, so 18″.

Each bed required:

  • Beds
    • (12) corner blocks
    • (2) 1/2″ by 10′ rebar cut into 4 x 36″ pieces.
      • This leaves about 18″ of support in the ground.
    • (8) pressure treated 2″ x 6″ x 16′
      • 3 for each of the long sides
      • 2 for the long sides of the top frame
    • (4) pressure treated 2″ x 6″ x 10′
      • 3 cut at about half to form the short sides
      • 1 cut at about half to form the short sides of the top frame
    • Misc scrap wood for supports and accouterments
    • Screws
    • A dozen or so standard poly feed bags (scraps from animal feed).
    • Staples
    • Wood chips
    • Soil
  • Waterers:
    • (3) 10′ lengths of 4″ solid drain pipe (with flared ends)
    • (3) 90° drain pipe or schedule 40 PVC elbows
    • (2) 4″ PVC caps
    • PVC glue (primer probably not necessary here)

Tools needed

  • Circular saw
  • Small sledgehammer
  • Digging tools for leveling
  • Cordless driver (for screws)
  • Tape measure
  • Level
  • Tri-square
  • Stapler
  • Tractor (filling 15′ x 5′ beds without a tractor is no fun!)

The process

We live on a mountain. Nothing is easy when you live on a mountain, except falling down the mountain or finding runoff water where it ought not be! Garden on a mountain long enough and you’re likely to have one leg grow longer than the other 😉

Our first step in building the beds was to excavate flat areas for our beds. Oh, what fun. We used a combination of a backhoe, shovels, and many curse words.

Once excavated, we placed the corner blocks, leveled and plumbed them to one another, then drove rebar into the ground through the first block of each corner, leaving ample rebar for the two additional courses. You don’t pound that all the way in until the bed is complete and ready for the top “frame”.

We then proceeded to place the lumber into the “grooves” of the blocks. They just sit in there in slots – no screws or nails. We added three “courses” to get the depth we desired. We used scrap wood to cut braces to screw vertically to the long sides to keep them tightly together (see two in the picture below).

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We used the scraps cut off of the top layer (45 degree cuts) to make side supports as seen on the end here. We fastened it all together with screws.

Now, we still have our concerns about pressure-treated lumber and want to minimize exposure as much as we can…

We have a massive amount of used feed bags so we decided to line the bed with them to keep the soil from having any direct contact with the pressure-treated wood.

Unfortunately, we got the idea for that after we put the top frame of the beds on, so we took the top frame off, stapled them to the top edge of the last course of the bed, and also to the bottom of the sides, making it nice and tight. We did it this way to keep the feed bags firmly attached and to avoid ugly fraying bag edges. Once the top frame goes on (with screws), it makes a nice clamp for the bags. The bed material holds the bags up against the wood.

IMG_1466

Here you can see the feed bags lining the bed. Only the brick edges have contact with the soil

Watering

We had prior experience with copious watering chores and decided to borrow some concepts from Hügelkultr for our raised beds. We did so first by adding about 1-2″ of wood chips to the bottom of the bed. This does a few things…. first, it helps to wick water up into the bed when it does rain, and then it holds it there quite well since there isn’t any easy opportunity for evaporation.

To be double sure that we would have drought-tolerant beds, we also added custom waterers made from 4″ PVC drain pipe. We drilled 1/32″ holes every few inches along each side of the pipe. If you were to look down the pipe so that the opening appeared as a circle, we drilled holes at about the 4 o’clock and 8 o’clock positions, and about every 6 inches down the length of the pipe.

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The pipe is capped at one end and then has a small stack that extends out of the bed and is loosely capped for filling. Then the whole waterer pipe gets buried with wood chips.  We added wood chips to about the top of the pipe.

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Note: We added horizontal braces (made of cedar) to add support across the bed and prevent the sides from bowing under the weight of the soil. These also pin the waterer down firmly.

This approach results in about 6″ of woodchips in the bottom of the bed and an expedited way to get water into them. This left us with a remaining 12″ or so of planting depth.

The idea is to saturate the woodchips so that they retain moisture and cause deep rooting of plants. It is also to prevent water loss. For what it’s worth, just from our common rainfall, we did not do any watering on these beds this year and had pretty nice results. We used these waterers in other beds with the same result – NO watering.

Most people water the surface of their garden beds, which requires a substantial volume to soak the root zone of the plants. It’s a waste of water and time in our experience. It also discourages deep rooting. This approach soaks all the rainfall UP into the bed as well as down from the surface and saturates the wood chips. From there it maintains an even moisture level that is almost impervious to evaporation through heat and wind. It is a huge time saver even without the watering pipes. We just used those so that if ever there is a long drought, we can maximize our watering efforts.

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Finally, we add a mix of mushroom soil, topsoil, and rabbit manure to the beds, followed by another thin layer of wood chips (to prevent evaporation and for mulch). All that was left was to plant them.

Note: We’ve had plenty of armchair gardeners critique using woodchips like this, but we can’t argue with our success. Forests don’t demand only composted leaf litter and debris take residence on the forest floor so demanding only heavily composted wood chips in garden beds is erroneous. We aren’t using as a soil amendment but as a soil cover. Even a cover of newspapers would be better than nothing! I’d rather have a cover of plastic than no cover. There is a word for uncovered landscapes – “desert”.

Lastly, we stained the outside exposed wood using Cabot Australian Timber Oil (“Carribean Walnut”). We love that stuff!

Summary

If we die of cancer, you the reader can point to this post and proclaim where we went wrong. If we don’t, we can at least enjoy these beds for a a decade or more and the fruits that we grow in them.

Please feel free to ask questions or share your experiences with pressure treated garden beds in the comments below.

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